//! Traits, helpers, and type definitions for core I/O functionality.
//!
//! The `std::io` module contains a number of common things you'll need
//! when doing input and output. The most core part of this module is
//! the [`Read`] and [`Write`] traits, which provide the
//! most general interface for reading and writing input and output.
//!
//! ## Read and Write
//!
//! Because they are traits, [`Read`] and [`Write`] are implemented by a number
//! of other types, and you can implement them for your types too. As such,
//! you'll see a few different types of I/O throughout the documentation in
//! this module: [`File`]s, [`TcpStream`]s, and sometimes even [`Vec<T>`]s. For
//! example, [`Read`] adds a [`read`][`Read::read`] method, which we can use on
//! [`File`]s:
//!
//! ```no_run
//! use std::io;
//! use std::io::prelude::*;
//! use std::fs::File;
//!
//! fn main() -> io::Result<()> {
//!     let mut f = File::open("foo.txt")?;
//!     let mut buffer = [0; 10];
//!
//!     // read up to 10 bytes
//!     let n = f.read(&mut buffer)?;
//!
//!     println!("The bytes: {:?}", &buffer[..n]);
//!     Ok(())
//! }
//! ```
//!
//! [`Read`] and [`Write`] are so important, implementors of the two traits have a
//! nickname: readers and writers. So you'll sometimes see 'a reader' instead
//! of 'a type that implements the [`Read`] trait'. Much easier!
//!
//! ## Seek and BufRead
//!
//! Beyond that, there are two important traits that are provided: [`Seek`]
//! and [`BufRead`]. Both of these build on top of a reader to control
//! how the reading happens. [`Seek`] lets you control where the next byte is
//! coming from:
//!
//! ```no_run
//! use std::io;
//! use std::io::prelude::*;
//! use std::io::SeekFrom;
//! use std::fs::File;
//!
//! fn main() -> io::Result<()> {
//!     let mut f = File::open("foo.txt")?;
//!     let mut buffer = [0; 10];
//!
//!     // skip to the last 10 bytes of the file
//!     f.seek(SeekFrom::End(-10))?;
//!
//!     // read up to 10 bytes
//!     let n = f.read(&mut buffer)?;
//!
//!     println!("The bytes: {:?}", &buffer[..n]);
//!     Ok(())
//! }
//! ```
//!
//! [`BufRead`] uses an internal buffer to provide a number of other ways to read, but
//! to show it off, we'll need to talk about buffers in general. Keep reading!
//!
//! ## BufReader and BufWriter
//!
//! Byte-based interfaces are unwieldy and can be inefficient, as we'd need to be
//! making near-constant calls to the operating system. To help with this,
//! `std::io` comes with two structs, [`BufReader`] and [`BufWriter`], which wrap
//! readers and writers. The wrapper uses a buffer, reducing the number of
//! calls and providing nicer methods for accessing exactly what you want.
//!
//! For example, [`BufReader`] works with the [`BufRead`] trait to add extra
//! methods to any reader:
//!
//! ```no_run
//! use std::io;
//! use std::io::prelude::*;
//! use std::io::BufReader;
//! use std::fs::File;
//!
//! fn main() -> io::Result<()> {
//!     let f = File::open("foo.txt")?;
//!     let mut reader = BufReader::new(f);
//!     let mut buffer = String::new();
//!
//!     // read a line into buffer
//!     reader.read_line(&mut buffer)?;
//!
//!     println!("{buffer}");
//!     Ok(())
//! }
//! ```
//!
//! [`BufWriter`] doesn't add any new ways of writing; it just buffers every call
//! to [`write`][`Write::write`]:
//!
//! ```no_run
//! use std::io;
//! use std::io::prelude::*;
//! use std::io::BufWriter;
//! use std::fs::File;
//!
//! fn main() -> io::Result<()> {
//!     let f = File::create("foo.txt")?;
//!     {
//!         let mut writer = BufWriter::new(f);
//!
//!         // write a byte to the buffer
//!         writer.write(&[42])?;
//!
//!     } // the buffer is flushed once writer goes out of scope
//!
//!     Ok(())
//! }
//! ```
//!
//! ## Standard input and output
//!
//! A very common source of input is standard input:
//!
//! ```no_run
//! use std::io;
//!
//! fn main() -> io::Result<()> {
//!     let mut input = String::new();
//!
//!     io::stdin().read_line(&mut input)?;
//!
//!     println!("You typed: {}", input.trim());
//!     Ok(())
//! }
//! ```
//!
//! Note that you cannot use the [`?` operator] in functions that do not return
//! a [`Result<T, E>`][`Result`]. Instead, you can call [`.unwrap()`]
//! or `match` on the return value to catch any possible errors:
//!
//! ```no_run
//! use std::io;
//!
//! let mut input = String::new();
//!
//! io::stdin().read_line(&mut input).unwrap();
//! ```
//!
//! And a very common source of output is standard output:
//!
//! ```no_run
//! use std::io;
//! use std::io::prelude::*;
//!
//! fn main() -> io::Result<()> {
//!     io::stdout().write(&[42])?;
//!     Ok(())
//! }
//! ```
//!
//! Of course, using [`io::stdout`] directly is less common than something like
//! [`println!`].
//!
//! ## Iterator types
//!
//! A large number of the structures provided by `std::io` are for various
//! ways of iterating over I/O. For example, [`Lines`] is used to split over
//! lines:
//!
//! ```no_run
//! use std::io;
//! use std::io::prelude::*;
//! use std::io::BufReader;
//! use std::fs::File;
//!
//! fn main() -> io::Result<()> {
//!     let f = File::open("foo.txt")?;
//!     let reader = BufReader::new(f);
//!
//!     for line in reader.lines() {
//!         println!("{}", line?);
//!     }
//!     Ok(())
//! }
//! ```
//!
//! ## Functions
//!
//! There are a number of [functions][functions-list] that offer access to various
//! features. For example, we can use three of these functions to copy everything
//! from standard input to standard output:
//!
//! ```no_run
//! use std::io;
//!
//! fn main() -> io::Result<()> {
//!     io::copy(&mut io::stdin(), &mut io::stdout())?;
//!     Ok(())
//! }
//! ```
//!
//! [functions-list]: #functions-1
//!
//! ## io::Result
//!
//! Last, but certainly not least, is [`io::Result`]. This type is used
//! as the return type of many `std::io` functions that can cause an error, and
//! can be returned from your own functions as well. Many of the examples in this
//! module use the [`?` operator]:
//!
//! ```
//! use std::io;
//!
//! fn read_input() -> io::Result<()> {
//!     let mut input = String::new();
//!
//!     io::stdin().read_line(&mut input)?;
//!
//!     println!("You typed: {}", input.trim());
//!
//!     Ok(())
//! }
//! ```
//!
//! The return type of `read_input()`, [`io::Result<()>`][`io::Result`], is a very
//! common type for functions which don't have a 'real' return value, but do want to
//! return errors if they happen. In this case, the only purpose of this function is
//! to read the line and print it, so we use `()`.
//!
//! ## Platform-specific behavior
//!
//! Many I/O functions throughout the standard library are documented to indicate
//! what various library or syscalls they are delegated to. This is done to help
//! applications both understand what's happening under the hood as well as investigate
//! any possibly unclear semantics. Note, however, that this is informative, not a binding
//! contract. The implementation of many of these functions are subject to change over
//! time and may call fewer or more syscalls/library functions.
//!
//! ## I/O Safety
//!
//! Rust follows an I/O safety discipline that is comparable to its memory safety discipline. This
//! means that file descriptors can be *exclusively owned*. (Here, "file descriptor" is meant to
//! subsume similar concepts that exist across a wide range of operating systems even if they might
//! use a different name, such as "handle".) An exclusively owned file descriptor is one that no
//! other code is allowed to access in any way, but the owner is allowed to access and even close
//! it any time. A type that owns its file descriptor should usually close it in its `drop`
//! function. Types like [`File`] own their file descriptor. Similarly, file descriptors
//! can be *borrowed*, granting the temporary right to perform operations on this file descriptor.
//! This indicates that the file descriptor will not be closed for the lifetime of the borrow, but
//! it does *not* imply any right to close this file descriptor, since it will likely be owned by
//! someone else.
//!
//! The platform-specific parts of the Rust standard library expose types that reflect these
//! concepts, see [`os::unix`] and [`os::windows`].
//!
//! To uphold I/O safety, it is crucial that no code acts on file descriptors it does not own or
//! borrow, and no code closes file descriptors it does not own. In other words, a safe function
//! that takes a regular integer, treats it as a file descriptor, and acts on it, is *unsound*.
//!
//! Not upholding I/O safety and acting on a file descriptor without proof of ownership can lead to
//! misbehavior and even Undefined Behavior in code that relies on ownership of its file
//! descriptors: a closed file descriptor could be re-allocated, so the original owner of that file
//! descriptor is now working on the wrong file. Some code might even rely on fully encapsulating
//! its file descriptors with no operations being performed by any other part of the program.
//!
//! Note that exclusive ownership of a file descriptor does *not* imply exclusive ownership of the
//! underlying kernel object that the file descriptor references (also called "open file description" on
//! some operating systems). File descriptors basically work like [`Arc`]: when you receive an owned
//! file descriptor, you cannot know whether there are any other file descriptors that reference the
//! same kernel object. However, when you create a new kernel object, you know that you are holding
//! the only reference to it. Just be careful not to lend it to anyone, since they can obtain a
//! clone and then you can no longer know what the reference count is! In that sense, [`OwnedFd`] is
//! like `Arc` and [`BorrowedFd<'a>`] is like `&'a Arc` (and similar for the Windows types). In
//! particular, given a `BorrowedFd<'a>`, you are not allowed to close the file descriptor -- just
//! like how, given a `&'a Arc`, you are not allowed to decrement the reference count and
//! potentially free the underlying object. There is no equivalent to `Box` for file descriptors in
//! the standard library (that would be a type that guarantees that the reference count is `1`),
//! however, it would be possible for a crate to define a type with those semantics.
//!
//! [`File`]: crate::fs::File
//! [`TcpStream`]: crate::net::TcpStream
//! [`io::stdout`]: stdout
//! [`io::Result`]: self::Result
//! [`?` operator]: ../../book/appendix-02-operators.html
//! [`Result`]: crate::result::Result
//! [`.unwrap()`]: crate::result::Result::unwrap
//! [`os::unix`]: ../os/unix/io/index.html
//! [`os::windows`]: ../os/windows/io/index.html
//! [`OwnedFd`]: ../os/fd/struct.OwnedFd.html
//! [`BorrowedFd<'a>`]: ../os/fd/struct.BorrowedFd.html
//! [`Arc`]: crate::sync::Arc

#![stable(feature = "rust1", since = "1.0.0")]

#[cfg(test)]
mod tests;

use crate::cmp;
use crate::fmt;
use crate::mem::take;
use crate::ops::{Deref, DerefMut};
use crate::slice;
use crate::str;
use crate::sys;
use core::slice::memchr;

#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
pub use self::buffered::WriterPanicked;
#[unstable(feature = "raw_os_error_ty", issue = "107792")]
pub use self::error::RawOsError;
pub(crate) use self::stdio::attempt_print_to_stderr;
#[stable(feature = "is_terminal", since = "1.70.0")]
pub use self::stdio::IsTerminal;
#[unstable(feature = "print_internals", issue = "none")]
#[doc(hidden)]
pub use self::stdio::{_eprint, _print};
#[unstable(feature = "internal_output_capture", issue = "none")]
#[doc(no_inline, hidden)]
pub use self::stdio::{set_output_capture, try_set_output_capture};
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::{
    buffered::{BufReader, BufWriter, IntoInnerError, LineWriter},
    copy::copy,
    cursor::Cursor,
    error::{Error, ErrorKind, Result},
    stdio::{stderr, stdin, stdout, Stderr, StderrLock, Stdin, StdinLock, Stdout, StdoutLock},
    util::{empty, repeat, sink, Empty, Repeat, Sink},
};

#[unstable(feature = "read_buf", issue = "78485")]
pub use core::io::{BorrowedBuf, BorrowedCursor};
pub(crate) use error::const_io_error;

mod buffered;
pub(crate) mod copy;
mod cursor;
mod error;
mod impls;
pub mod prelude;
mod stdio;
mod util;

const DEFAULT_BUF_SIZE: usize = crate::sys_common::io::DEFAULT_BUF_SIZE;

pub(crate) use stdio::cleanup;

struct Guard<'a> {
    buf: &'a mut Vec<u8>,
    len: usize,
}

impl Drop for Guard<'_> {
    fn drop(&mut self) {
        unsafe {
            self.buf.set_len(self.len);
        }
    }
}

// Several `read_to_string` and `read_line` methods in the standard library will
// append data into a `String` buffer, but we need to be pretty careful when
// doing this. The implementation will just call `.as_mut_vec()` and then
// delegate to a byte-oriented reading method, but we must ensure that when
// returning we never leave `buf` in a state such that it contains invalid UTF-8
// in its bounds.
//
// To this end, we use an RAII guard (to protect against panics) which updates
// the length of the string when it is dropped. This guard initially truncates
// the string to the prior length and only after we've validated that the
// new contents are valid UTF-8 do we allow it to set a longer length.
//
// The unsafety in this function is twofold:
//
// 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
//    checks.
// 2. We're passing a raw buffer to the function `f`, and it is expected that
//    the function only *appends* bytes to the buffer. We'll get undefined
//    behavior if existing bytes are overwritten to have non-UTF-8 data.
pub(crate) unsafe fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
where
    F: FnOnce(&mut Vec<u8>) -> Result<usize>,
{
    let mut g = Guard { len: buf.len(), buf: buf.as_mut_vec() };
    let ret = f(g.buf);

    // SAFETY: the caller promises to only append data to `buf`
    let appended = g.buf.get_unchecked(g.len..);
    if str::from_utf8(appended).is_err() {
        ret.and_then(|_| Err(Error::INVALID_UTF8))
    } else {
        g.len = g.buf.len();
        ret
    }
}

// Here we must serve many masters with conflicting goals:
//
// - avoid allocating unless necessary
// - avoid overallocating if we know the exact size (#89165)
// - avoid passing large buffers to readers that always initialize the free capacity if they perform short reads (#23815, #23820)
// - pass large buffers to readers that do not initialize the spare capacity. this can amortize per-call overheads
// - and finally pass not-too-small and not-too-large buffers to Windows read APIs because they manage to suffer from both problems
//   at the same time, i.e. small reads suffer from syscall overhead, all reads incur initialization cost
//   proportional to buffer size (#110650)
//
pub(crate) fn default_read_to_end<R: Read + ?Sized>(
    r: &mut R,
    buf: &mut Vec<u8>,
    size_hint: Option<usize>,
) -> Result<usize> {
    let start_len = buf.len();
    let start_cap = buf.capacity();
    // Optionally limit the maximum bytes read on each iteration.
    // This adds an arbitrary fiddle factor to allow for more data than we expect.
    let mut max_read_size = size_hint
        .and_then(|s| s.checked_add(1024)?.checked_next_multiple_of(DEFAULT_BUF_SIZE))
        .unwrap_or(DEFAULT_BUF_SIZE);

    let mut initialized = 0; // Extra initialized bytes from previous loop iteration

    const PROBE_SIZE: usize = 32;

    fn small_probe_read<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
        let mut probe = [0u8; PROBE_SIZE];

        loop {
            match r.read(&mut probe) {
                Ok(n) => {
                    // there is no way to recover from allocation failure here
                    // because the data has already been read.
                    buf.extend_from_slice(&probe[..n]);
                    return Ok(n);
                }
                Err(ref e) if e.is_interrupted() => continue,
                Err(e) => return Err(e),
            }
        }
    }

    // avoid inflating empty/small vecs before we have determined that there's anything to read
    if (size_hint.is_none() || size_hint == Some(0)) && buf.capacity() - buf.len() < PROBE_SIZE {
        let read = small_probe_read(r, buf)?;

        if read == 0 {
            return Ok(0);
        }
    }

    loop {
        if buf.len() == buf.capacity() && buf.capacity() == start_cap {
            // The buffer might be an exact fit. Let's read into a probe buffer
            // and see if it returns `Ok(0)`. If so, we've avoided an
            // unnecessary doubling of the capacity. But if not, append the
            // probe buffer to the primary buffer and let its capacity grow.
            let read = small_probe_read(r, buf)?;

            if read == 0 {
                return Ok(buf.len() - start_len);
            }
        }

        if buf.len() == buf.capacity() {
            // buf is full, need more space
            buf.try_reserve(PROBE_SIZE)?;
        }

        let mut spare = buf.spare_capacity_mut();
        let buf_len = cmp::min(spare.len(), max_read_size);
        spare = &mut spare[..buf_len];
        let mut read_buf: BorrowedBuf<'_> = spare.into();

        // SAFETY: These bytes were initialized but not filled in the previous loop
        unsafe {
            read_buf.set_init(initialized);
        }

        let mut cursor = read_buf.unfilled();
        loop {
            match r.read_buf(cursor.reborrow()) {
                Ok(()) => break,
                Err(e) if e.is_interrupted() => continue,
                Err(e) => return Err(e),
            }
        }

        let unfilled_but_initialized = cursor.init_ref().len();
        let bytes_read = cursor.written();
        let was_fully_initialized = read_buf.init_len() == buf_len;

        if bytes_read == 0 {
            return Ok(buf.len() - start_len);
        }

        // store how much was initialized but not filled
        initialized = unfilled_but_initialized;

        // SAFETY: BorrowedBuf's invariants mean this much memory is initialized.
        unsafe {
            let new_len = bytes_read + buf.len();
            buf.set_len(new_len);
        }

        // Use heuristics to determine the max read size if no initial size hint was provided
        if size_hint.is_none() {
            // The reader is returning short reads but it doesn't call ensure_init().
            // In that case we no longer need to restrict read sizes to avoid
            // initialization costs.
            if !was_fully_initialized {
                max_read_size = usize::MAX;
            }

            // we have passed a larger buffer than previously and the
            // reader still hasn't returned a short read
            if buf_len >= max_read_size && bytes_read == buf_len {
                max_read_size = max_read_size.saturating_mul(2);
            }
        }
    }
}

pub(crate) fn default_read_to_string<R: Read + ?Sized>(
    r: &mut R,
    buf: &mut String,
    size_hint: Option<usize>,
) -> Result<usize> {
    // Note that we do *not* call `r.read_to_end()` here. We are passing
    // `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
    // method to fill it up. An arbitrary implementation could overwrite the
    // entire contents of the vector, not just append to it (which is what
    // we are expecting).
    //
    // To prevent extraneously checking the UTF-8-ness of the entire buffer
    // we pass it to our hardcoded `default_read_to_end` implementation which
    // we know is guaranteed to only read data into the end of the buffer.
    unsafe { append_to_string(buf, |b| default_read_to_end(r, b, size_hint)) }
}

pub(crate) fn default_read_vectored<F>(read: F, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
where
    F: FnOnce(&mut [u8]) -> Result<usize>,
{
    let buf = bufs.iter_mut().find(|b| !b.is_empty()).map_or(&mut [][..], |b| &mut **b);
    read(buf)
}

pub(crate) fn default_write_vectored<F>(write: F, bufs: &[IoSlice<'_>]) -> Result<usize>
where
    F: FnOnce(&[u8]) -> Result<usize>,
{
    let buf = bufs.iter().find(|b| !b.is_empty()).map_or(&[][..], |b| &**b);
    write(buf)
}

pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [u8]) -> Result<()> {
    while !buf.is_empty() {
        match this.read(buf) {
            Ok(0) => break,
            Ok(n) => {
                buf = &mut buf[n..];
            }
            Err(ref e) if e.is_interrupted() => {}
            Err(e) => return Err(e),
        }
    }
    if !buf.is_empty() { Err(Error::READ_EXACT_EOF) } else { Ok(()) }
}

pub(crate) fn default_read_buf<F>(read: F, mut cursor: BorrowedCursor<'_>) -> Result<()>
where
    F: FnOnce(&mut [u8]) -> Result<usize>,
{
    let n = read(cursor.ensure_init().init_mut())?;
    cursor.advance(n);
    Ok(())
}

pub(crate) fn default_read_buf_exact<R: Read + ?Sized>(
    this: &mut R,
    mut cursor: BorrowedCursor<'_>,
) -> Result<()> {
    while cursor.capacity() > 0 {
        let prev_written = cursor.written();
        match this.read_buf(cursor.reborrow()) {
            Ok(()) => {}
            Err(e) if e.is_interrupted() => continue,
            Err(e) => return Err(e),
        }

        if cursor.written() == prev_written {
            return Err(Error::READ_EXACT_EOF);
        }
    }

    Ok(())
}

/// The `Read` trait allows for reading bytes from a source.
///
/// Implementors of the `Read` trait are called 'readers'.
///
/// Readers are defined by one required method, [`read()`]. Each call to [`read()`]
/// will attempt to pull bytes from this source into a provided buffer. A
/// number of other methods are implemented in terms of [`read()`], giving
/// implementors a number of ways to read bytes while only needing to implement
/// a single method.
///
/// Readers are intended to be composable with one another. Many implementors
/// throughout [`std::io`] take and provide types which implement the `Read`
/// trait.
///
/// Please note that each call to [`read()`] may involve a system call, and
/// therefore, using something that implements [`BufRead`], such as
/// [`BufReader`], will be more efficient.
///
/// Repeated calls to the reader use the same cursor, so for example
/// calling `read_to_end` twice on a [`File`] will only return the file's
/// contents once. It's recommended to first call `rewind()` in that case.
///
/// # Examples
///
/// [`File`]s implement `Read`:
///
/// ```no_run
/// use std::io;
/// use std::io::prelude::*;
/// use std::fs::File;
///
/// fn main() -> io::Result<()> {
///     let mut f = File::open("foo.txt")?;
///     let mut buffer = [0; 10];
///
///     // read up to 10 bytes
///     f.read(&mut buffer)?;
///
///     let mut buffer = Vec::new();
///     // read the whole file
///     f.read_to_end(&mut buffer)?;
///
///     // read into a String, so that you don't need to do the conversion.
///     let mut buffer = String::new();
///     f.read_to_string(&mut buffer)?;
///
///     // and more! See the other methods for more details.
///     Ok(())
/// }
/// ```
///
/// Read from [`&str`] because [`&[u8]`][prim@slice] implements `Read`:
///
/// ```no_run
/// # use std::io;
/// use std::io::prelude::*;
///
/// fn main() -> io::Result<()> {
///     let mut b = "This string will be read".as_bytes();
///     let mut buffer = [0; 10];
///
///     // read up to 10 bytes
///     b.read(&mut buffer)?;
///
///     // etc... it works exactly as a File does!
///     Ok(())
/// }
/// ```
///
/// [`read()`]: Read::read
/// [`&str`]: prim@str
/// [`std::io`]: self
/// [`File`]: crate::fs::File
#[stable(feature = "rust1", since = "1.0.0")]
#[doc(notable_trait)]
#[cfg_attr(not(test), rustc_diagnostic_item = "IoRead")]
pub trait Read {
    /// Pull some bytes from this source into the specified buffer, returning
    /// how many bytes were read.
    ///
    /// This function does not provide any guarantees about whether it blocks
    /// waiting for data, but if an object needs to block for a read and cannot,
    /// it will typically signal this via an [`Err`] return value.
    ///
    /// If the return value of this method is [`Ok(n)`], then implementations must
    /// guarantee that `0 <= n <= buf.len()`. A nonzero `n` value indicates
    /// that the buffer `buf` has been filled in with `n` bytes of data from this
    /// source. If `n` is `0`, then it can indicate one of two scenarios:
    ///
    /// 1. This reader has reached its "end of file" and will likely no longer
    ///    be able to produce bytes. Note that this does not mean that the
    ///    reader will *always* no longer be able to produce bytes. As an example,
    ///    on Linux, this method will call the `recv` syscall for a [`TcpStream`],
    ///    where returning zero indicates the connection was shut down correctly. While
    ///    for [`File`], it is possible to reach the end of file and get zero as result,
    ///    but if more data is appended to the file, future calls to `read` will return
    ///    more data.
    /// 2. The buffer specified was 0 bytes in length.
    ///
    /// It is not an error if the returned value `n` is smaller than the buffer size,
    /// even when the reader is not at the end of the stream yet.
    /// This may happen for example because fewer bytes are actually available right now
    /// (e. g. being close to end-of-file) or because read() was interrupted by a signal.
    ///
    /// As this trait is safe to implement, callers in unsafe code cannot rely on
    /// `n <= buf.len()` for safety.
    /// Extra care needs to be taken when `unsafe` functions are used to access the read bytes.
    /// Callers have to ensure that no unchecked out-of-bounds accesses are possible even if
    /// `n > buf.len()`.
    ///
    /// *Implementations* of this method can make no assumptions about the contents of `buf` when
    /// this function is called. It is recommended that implementations only write data to `buf`
    /// instead of reading its contents.
    ///
    /// Correspondingly, however, *callers* of this method in unsafe code must not assume
    /// any guarantees about how the implementation uses `buf`. The trait is safe to implement,
    /// so it is possible that the code that's supposed to write to the buffer might also read
    /// from it. It is your responsibility to make sure that `buf` is initialized
    /// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one
    /// obtains via [`MaybeUninit<T>`]) is not safe, and can lead to undefined behavior.
    ///
    /// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
    ///
    /// # Errors
    ///
    /// If this function encounters any form of I/O or other error, an error
    /// variant will be returned. If an error is returned then it must be
    /// guaranteed that no bytes were read.
    ///
    /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the read
    /// operation should be retried if there is nothing else to do.
    ///
    /// # Examples
    ///
    /// [`File`]s implement `Read`:
    ///
    /// [`Ok(n)`]: Ok
    /// [`File`]: crate::fs::File
    /// [`TcpStream`]: crate::net::TcpStream
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = File::open("foo.txt")?;
    ///     let mut buffer = [0; 10];
    ///
    ///     // read up to 10 bytes
    ///     let n = f.read(&mut buffer[..])?;
    ///
    ///     println!("The bytes: {:?}", &buffer[..n]);
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn read(&mut self, buf: &mut [u8]) -> Result<usize>;

    /// Like `read`, except that it reads into a slice of buffers.
    ///
    /// Data is copied to fill each buffer in order, with the final buffer
    /// written to possibly being only partially filled. This method must
    /// behave equivalently to a single call to `read` with concatenated
    /// buffers.
    ///
    /// The default implementation calls `read` with either the first nonempty
    /// buffer provided, or an empty one if none exists.
    #[stable(feature = "iovec", since = "1.36.0")]
    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
        default_read_vectored(|b| self.read(b), bufs)
    }

    /// Determines if this `Read`er has an efficient `read_vectored`
    /// implementation.
    ///
    /// If a `Read`er does not override the default `read_vectored`
    /// implementation, code using it may want to avoid the method all together
    /// and coalesce writes into a single buffer for higher performance.
    ///
    /// The default implementation returns `false`.
    #[unstable(feature = "can_vector", issue = "69941")]
    fn is_read_vectored(&self) -> bool {
        false
    }

    /// Read all bytes until EOF in this source, placing them into `buf`.
    ///
    /// All bytes read from this source will be appended to the specified buffer
    /// `buf`. This function will continuously call [`read()`] to append more data to
    /// `buf` until [`read()`] returns either [`Ok(0)`] or an error of
    /// non-[`ErrorKind::Interrupted`] kind.
    ///
    /// If successful, this function will return the total number of bytes read.
    ///
    /// # Errors
    ///
    /// If this function encounters an error of the kind
    /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
    /// will continue.
    ///
    /// If any other read error is encountered then this function immediately
    /// returns. Any bytes which have already been read will be appended to
    /// `buf`.
    ///
    /// # Examples
    ///
    /// [`File`]s implement `Read`:
    ///
    /// [`read()`]: Read::read
    /// [`Ok(0)`]: Ok
    /// [`File`]: crate::fs::File
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = File::open("foo.txt")?;
    ///     let mut buffer = Vec::new();
    ///
    ///     // read the whole file
    ///     f.read_to_end(&mut buffer)?;
    ///     Ok(())
    /// }
    /// ```
    ///
    /// (See also the [`std::fs::read`] convenience function for reading from a
    /// file.)
    ///
    /// [`std::fs::read`]: crate::fs::read
    ///
    /// ## Implementing `read_to_end`
    ///
    /// When implementing the `io::Read` trait, it is recommended to allocate
    /// memory using [`Vec::try_reserve`]. However, this behavior is not guaranteed
    /// by all implementations, and `read_to_end` may not handle out-of-memory
    /// situations gracefully.
    ///
    /// ```no_run
    /// # use std::io::{self, BufRead};
    /// # struct Example { example_datasource: io::Empty } impl Example {
    /// # fn get_some_data_for_the_example(&self) -> &'static [u8] { &[] }
    /// fn read_to_end(&mut self, dest_vec: &mut Vec<u8>) -> io::Result<usize> {
    ///     let initial_vec_len = dest_vec.len();
    ///     loop {
    ///         let src_buf = self.example_datasource.fill_buf()?;
    ///         if src_buf.is_empty() {
    ///             break;
    ///         }
    ///         dest_vec.try_reserve(src_buf.len())?;
    ///         dest_vec.extend_from_slice(src_buf);
    ///
    ///         // Any irreversible side effects should happen after `try_reserve` succeeds,
    ///         // to avoid losing data on allocation error.
    ///         let read = src_buf.len();
    ///         self.example_datasource.consume(read);
    ///     }
    ///     Ok(dest_vec.len() - initial_vec_len)
    /// }
    /// # }
    /// ```
    ///
    /// [`Vec::try_reserve`]: crate::vec::Vec::try_reserve
    #[stable(feature = "rust1", since = "1.0.0")]
    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
        default_read_to_end(self, buf, None)
    }

    /// Read all bytes until EOF in this source, appending them to `buf`.
    ///
    /// If successful, this function returns the number of bytes which were read
    /// and appended to `buf`.
    ///
    /// # Errors
    ///
    /// If the data in this stream is *not* valid UTF-8 then an error is
    /// returned and `buf` is unchanged.
    ///
    /// See [`read_to_end`] for other error semantics.
    ///
    /// [`read_to_end`]: Read::read_to_end
    ///
    /// # Examples
    ///
    /// [`File`]s implement `Read`:
    ///
    /// [`File`]: crate::fs::File
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = File::open("foo.txt")?;
    ///     let mut buffer = String::new();
    ///
    ///     f.read_to_string(&mut buffer)?;
    ///     Ok(())
    /// }
    /// ```
    ///
    /// (See also the [`std::fs::read_to_string`] convenience function for
    /// reading from a file.)
    ///
    /// [`std::fs::read_to_string`]: crate::fs::read_to_string
    #[stable(feature = "rust1", since = "1.0.0")]
    fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
        default_read_to_string(self, buf, None)
    }

    /// Read the exact number of bytes required to fill `buf`.
    ///
    /// This function reads as many bytes as necessary to completely fill the
    /// specified buffer `buf`.
    ///
    /// *Implementations* of this method can make no assumptions about the contents of `buf` when
    /// this function is called. It is recommended that implementations only write data to `buf`
    /// instead of reading its contents. The documentation on [`read`] has a more detailed
    /// explanation of this subject.
    ///
    /// # Errors
    ///
    /// If this function encounters an error of the kind
    /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
    /// will continue.
    ///
    /// If this function encounters an "end of file" before completely filling
    /// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
    /// The contents of `buf` are unspecified in this case.
    ///
    /// If any other read error is encountered then this function immediately
    /// returns. The contents of `buf` are unspecified in this case.
    ///
    /// If this function returns an error, it is unspecified how many bytes it
    /// has read, but it will never read more than would be necessary to
    /// completely fill the buffer.
    ///
    /// # Examples
    ///
    /// [`File`]s implement `Read`:
    ///
    /// [`read`]: Read::read
    /// [`File`]: crate::fs::File
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = File::open("foo.txt")?;
    ///     let mut buffer = [0; 10];
    ///
    ///     // read exactly 10 bytes
    ///     f.read_exact(&mut buffer)?;
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "read_exact", since = "1.6.0")]
    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
        default_read_exact(self, buf)
    }

    /// Pull some bytes from this source into the specified buffer.
    ///
    /// This is equivalent to the [`read`](Read::read) method, except that it is passed a [`BorrowedCursor`] rather than `[u8]` to allow use
    /// with uninitialized buffers. The new data will be appended to any existing contents of `buf`.
    ///
    /// The default implementation delegates to `read`.
    #[unstable(feature = "read_buf", issue = "78485")]
    fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<()> {
        default_read_buf(|b| self.read(b), buf)
    }

    /// Read the exact number of bytes required to fill `cursor`.
    ///
    /// This is similar to the [`read_exact`](Read::read_exact) method, except
    /// that it is passed a [`BorrowedCursor`] rather than `[u8]` to allow use
    /// with uninitialized buffers.
    ///
    /// # Errors
    ///
    /// If this function encounters an error of the kind [`ErrorKind::Interrupted`]
    /// then the error is ignored and the operation will continue.
    ///
    /// If this function encounters an "end of file" before completely filling
    /// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
    ///
    /// If any other read error is encountered then this function immediately
    /// returns.
    ///
    /// If this function returns an error, all bytes read will be appended to `cursor`.
    #[unstable(feature = "read_buf", issue = "78485")]
    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()> {
        default_read_buf_exact(self, cursor)
    }

    /// Creates a "by reference" adaptor for this instance of `Read`.
    ///
    /// The returned adapter also implements `Read` and will simply borrow this
    /// current reader.
    ///
    /// # Examples
    ///
    /// [`File`]s implement `Read`:
    ///
    /// [`File`]: crate::fs::File
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::Read;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = File::open("foo.txt")?;
    ///     let mut buffer = Vec::new();
    ///     let mut other_buffer = Vec::new();
    ///
    ///     {
    ///         let reference = f.by_ref();
    ///
    ///         // read at most 5 bytes
    ///         reference.take(5).read_to_end(&mut buffer)?;
    ///
    ///     } // drop our &mut reference so we can use f again
    ///
    ///     // original file still usable, read the rest
    ///     f.read_to_end(&mut other_buffer)?;
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn by_ref(&mut self) -> &mut Self
    where
        Self: Sized,
    {
        self
    }

    /// Transforms this `Read` instance to an [`Iterator`] over its bytes.
    ///
    /// The returned type implements [`Iterator`] where the [`Item`] is
    /// <code>[Result]<[u8], [io::Error]></code>.
    /// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
    /// otherwise. EOF is mapped to returning [`None`] from this iterator.
    ///
    /// The default implementation calls `read` for each byte,
    /// which can be very inefficient for data that's not in memory,
    /// such as [`File`]. Consider using a [`BufReader`] in such cases.
    ///
    /// # Examples
    ///
    /// [`File`]s implement `Read`:
    ///
    /// [`Item`]: Iterator::Item
    /// [`File`]: crate::fs::File "fs::File"
    /// [Result]: crate::result::Result "Result"
    /// [io::Error]: self::Error "io::Error"
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::io::BufReader;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let f = BufReader::new(File::open("foo.txt")?);
    ///
    ///     for byte in f.bytes() {
    ///         println!("{}", byte.unwrap());
    ///     }
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn bytes(self) -> Bytes<Self>
    where
        Self: Sized,
    {
        Bytes { inner: self }
    }

    /// Creates an adapter which will chain this stream with another.
    ///
    /// The returned `Read` instance will first read all bytes from this object
    /// until EOF is encountered. Afterwards the output is equivalent to the
    /// output of `next`.
    ///
    /// # Examples
    ///
    /// [`File`]s implement `Read`:
    ///
    /// [`File`]: crate::fs::File
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let f1 = File::open("foo.txt")?;
    ///     let f2 = File::open("bar.txt")?;
    ///
    ///     let mut handle = f1.chain(f2);
    ///     let mut buffer = String::new();
    ///
    ///     // read the value into a String. We could use any Read method here,
    ///     // this is just one example.
    ///     handle.read_to_string(&mut buffer)?;
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn chain<R: Read>(self, next: R) -> Chain<Self, R>
    where
        Self: Sized,
    {
        Chain { first: self, second: next, done_first: false }
    }

    /// Creates an adapter which will read at most `limit` bytes from it.
    ///
    /// This function returns a new instance of `Read` which will read at most
    /// `limit` bytes, after which it will always return EOF ([`Ok(0)`]). Any
    /// read errors will not count towards the number of bytes read and future
    /// calls to [`read()`] may succeed.
    ///
    /// # Examples
    ///
    /// [`File`]s implement `Read`:
    ///
    /// [`File`]: crate::fs::File
    /// [`Ok(0)`]: Ok
    /// [`read()`]: Read::read
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let f = File::open("foo.txt")?;
    ///     let mut buffer = [0; 5];
    ///
    ///     // read at most five bytes
    ///     let mut handle = f.take(5);
    ///
    ///     handle.read(&mut buffer)?;
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn take(self, limit: u64) -> Take<Self>
    where
        Self: Sized,
    {
        Take { inner: self, limit }
    }
}

/// Read all bytes from a [reader][Read] into a new [`String`].
///
/// This is a convenience function for [`Read::read_to_string`]. Using this
/// function avoids having to create a variable first and provides more type
/// safety since you can only get the buffer out if there were no errors. (If you
/// use [`Read::read_to_string`] you have to remember to check whether the read
/// succeeded because otherwise your buffer will be empty or only partially full.)
///
/// # Performance
///
/// The downside of this function's increased ease of use and type safety is
/// that it gives you less control over performance. For example, you can't
/// pre-allocate memory like you can using [`String::with_capacity`] and
/// [`Read::read_to_string`]. Also, you can't re-use the buffer if an error
/// occurs while reading.
///
/// In many cases, this function's performance will be adequate and the ease of use
/// and type safety tradeoffs will be worth it. However, there are cases where you
/// need more control over performance, and in those cases you should definitely use
/// [`Read::read_to_string`] directly.
///
/// Note that in some special cases, such as when reading files, this function will
/// pre-allocate memory based on the size of the input it is reading. In those
/// cases, the performance should be as good as if you had used
/// [`Read::read_to_string`] with a manually pre-allocated buffer.
///
/// # Errors
///
/// This function forces you to handle errors because the output (the `String`)
/// is wrapped in a [`Result`]. See [`Read::read_to_string`] for the errors
/// that can occur. If any error occurs, you will get an [`Err`], so you
/// don't have to worry about your buffer being empty or partially full.
///
/// # Examples
///
/// ```no_run
/// # use std::io;
/// fn main() -> io::Result<()> {
///     let stdin = io::read_to_string(io::stdin())?;
///     println!("Stdin was:");
///     println!("{stdin}");
///     Ok(())
/// }
/// ```
#[stable(feature = "io_read_to_string", since = "1.65.0")]
pub fn read_to_string<R: Read>(mut reader: R) -> Result<String> {
    let mut buf = String::new();
    reader.read_to_string(&mut buf)?;
    Ok(buf)
}

/// A buffer type used with `Read::read_vectored`.
///
/// It is semantically a wrapper around an `&mut [u8]`, but is guaranteed to be
/// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
/// Windows.
#[stable(feature = "iovec", since = "1.36.0")]
#[repr(transparent)]
pub struct IoSliceMut<'a>(sys::io::IoSliceMut<'a>);

#[stable(feature = "iovec_send_sync", since = "1.44.0")]
unsafe impl<'a> Send for IoSliceMut<'a> {}

#[stable(feature = "iovec_send_sync", since = "1.44.0")]
unsafe impl<'a> Sync for IoSliceMut<'a> {}

#[stable(feature = "iovec", since = "1.36.0")]
impl<'a> fmt::Debug for IoSliceMut<'a> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self.0.as_slice(), fmt)
    }
}

impl<'a> IoSliceMut<'a> {
    /// Creates a new `IoSliceMut` wrapping a byte slice.
    ///
    /// # Panics
    ///
    /// Panics on Windows if the slice is larger than 4GB.
    #[stable(feature = "iovec", since = "1.36.0")]
    #[inline]
    pub fn new(buf: &'a mut [u8]) -> IoSliceMut<'a> {
        IoSliceMut(sys::io::IoSliceMut::new(buf))
    }

    /// Advance the internal cursor of the slice.
    ///
    /// Also see [`IoSliceMut::advance_slices`] to advance the cursors of
    /// multiple buffers.
    ///
    /// # Panics
    ///
    /// Panics when trying to advance beyond the end of the slice.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(io_slice_advance)]
    ///
    /// use std::io::IoSliceMut;
    /// use std::ops::Deref;
    ///
    /// let mut data = [1; 8];
    /// let mut buf = IoSliceMut::new(&mut data);
    ///
    /// // Mark 3 bytes as read.
    /// buf.advance(3);
    /// assert_eq!(buf.deref(), [1; 5].as_ref());
    /// ```
    #[unstable(feature = "io_slice_advance", issue = "62726")]
    #[inline]
    pub fn advance(&mut self, n: usize) {
        self.0.advance(n)
    }

    /// Advance a slice of slices.
    ///
    /// Shrinks the slice to remove any `IoSliceMut`s that are fully advanced over.
    /// If the cursor ends up in the middle of an `IoSliceMut`, it is modified
    /// to start at that cursor.
    ///
    /// For example, if we have a slice of two 8-byte `IoSliceMut`s, and we advance by 10 bytes,
    /// the result will only include the second `IoSliceMut`, advanced by 2 bytes.
    ///
    /// # Panics
    ///
    /// Panics when trying to advance beyond the end of the slices.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(io_slice_advance)]
    ///
    /// use std::io::IoSliceMut;
    /// use std::ops::Deref;
    ///
    /// let mut buf1 = [1; 8];
    /// let mut buf2 = [2; 16];
    /// let mut buf3 = [3; 8];
    /// let mut bufs = &mut [
    ///     IoSliceMut::new(&mut buf1),
    ///     IoSliceMut::new(&mut buf2),
    ///     IoSliceMut::new(&mut buf3),
    /// ][..];
    ///
    /// // Mark 10 bytes as read.
    /// IoSliceMut::advance_slices(&mut bufs, 10);
    /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
    /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
    /// ```
    #[unstable(feature = "io_slice_advance", issue = "62726")]
    #[inline]
    pub fn advance_slices(bufs: &mut &mut [IoSliceMut<'a>], n: usize) {
        // Number of buffers to remove.
        let mut remove = 0;
        // Remaining length before reaching n.
        let mut left = n;
        for buf in bufs.iter() {
            if let Some(remainder) = left.checked_sub(buf.len()) {
                left = remainder;
                remove += 1;
            } else {
                break;
            }
        }

        *bufs = &mut take(bufs)[remove..];
        if bufs.is_empty() {
            assert!(left == 0, "advancing io slices beyond their length");
        } else {
            bufs[0].advance(left);
        }
    }
}

#[stable(feature = "iovec", since = "1.36.0")]
impl<'a> Deref for IoSliceMut<'a> {
    type Target = [u8];

    #[inline]
    fn deref(&self) -> &[u8] {
        self.0.as_slice()
    }
}

#[stable(feature = "iovec", since = "1.36.0")]
impl<'a> DerefMut for IoSliceMut<'a> {
    #[inline]
    fn deref_mut(&mut self) -> &mut [u8] {
        self.0.as_mut_slice()
    }
}

/// A buffer type used with `Write::write_vectored`.
///
/// It is semantically a wrapper around a `&[u8]`, but is guaranteed to be
/// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
/// Windows.
#[stable(feature = "iovec", since = "1.36.0")]
#[derive(Copy, Clone)]
#[repr(transparent)]
pub struct IoSlice<'a>(sys::io::IoSlice<'a>);

#[stable(feature = "iovec_send_sync", since = "1.44.0")]
unsafe impl<'a> Send for IoSlice<'a> {}

#[stable(feature = "iovec_send_sync", since = "1.44.0")]
unsafe impl<'a> Sync for IoSlice<'a> {}

#[stable(feature = "iovec", since = "1.36.0")]
impl<'a> fmt::Debug for IoSlice<'a> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self.0.as_slice(), fmt)
    }
}

impl<'a> IoSlice<'a> {
    /// Creates a new `IoSlice` wrapping a byte slice.
    ///
    /// # Panics
    ///
    /// Panics on Windows if the slice is larger than 4GB.
    #[stable(feature = "iovec", since = "1.36.0")]
    #[must_use]
    #[inline]
    pub fn new(buf: &'a [u8]) -> IoSlice<'a> {
        IoSlice(sys::io::IoSlice::new(buf))
    }

    /// Advance the internal cursor of the slice.
    ///
    /// Also see [`IoSlice::advance_slices`] to advance the cursors of multiple
    /// buffers.
    ///
    /// # Panics
    ///
    /// Panics when trying to advance beyond the end of the slice.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(io_slice_advance)]
    ///
    /// use std::io::IoSlice;
    /// use std::ops::Deref;
    ///
    /// let data = [1; 8];
    /// let mut buf = IoSlice::new(&data);
    ///
    /// // Mark 3 bytes as read.
    /// buf.advance(3);
    /// assert_eq!(buf.deref(), [1; 5].as_ref());
    /// ```
    #[unstable(feature = "io_slice_advance", issue = "62726")]
    #[inline]
    pub fn advance(&mut self, n: usize) {
        self.0.advance(n)
    }

    /// Advance a slice of slices.
    ///
    /// Shrinks the slice to remove any `IoSlice`s that are fully advanced over.
    /// If the cursor ends up in the middle of an `IoSlice`, it is modified
    /// to start at that cursor.
    ///
    /// For example, if we have a slice of two 8-byte `IoSlice`s, and we advance by 10 bytes,
    /// the result will only include the second `IoSlice`, advanced by 2 bytes.
    ///
    /// # Panics
    ///
    /// Panics when trying to advance beyond the end of the slices.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(io_slice_advance)]
    ///
    /// use std::io::IoSlice;
    /// use std::ops::Deref;
    ///
    /// let buf1 = [1; 8];
    /// let buf2 = [2; 16];
    /// let buf3 = [3; 8];
    /// let mut bufs = &mut [
    ///     IoSlice::new(&buf1),
    ///     IoSlice::new(&buf2),
    ///     IoSlice::new(&buf3),
    /// ][..];
    ///
    /// // Mark 10 bytes as written.
    /// IoSlice::advance_slices(&mut bufs, 10);
    /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
    /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
    #[unstable(feature = "io_slice_advance", issue = "62726")]
    #[inline]
    pub fn advance_slices(bufs: &mut &mut [IoSlice<'a>], n: usize) {
        // Number of buffers to remove.
        let mut remove = 0;
        // Remaining length before reaching n. This prevents overflow
        // that could happen if the length of slices in `bufs` were instead
        // accumulated. Those slice may be aliased and, if they are large
        // enough, their added length may overflow a `usize`.
        let mut left = n;
        for buf in bufs.iter() {
            if let Some(remainder) = left.checked_sub(buf.len()) {
                left = remainder;
                remove += 1;
            } else {
                break;
            }
        }

        *bufs = &mut take(bufs)[remove..];
        if bufs.is_empty() {
            assert!(left == 0, "advancing io slices beyond their length");
        } else {
            bufs[0].advance(left);
        }
    }
}

#[stable(feature = "iovec", since = "1.36.0")]
impl<'a> Deref for IoSlice<'a> {
    type Target = [u8];

    #[inline]
    fn deref(&self) -> &[u8] {
        self.0.as_slice()
    }
}

/// A trait for objects which are byte-oriented sinks.
///
/// Implementors of the `Write` trait are sometimes called 'writers'.
///
/// Writers are defined by two required methods, [`write`] and [`flush`]:
///
/// * The [`write`] method will attempt to write some data into the object,
///   returning how many bytes were successfully written.
///
/// * The [`flush`] method is useful for adapters and explicit buffers
///   themselves for ensuring that all buffered data has been pushed out to the
///   'true sink'.
///
/// Writers are intended to be composable with one another. Many implementors
/// throughout [`std::io`] take and provide types which implement the `Write`
/// trait.
///
/// [`write`]: Write::write
/// [`flush`]: Write::flush
/// [`std::io`]: self
///
/// # Examples
///
/// ```no_run
/// use std::io::prelude::*;
/// use std::fs::File;
///
/// fn main() -> std::io::Result<()> {
///     let data = b"some bytes";
///
///     let mut pos = 0;
///     let mut buffer = File::create("foo.txt")?;
///
///     while pos < data.len() {
///         let bytes_written = buffer.write(&data[pos..])?;
///         pos += bytes_written;
///     }
///     Ok(())
/// }
/// ```
///
/// The trait also provides convenience methods like [`write_all`], which calls
/// `write` in a loop until its entire input has been written.
///
/// [`write_all`]: Write::write_all
#[stable(feature = "rust1", since = "1.0.0")]
#[doc(notable_trait)]
#[cfg_attr(not(test), rustc_diagnostic_item = "IoWrite")]
pub trait Write {
    /// Write a buffer into this writer, returning how many bytes were written.
    ///
    /// This function will attempt to write the entire contents of `buf`, but
    /// the entire write might not succeed, or the write may also generate an
    /// error. Typically, a call to `write` represents one attempt to write to
    /// any wrapped object.
    ///
    /// Calls to `write` are not guaranteed to block waiting for data to be
    /// written, and a write which would otherwise block can be indicated through
    /// an [`Err`] variant.
    ///
    /// If this method consumed `n > 0` bytes of `buf` it must return [`Ok(n)`].
    /// If the return value is `Ok(n)` then `n` must satisfy `n <= buf.len()`.
    /// A return value of `Ok(0)` typically means that the underlying object is
    /// no longer able to accept bytes and will likely not be able to in the
    /// future as well, or that the buffer provided is empty.
    ///
    /// # Errors
    ///
    /// Each call to `write` may generate an I/O error indicating that the
    /// operation could not be completed. If an error is returned then no bytes
    /// in the buffer were written to this writer.
    ///
    /// It is **not** considered an error if the entire buffer could not be
    /// written to this writer.
    ///
    /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the
    /// write operation should be retried if there is nothing else to do.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> std::io::Result<()> {
    ///     let mut buffer = File::create("foo.txt")?;
    ///
    ///     // Writes some prefix of the byte string, not necessarily all of it.
    ///     buffer.write(b"some bytes")?;
    ///     Ok(())
    /// }
    /// ```
    ///
    /// [`Ok(n)`]: Ok
    #[stable(feature = "rust1", since = "1.0.0")]
    fn write(&mut self, buf: &[u8]) -> Result<usize>;

    /// Like [`write`], except that it writes from a slice of buffers.
    ///
    /// Data is copied from each buffer in order, with the final buffer
    /// read from possibly being only partially consumed. This method must
    /// behave as a call to [`write`] with the buffers concatenated would.
    ///
    /// The default implementation calls [`write`] with either the first nonempty
    /// buffer provided, or an empty one if none exists.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io::IoSlice;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> std::io::Result<()> {
    ///     let data1 = [1; 8];
    ///     let data2 = [15; 8];
    ///     let io_slice1 = IoSlice::new(&data1);
    ///     let io_slice2 = IoSlice::new(&data2);
    ///
    ///     let mut buffer = File::create("foo.txt")?;
    ///
    ///     // Writes some prefix of the byte string, not necessarily all of it.
    ///     buffer.write_vectored(&[io_slice1, io_slice2])?;
    ///     Ok(())
    /// }
    /// ```
    ///
    /// [`write`]: Write::write
    #[stable(feature = "iovec", since = "1.36.0")]
    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
        default_write_vectored(|b| self.write(b), bufs)
    }

    /// Determines if this `Write`r has an efficient [`write_vectored`]
    /// implementation.
    ///
    /// If a `Write`r does not override the default [`write_vectored`]
    /// implementation, code using it may want to avoid the method all together
    /// and coalesce writes into a single buffer for higher performance.
    ///
    /// The default implementation returns `false`.
    ///
    /// [`write_vectored`]: Write::write_vectored
    #[unstable(feature = "can_vector", issue = "69941")]
    fn is_write_vectored(&self) -> bool {
        false
    }

    /// Flush this output stream, ensuring that all intermediately buffered
    /// contents reach their destination.
    ///
    /// # Errors
    ///
    /// It is considered an error if not all bytes could be written due to
    /// I/O errors or EOF being reached.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io::prelude::*;
    /// use std::io::BufWriter;
    /// use std::fs::File;
    ///
    /// fn main() -> std::io::Result<()> {
    ///     let mut buffer = BufWriter::new(File::create("foo.txt")?);
    ///
    ///     buffer.write_all(b"some bytes")?;
    ///     buffer.flush()?;
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn flush(&mut self) -> Result<()>;

    /// Attempts to write an entire buffer into this writer.
    ///
    /// This method will continuously call [`write`] until there is no more data
    /// to be written or an error of non-[`ErrorKind::Interrupted`] kind is
    /// returned. This method will not return until the entire buffer has been
    /// successfully written or such an error occurs. The first error that is
    /// not of [`ErrorKind::Interrupted`] kind generated from this method will be
    /// returned.
    ///
    /// If the buffer contains no data, this will never call [`write`].
    ///
    /// # Errors
    ///
    /// This function will return the first error of
    /// non-[`ErrorKind::Interrupted`] kind that [`write`] returns.
    ///
    /// [`write`]: Write::write
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> std::io::Result<()> {
    ///     let mut buffer = File::create("foo.txt")?;
    ///
    ///     buffer.write_all(b"some bytes")?;
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn write_all(&mut self, mut buf: &[u8]) -> Result<()> {
        while !buf.is_empty() {
            match self.write(buf) {
                Ok(0) => {
                    return Err(Error::WRITE_ALL_EOF);
                }
                Ok(n) => buf = &buf[n..],
                Err(ref e) if e.is_interrupted() => {}
                Err(e) => return Err(e),
            }
        }
        Ok(())
    }

    /// Attempts to write multiple buffers into this writer.
    ///
    /// This method will continuously call [`write_vectored`] until there is no
    /// more data to be written or an error of non-[`ErrorKind::Interrupted`]
    /// kind is returned. This method will not return until all buffers have
    /// been successfully written or such an error occurs. The first error that
    /// is not of [`ErrorKind::Interrupted`] kind generated from this method
    /// will be returned.
    ///
    /// If the buffer contains no data, this will never call [`write_vectored`].
    ///
    /// # Notes
    ///
    /// Unlike [`write_vectored`], this takes a *mutable* reference to
    /// a slice of [`IoSlice`]s, not an immutable one. That's because we need to
    /// modify the slice to keep track of the bytes already written.
    ///
    /// Once this function returns, the contents of `bufs` are unspecified, as
    /// this depends on how many calls to [`write_vectored`] were necessary. It is
    /// best to understand this function as taking ownership of `bufs` and to
    /// not use `bufs` afterwards. The underlying buffers, to which the
    /// [`IoSlice`]s point (but not the [`IoSlice`]s themselves), are unchanged and
    /// can be reused.
    ///
    /// [`write_vectored`]: Write::write_vectored
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(write_all_vectored)]
    /// # fn main() -> std::io::Result<()> {
    ///
    /// use std::io::{Write, IoSlice};
    ///
    /// let mut writer = Vec::new();
    /// let bufs = &mut [
    ///     IoSlice::new(&[1]),
    ///     IoSlice::new(&[2, 3]),
    ///     IoSlice::new(&[4, 5, 6]),
    /// ];
    ///
    /// writer.write_all_vectored(bufs)?;
    /// // Note: the contents of `bufs` is now undefined, see the Notes section.
    ///
    /// assert_eq!(writer, &[1, 2, 3, 4, 5, 6]);
    /// # Ok(()) }
    /// ```
    #[unstable(feature = "write_all_vectored", issue = "70436")]
    fn write_all_vectored(&mut self, mut bufs: &mut [IoSlice<'_>]) -> Result<()> {
        // Guarantee that bufs is empty if it contains no data,
        // to avoid calling write_vectored if there is no data to be written.
        IoSlice::advance_slices(&mut bufs, 0);
        while !bufs.is_empty() {
            match self.write_vectored(bufs) {
                Ok(0) => {
                    return Err(Error::WRITE_ALL_EOF);
                }
                Ok(n) => IoSlice::advance_slices(&mut bufs, n),
                Err(ref e) if e.is_interrupted() => {}
                Err(e) => return Err(e),
            }
        }
        Ok(())
    }

    /// Writes a formatted string into this writer, returning any error
    /// encountered.
    ///
    /// This method is primarily used to interface with the
    /// [`format_args!()`] macro, and it is rare that this should
    /// explicitly be called. The [`write!()`] macro should be favored to
    /// invoke this method instead.
    ///
    /// This function internally uses the [`write_all`] method on
    /// this trait and hence will continuously write data so long as no errors
    /// are received. This also means that partial writes are not indicated in
    /// this signature.
    ///
    /// [`write_all`]: Write::write_all
    ///
    /// # Errors
    ///
    /// This function will return any I/O error reported while formatting.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> std::io::Result<()> {
    ///     let mut buffer = File::create("foo.txt")?;
    ///
    ///     // this call
    ///     write!(buffer, "{:.*}", 2, 1.234567)?;
    ///     // turns into this:
    ///     buffer.write_fmt(format_args!("{:.*}", 2, 1.234567))?;
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
        // Create a shim which translates a Write to a fmt::Write and saves
        // off I/O errors. instead of discarding them
        struct Adapter<'a, T: ?Sized + 'a> {
            inner: &'a mut T,
            error: Result<()>,
        }

        impl<T: Write + ?Sized> fmt::Write for Adapter<'_, T> {
            fn write_str(&mut self, s: &str) -> fmt::Result {
                match self.inner.write_all(s.as_bytes()) {
                    Ok(()) => Ok(()),
                    Err(e) => {
                        self.error = Err(e);
                        Err(fmt::Error)
                    }
                }
            }
        }

        let mut output = Adapter { inner: self, error: Ok(()) };
        match fmt::write(&mut output, fmt) {
            Ok(()) => Ok(()),
            Err(..) => {
                // check if the error came from the underlying `Write` or not
                if output.error.is_err() {
                    output.error
                } else {
                    Err(error::const_io_error!(ErrorKind::Uncategorized, "formatter error"))
                }
            }
        }
    }

    /// Creates a "by reference" adapter for this instance of `Write`.
    ///
    /// The returned adapter also implements `Write` and will simply borrow this
    /// current writer.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io::Write;
    /// use std::fs::File;
    ///
    /// fn main() -> std::io::Result<()> {
    ///     let mut buffer = File::create("foo.txt")?;
    ///
    ///     let reference = buffer.by_ref();
    ///
    ///     // we can use reference just like our original buffer
    ///     reference.write_all(b"some bytes")?;
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn by_ref(&mut self) -> &mut Self
    where
        Self: Sized,
    {
        self
    }
}

/// The `Seek` trait provides a cursor which can be moved within a stream of
/// bytes.
///
/// The stream typically has a fixed size, allowing seeking relative to either
/// end or the current offset.
///
/// # Examples
///
/// [`File`]s implement `Seek`:
///
/// [`File`]: crate::fs::File
///
/// ```no_run
/// use std::io;
/// use std::io::prelude::*;
/// use std::fs::File;
/// use std::io::SeekFrom;
///
/// fn main() -> io::Result<()> {
///     let mut f = File::open("foo.txt")?;
///
///     // move the cursor 42 bytes from the start of the file
///     f.seek(SeekFrom::Start(42))?;
///     Ok(())
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "IoSeek")]
pub trait Seek {
    /// Seek to an offset, in bytes, in a stream.
    ///
    /// A seek beyond the end of a stream is allowed, but behavior is defined
    /// by the implementation.
    ///
    /// If the seek operation completed successfully,
    /// this method returns the new position from the start of the stream.
    /// That position can be used later with [`SeekFrom::Start`].
    ///
    /// # Errors
    ///
    /// Seeking can fail, for example because it might involve flushing a buffer.
    ///
    /// Seeking to a negative offset is considered an error.
    #[stable(feature = "rust1", since = "1.0.0")]
    fn seek(&mut self, pos: SeekFrom) -> Result<u64>;

    /// Rewind to the beginning of a stream.
    ///
    /// This is a convenience method, equivalent to `seek(SeekFrom::Start(0))`.
    ///
    /// # Errors
    ///
    /// Rewinding can fail, for example because it might involve flushing a buffer.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use std::io::{Read, Seek, Write};
    /// use std::fs::OpenOptions;
    ///
    /// let mut f = OpenOptions::new()
    ///     .write(true)
    ///     .read(true)
    ///     .create(true)
    ///     .open("foo.txt").unwrap();
    ///
    /// let hello = "Hello!\n";
    /// write!(f, "{hello}").unwrap();
    /// f.rewind().unwrap();
    ///
    /// let mut buf = String::new();
    /// f.read_to_string(&mut buf).unwrap();
    /// assert_eq!(&buf, hello);
    /// ```
    #[stable(feature = "seek_rewind", since = "1.55.0")]
    fn rewind(&mut self) -> Result<()> {
        self.seek(SeekFrom::Start(0))?;
        Ok(())
    }

    /// Returns the length of this stream (in bytes).
    ///
    /// This method is implemented using up to three seek operations. If this
    /// method returns successfully, the seek position is unchanged (i.e. the
    /// position before calling this method is the same as afterwards).
    /// However, if this method returns an error, the seek position is
    /// unspecified.
    ///
    /// If you need to obtain the length of *many* streams and you don't care
    /// about the seek position afterwards, you can reduce the number of seek
    /// operations by simply calling `seek(SeekFrom::End(0))` and using its
    /// return value (it is also the stream length).
    ///
    /// Note that length of a stream can change over time (for example, when
    /// data is appended to a file). So calling this method multiple times does
    /// not necessarily return the same length each time.
    ///
    /// # Example
    ///
    /// ```no_run
    /// #![feature(seek_stream_len)]
    /// use std::{
    ///     io::{self, Seek},
    ///     fs::File,
    /// };
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = File::open("foo.txt")?;
    ///
    ///     let len = f.stream_len()?;
    ///     println!("The file is currently {len} bytes long");
    ///     Ok(())
    /// }
    /// ```
    #[unstable(feature = "seek_stream_len", issue = "59359")]
    fn stream_len(&mut self) -> Result<u64> {
        let old_pos = self.stream_position()?;
        let len = self.seek(SeekFrom::End(0))?;

        // Avoid seeking a third time when we were already at the end of the
        // stream. The branch is usually way cheaper than a seek operation.
        if old_pos != len {
            self.seek(SeekFrom::Start(old_pos))?;
        }

        Ok(len)
    }

    /// Returns the current seek position from the start of the stream.
    ///
    /// This is equivalent to `self.seek(SeekFrom::Current(0))`.
    ///
    /// # Example
    ///
    /// ```no_run
    /// use std::{
    ///     io::{self, BufRead, BufReader, Seek},
    ///     fs::File,
    /// };
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = BufReader::new(File::open("foo.txt")?);
    ///
    ///     let before = f.stream_position()?;
    ///     f.read_line(&mut String::new())?;
    ///     let after = f.stream_position()?;
    ///
    ///     println!("The first line was {} bytes long", after - before);
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "seek_convenience", since = "1.51.0")]
    fn stream_position(&mut self) -> Result<u64> {
        self.seek(SeekFrom::Current(0))
    }

    /// Seeks relative to the current position.
    ///
    /// This is equivalent to `self.seek(SeekFrom::Current(offset))` but
    /// doesn't return the new position which can allow some implementations
    /// such as [`BufReader`] to perform more efficient seeks.
    ///
    /// # Example
    ///
    /// ```no_run
    /// #![feature(seek_seek_relative)]
    /// use std::{
    ///     io::{self, Seek},
    ///     fs::File,
    /// };
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = File::open("foo.txt")?;
    ///     f.seek_relative(10)?;
    ///     assert_eq!(f.stream_position()?, 10);
    ///     Ok(())
    /// }
    /// ```
    ///
    /// [`BufReader`]: crate::io::BufReader
    #[unstable(feature = "seek_seek_relative", issue = "117374")]
    fn seek_relative(&mut self, offset: i64) -> Result<()> {
        self.seek(SeekFrom::Current(offset))?;
        Ok(())
    }
}

/// Enumeration of possible methods to seek within an I/O object.
///
/// It is used by the [`Seek`] trait.
#[derive(Copy, PartialEq, Eq, Clone, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub enum SeekFrom {
    /// Sets the offset to the provided number of bytes.
    #[stable(feature = "rust1", since = "1.0.0")]
    Start(#[stable(feature = "rust1", since = "1.0.0")] u64),

    /// Sets the offset to the size of this object plus the specified number of
    /// bytes.
    ///
    /// It is possible to seek beyond the end of an object, but it's an error to
    /// seek before byte 0.
    #[stable(feature = "rust1", since = "1.0.0")]
    End(#[stable(feature = "rust1", since = "1.0.0")] i64),

    /// Sets the offset to the current position plus the specified number of
    /// bytes.
    ///
    /// It is possible to seek beyond the end of an object, but it's an error to
    /// seek before byte 0.
    #[stable(feature = "rust1", since = "1.0.0")]
    Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
}

fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> Result<usize> {
    let mut read = 0;
    loop {
        let (done, used) = {
            let available = match r.fill_buf() {
                Ok(n) => n,
                Err(ref e) if e.is_interrupted() => continue,
                Err(e) => return Err(e),
            };
            match memchr::memchr(delim, available) {
                Some(i) => {
                    buf.extend_from_slice(&available[..=i]);
                    (true, i + 1)
                }
                None => {
                    buf.extend_from_slice(available);
                    (false, available.len())
                }
            }
        };
        r.consume(used);
        read += used;
        if done || used == 0 {
            return Ok(read);
        }
    }
}

fn skip_until<R: BufRead + ?Sized>(r: &mut R, delim: u8) -> Result<usize> {
    let mut read = 0;
    loop {
        let (done, used) = {
            let available = match r.fill_buf() {
                Ok(n) => n,
                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                Err(e) => return Err(e),
            };
            match memchr::memchr(delim, available) {
                Some(i) => (true, i + 1),
                None => (false, available.len()),
            }
        };
        r.consume(used);
        read += used;
        if done || used == 0 {
            return Ok(read);
        }
    }
}

/// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
/// to perform extra ways of reading.
///
/// For example, reading line-by-line is inefficient without using a buffer, so
/// if you want to read by line, you'll need `BufRead`, which includes a
/// [`read_line`] method as well as a [`lines`] iterator.
///
/// # Examples
///
/// A locked standard input implements `BufRead`:
///
/// ```no_run
/// use std::io;
/// use std::io::prelude::*;
///
/// let stdin = io::stdin();
/// for line in stdin.lock().lines() {
///     println!("{}", line.unwrap());
/// }
/// ```
///
/// If you have something that implements [`Read`], you can use the [`BufReader`
/// type][`BufReader`] to turn it into a `BufRead`.
///
/// For example, [`File`] implements [`Read`], but not `BufRead`.
/// [`BufReader`] to the rescue!
///
/// [`File`]: crate::fs::File
/// [`read_line`]: BufRead::read_line
/// [`lines`]: BufRead::lines
///
/// ```no_run
/// use std::io::{self, BufReader};
/// use std::io::prelude::*;
/// use std::fs::File;
///
/// fn main() -> io::Result<()> {
///     let f = File::open("foo.txt")?;
///     let f = BufReader::new(f);
///
///     for line in f.lines() {
///         println!("{}", line.unwrap());
///     }
///
///     Ok(())
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub trait BufRead: Read {
    /// Returns the contents of the internal buffer, filling it with more data
    /// from the inner reader if it is empty.
    ///
    /// This function is a lower-level call. It needs to be paired with the
    /// [`consume`] method to function properly. When calling this
    /// method, none of the contents will be "read" in the sense that later
    /// calling `read` may return the same contents. As such, [`consume`] must
    /// be called with the number of bytes that are consumed from this buffer to
    /// ensure that the bytes are never returned twice.
    ///
    /// [`consume`]: BufRead::consume
    ///
    /// An empty buffer returned indicates that the stream has reached EOF.
    ///
    /// # Errors
    ///
    /// This function will return an I/O error if the underlying reader was
    /// read, but returned an error.
    ///
    /// # Examples
    ///
    /// A locked standard input implements `BufRead`:
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    ///
    /// let stdin = io::stdin();
    /// let mut stdin = stdin.lock();
    ///
    /// let buffer = stdin.fill_buf().unwrap();
    ///
    /// // work with buffer
    /// println!("{buffer:?}");
    ///
    /// // ensure the bytes we worked with aren't returned again later
    /// let length = buffer.len();
    /// stdin.consume(length);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn fill_buf(&mut self) -> Result<&[u8]>;

    /// Tells this buffer that `amt` bytes have been consumed from the buffer,
    /// so they should no longer be returned in calls to `read`.
    ///
    /// This function is a lower-level call. It needs to be paired with the
    /// [`fill_buf`] method to function properly. This function does
    /// not perform any I/O, it simply informs this object that some amount of
    /// its buffer, returned from [`fill_buf`], has been consumed and should
    /// no longer be returned. As such, this function may do odd things if
    /// [`fill_buf`] isn't called before calling it.
    ///
    /// The `amt` must be `<=` the number of bytes in the buffer returned by
    /// [`fill_buf`].
    ///
    /// # Examples
    ///
    /// Since `consume()` is meant to be used with [`fill_buf`],
    /// that method's example includes an example of `consume()`.
    ///
    /// [`fill_buf`]: BufRead::fill_buf
    #[stable(feature = "rust1", since = "1.0.0")]
    fn consume(&mut self, amt: usize);

    /// Check if the underlying `Read` has any data left to be read.
    ///
    /// This function may fill the buffer to check for data,
    /// so this functions returns `Result<bool>`, not `bool`.
    ///
    /// Default implementation calls `fill_buf` and checks that
    /// returned slice is empty (which means that there is no data left,
    /// since EOF is reached).
    ///
    /// Examples
    ///
    /// ```
    /// #![feature(buf_read_has_data_left)]
    /// use std::io;
    /// use std::io::prelude::*;
    ///
    /// let stdin = io::stdin();
    /// let mut stdin = stdin.lock();
    ///
    /// while stdin.has_data_left().unwrap() {
    ///     let mut line = String::new();
    ///     stdin.read_line(&mut line).unwrap();
    ///     // work with line
    ///     println!("{line:?}");
    /// }
    /// ```
    #[unstable(feature = "buf_read_has_data_left", reason = "recently added", issue = "86423")]
    fn has_data_left(&mut self) -> Result<bool> {
        self.fill_buf().map(|b| !b.is_empty())
    }

    /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
    ///
    /// This function will read bytes from the underlying stream until the
    /// delimiter or EOF is found. Once found, all bytes up to, and including,
    /// the delimiter (if found) will be appended to `buf`.
    ///
    /// If successful, this function will return the total number of bytes read.
    ///
    /// This function is blocking and should be used carefully: it is possible for
    /// an attacker to continuously send bytes without ever sending the delimiter
    /// or EOF.
    ///
    /// # Errors
    ///
    /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
    /// will otherwise return any errors returned by [`fill_buf`].
    ///
    /// If an I/O error is encountered then all bytes read so far will be
    /// present in `buf` and its length will have been adjusted appropriately.
    ///
    /// [`fill_buf`]: BufRead::fill_buf
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to read all the bytes in a byte slice
    /// in hyphen delimited segments:
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let mut cursor = io::Cursor::new(b"lorem-ipsum");
    /// let mut buf = vec![];
    ///
    /// // cursor is at 'l'
    /// let num_bytes = cursor.read_until(b'-', &mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 6);
    /// assert_eq!(buf, b"lorem-");
    /// buf.clear();
    ///
    /// // cursor is at 'i'
    /// let num_bytes = cursor.read_until(b'-', &mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 5);
    /// assert_eq!(buf, b"ipsum");
    /// buf.clear();
    ///
    /// // cursor is at EOF
    /// let num_bytes = cursor.read_until(b'-', &mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 0);
    /// assert_eq!(buf, b"");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
        read_until(self, byte, buf)
    }

    /// Skip all bytes until the delimiter `byte` or EOF is reached.
    ///
    /// This function will read (and discard) bytes from the underlying stream until the
    /// delimiter or EOF is found.
    ///
    /// If successful, this function will return the total number of bytes read,
    /// including the delimiter byte.
    ///
    /// This is useful for efficiently skipping data such as NUL-terminated strings
    /// in binary file formats without buffering.
    ///
    /// This function is blocking and should be used carefully: it is possible for
    /// an attacker to continuously send bytes without ever sending the delimiter
    /// or EOF.
    ///
    /// # Errors
    ///
    /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
    /// will otherwise return any errors returned by [`fill_buf`].
    ///
    /// If an I/O error is encountered then all bytes read so far will be
    /// present in `buf` and its length will have been adjusted appropriately.
    ///
    /// [`fill_buf`]: BufRead::fill_buf
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to read some NUL-terminated information
    /// about Ferris from a binary string, skipping the fun fact:
    ///
    /// ```
    /// #![feature(bufread_skip_until)]
    ///
    /// use std::io::{self, BufRead};
    ///
    /// let mut cursor = io::Cursor::new(b"Ferris\0Likes long walks on the beach\0Crustacean\0");
    ///
    /// // read name
    /// let mut name = Vec::new();
    /// let num_bytes = cursor.read_until(b'\0', &mut name)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 7);
    /// assert_eq!(name, b"Ferris\0");
    ///
    /// // skip fun fact
    /// let num_bytes = cursor.skip_until(b'\0')
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 30);
    ///
    /// // read animal type
    /// let mut animal = Vec::new();
    /// let num_bytes = cursor.read_until(b'\0', &mut animal)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 11);
    /// assert_eq!(animal, b"Crustacean\0");
    /// ```
    #[unstable(feature = "bufread_skip_until", issue = "111735")]
    fn skip_until(&mut self, byte: u8) -> Result<usize> {
        skip_until(self, byte)
    }

    /// Read all bytes until a newline (the `0xA` byte) is reached, and append
    /// them to the provided `String` buffer.
    ///
    /// Previous content of the buffer will be preserved. To avoid appending to
    /// the buffer, you need to [`clear`] it first.
    ///
    /// This function will read bytes from the underlying stream until the
    /// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes
    /// up to, and including, the delimiter (if found) will be appended to
    /// `buf`.
    ///
    /// If successful, this function will return the total number of bytes read.
    ///
    /// If this function returns [`Ok(0)`], the stream has reached EOF.
    ///
    /// This function is blocking and should be used carefully: it is possible for
    /// an attacker to continuously send bytes without ever sending a newline
    /// or EOF. You can use [`take`] to limit the maximum number of bytes read.
    ///
    /// [`Ok(0)`]: Ok
    /// [`clear`]: String::clear
    /// [`take`]: crate::io::Read::take
    ///
    /// # Errors
    ///
    /// This function has the same error semantics as [`read_until`] and will
    /// also return an error if the read bytes are not valid UTF-8. If an I/O
    /// error is encountered then `buf` may contain some bytes already read in
    /// the event that all data read so far was valid UTF-8.
    ///
    /// [`read_until`]: BufRead::read_until
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to read all the lines in a byte slice:
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let mut cursor = io::Cursor::new(b"foo\nbar");
    /// let mut buf = String::new();
    ///
    /// // cursor is at 'f'
    /// let num_bytes = cursor.read_line(&mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 4);
    /// assert_eq!(buf, "foo\n");
    /// buf.clear();
    ///
    /// // cursor is at 'b'
    /// let num_bytes = cursor.read_line(&mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 3);
    /// assert_eq!(buf, "bar");
    /// buf.clear();
    ///
    /// // cursor is at EOF
    /// let num_bytes = cursor.read_line(&mut buf)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 0);
    /// assert_eq!(buf, "");
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn read_line(&mut self, buf: &mut String) -> Result<usize> {
        // Note that we are not calling the `.read_until` method here, but
        // rather our hardcoded implementation. For more details as to why, see
        // the comments in `read_to_end`.
        unsafe { append_to_string(buf, |b| read_until(self, b'\n', b)) }
    }

    /// Returns an iterator over the contents of this reader split on the byte
    /// `byte`.
    ///
    /// The iterator returned from this function will return instances of
    /// <code>[io::Result]<[Vec]\<u8>></code>. Each vector returned will *not* have
    /// the delimiter byte at the end.
    ///
    /// This function will yield errors whenever [`read_until`] would have
    /// also yielded an error.
    ///
    /// [io::Result]: self::Result "io::Result"
    /// [`read_until`]: BufRead::read_until
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to iterate over all hyphen delimited
    /// segments in a byte slice
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let cursor = io::Cursor::new(b"lorem-ipsum-dolor");
    ///
    /// let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
    /// assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
    /// assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
    /// assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
    /// assert_eq!(split_iter.next(), None);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn split(self, byte: u8) -> Split<Self>
    where
        Self: Sized,
    {
        Split { buf: self, delim: byte }
    }

    /// Returns an iterator over the lines of this reader.
    ///
    /// The iterator returned from this function will yield instances of
    /// <code>[io::Result]<[String]></code>. Each string returned will *not* have a newline
    /// byte (the `0xA` byte) or `CRLF` (`0xD`, `0xA` bytes) at the end.
    ///
    /// [io::Result]: self::Result "io::Result"
    ///
    /// # Examples
    ///
    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
    /// this example, we use [`Cursor`] to iterate over all the lines in a byte
    /// slice.
    ///
    /// ```
    /// use std::io::{self, BufRead};
    ///
    /// let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");
    ///
    /// let mut lines_iter = cursor.lines().map(|l| l.unwrap());
    /// assert_eq!(lines_iter.next(), Some(String::from("lorem")));
    /// assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
    /// assert_eq!(lines_iter.next(), Some(String::from("dolor")));
    /// assert_eq!(lines_iter.next(), None);
    /// ```
    ///
    /// # Errors
    ///
    /// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
    #[stable(feature = "rust1", since = "1.0.0")]
    fn lines(self) -> Lines<Self>
    where
        Self: Sized,
    {
        Lines { buf: self }
    }
}

/// Adapter to chain together two readers.
///
/// This struct is generally created by calling [`chain`] on a reader.
/// Please see the documentation of [`chain`] for more details.
///
/// [`chain`]: Read::chain
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct Chain<T, U> {
    first: T,
    second: U,
    done_first: bool,
}

impl<T, U> Chain<T, U> {
    /// Consumes the `Chain`, returning the wrapped readers.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut foo_file = File::open("foo.txt")?;
    ///     let mut bar_file = File::open("bar.txt")?;
    ///
    ///     let chain = foo_file.chain(bar_file);
    ///     let (foo_file, bar_file) = chain.into_inner();
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
    pub fn into_inner(self) -> (T, U) {
        (self.first, self.second)
    }

    /// Gets references to the underlying readers in this `Chain`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut foo_file = File::open("foo.txt")?;
    ///     let mut bar_file = File::open("bar.txt")?;
    ///
    ///     let chain = foo_file.chain(bar_file);
    ///     let (foo_file, bar_file) = chain.get_ref();
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
    pub fn get_ref(&self) -> (&T, &U) {
        (&self.first, &self.second)
    }

    /// Gets mutable references to the underlying readers in this `Chain`.
    ///
    /// Care should be taken to avoid modifying the internal I/O state of the
    /// underlying readers as doing so may corrupt the internal state of this
    /// `Chain`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut foo_file = File::open("foo.txt")?;
    ///     let mut bar_file = File::open("bar.txt")?;
    ///
    ///     let mut chain = foo_file.chain(bar_file);
    ///     let (foo_file, bar_file) = chain.get_mut();
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
    pub fn get_mut(&mut self) -> (&mut T, &mut U) {
        (&mut self.first, &mut self.second)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Read, U: Read> Read for Chain<T, U> {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        if !self.done_first {
            match self.first.read(buf)? {
                0 if !buf.is_empty() => self.done_first = true,
                n => return Ok(n),
            }
        }
        self.second.read(buf)
    }

    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
        if !self.done_first {
            match self.first.read_vectored(bufs)? {
                0 if bufs.iter().any(|b| !b.is_empty()) => self.done_first = true,
                n => return Ok(n),
            }
        }
        self.second.read_vectored(bufs)
    }

    #[inline]
    fn is_read_vectored(&self) -> bool {
        self.first.is_read_vectored() || self.second.is_read_vectored()
    }

    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
        let mut read = 0;
        if !self.done_first {
            read += self.first.read_to_end(buf)?;
            self.done_first = true;
        }
        read += self.second.read_to_end(buf)?;
        Ok(read)
    }

    // We don't override `read_to_string` here because an UTF-8 sequence could
    // be split between the two parts of the chain

    fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
        if buf.capacity() == 0 {
            return Ok(());
        }

        if !self.done_first {
            let old_len = buf.written();
            self.first.read_buf(buf.reborrow())?;

            if buf.written() != old_len {
                return Ok(());
            } else {
                self.done_first = true;
            }
        }
        self.second.read_buf(buf)
    }
}

#[stable(feature = "chain_bufread", since = "1.9.0")]
impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
    fn fill_buf(&mut self) -> Result<&[u8]> {
        if !self.done_first {
            match self.first.fill_buf()? {
                buf if buf.is_empty() => self.done_first = true,
                buf => return Ok(buf),
            }
        }
        self.second.fill_buf()
    }

    fn consume(&mut self, amt: usize) {
        if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
    }

    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
        let mut read = 0;
        if !self.done_first {
            let n = self.first.read_until(byte, buf)?;
            read += n;

            match buf.last() {
                Some(b) if *b == byte && n != 0 => return Ok(read),
                _ => self.done_first = true,
            }
        }
        read += self.second.read_until(byte, buf)?;
        Ok(read)
    }

    // We don't override `read_line` here because an UTF-8 sequence could be
    // split between the two parts of the chain
}

impl<T, U> SizeHint for Chain<T, U> {
    #[inline]
    fn lower_bound(&self) -> usize {
        SizeHint::lower_bound(&self.first) + SizeHint::lower_bound(&self.second)
    }

    #[inline]
    fn upper_bound(&self) -> Option<usize> {
        match (SizeHint::upper_bound(&self.first), SizeHint::upper_bound(&self.second)) {
            (Some(first), Some(second)) => first.checked_add(second),
            _ => None,
        }
    }
}

/// Reader adapter which limits the bytes read from an underlying reader.
///
/// This struct is generally created by calling [`take`] on a reader.
/// Please see the documentation of [`take`] for more details.
///
/// [`take`]: Read::take
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct Take<T> {
    inner: T,
    limit: u64,
}

impl<T> Take<T> {
    /// Returns the number of bytes that can be read before this instance will
    /// return EOF.
    ///
    /// # Note
    ///
    /// This instance may reach `EOF` after reading fewer bytes than indicated by
    /// this method if the underlying [`Read`] instance reaches EOF.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let f = File::open("foo.txt")?;
    ///
    ///     // read at most five bytes
    ///     let handle = f.take(5);
    ///
    ///     println!("limit: {}", handle.limit());
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn limit(&self) -> u64 {
        self.limit
    }

    /// Sets the number of bytes that can be read before this instance will
    /// return EOF. This is the same as constructing a new `Take` instance, so
    /// the amount of bytes read and the previous limit value don't matter when
    /// calling this method.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let f = File::open("foo.txt")?;
    ///
    ///     // read at most five bytes
    ///     let mut handle = f.take(5);
    ///     handle.set_limit(10);
    ///
    ///     assert_eq!(handle.limit(), 10);
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "take_set_limit", since = "1.27.0")]
    pub fn set_limit(&mut self, limit: u64) {
        self.limit = limit;
    }

    /// Consumes the `Take`, returning the wrapped reader.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut file = File::open("foo.txt")?;
    ///
    ///     let mut buffer = [0; 5];
    ///     let mut handle = file.take(5);
    ///     handle.read(&mut buffer)?;
    ///
    ///     let file = handle.into_inner();
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "io_take_into_inner", since = "1.15.0")]
    pub fn into_inner(self) -> T {
        self.inner
    }

    /// Gets a reference to the underlying reader.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut file = File::open("foo.txt")?;
    ///
    ///     let mut buffer = [0; 5];
    ///     let mut handle = file.take(5);
    ///     handle.read(&mut buffer)?;
    ///
    ///     let file = handle.get_ref();
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
    pub fn get_ref(&self) -> &T {
        &self.inner
    }

    /// Gets a mutable reference to the underlying reader.
    ///
    /// Care should be taken to avoid modifying the internal I/O state of the
    /// underlying reader as doing so may corrupt the internal limit of this
    /// `Take`.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut file = File::open("foo.txt")?;
    ///
    ///     let mut buffer = [0; 5];
    ///     let mut handle = file.take(5);
    ///     handle.read(&mut buffer)?;
    ///
    ///     let file = handle.get_mut();
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
    pub fn get_mut(&mut self) -> &mut T {
        &mut self.inner
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Read> Read for Take<T> {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        // Don't call into inner reader at all at EOF because it may still block
        if self.limit == 0 {
            return Ok(0);
        }

        let max = cmp::min(buf.len() as u64, self.limit) as usize;
        let n = self.inner.read(&mut buf[..max])?;
        assert!(n as u64 <= self.limit, "number of read bytes exceeds limit");
        self.limit -= n as u64;
        Ok(n)
    }

    fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
        // Don't call into inner reader at all at EOF because it may still block
        if self.limit == 0 {
            return Ok(());
        }

        if self.limit <= buf.capacity() as u64 {
            // if we just use an as cast to convert, limit may wrap around on a 32 bit target
            let limit = cmp::min(self.limit, usize::MAX as u64) as usize;

            let extra_init = cmp::min(limit as usize, buf.init_ref().len());

            // SAFETY: no uninit data is written to ibuf
            let ibuf = unsafe { &mut buf.as_mut()[..limit] };

            let mut sliced_buf: BorrowedBuf<'_> = ibuf.into();

            // SAFETY: extra_init bytes of ibuf are known to be initialized
            unsafe {
                sliced_buf.set_init(extra_init);
            }

            let mut cursor = sliced_buf.unfilled();
            self.inner.read_buf(cursor.reborrow())?;

            let new_init = cursor.init_ref().len();
            let filled = sliced_buf.len();

            // cursor / sliced_buf / ibuf must drop here

            unsafe {
                // SAFETY: filled bytes have been filled and therefore initialized
                buf.advance_unchecked(filled);
                // SAFETY: new_init bytes of buf's unfilled buffer have been initialized
                buf.set_init(new_init);
            }

            self.limit -= filled as u64;
        } else {
            let written = buf.written();
            self.inner.read_buf(buf.reborrow())?;
            self.limit -= (buf.written() - written) as u64;
        }

        Ok(())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: BufRead> BufRead for Take<T> {
    fn fill_buf(&mut self) -> Result<&[u8]> {
        // Don't call into inner reader at all at EOF because it may still block
        if self.limit == 0 {
            return Ok(&[]);
        }

        let buf = self.inner.fill_buf()?;
        let cap = cmp::min(buf.len() as u64, self.limit) as usize;
        Ok(&buf[..cap])
    }

    fn consume(&mut self, amt: usize) {
        // Don't let callers reset the limit by passing an overlarge value
        let amt = cmp::min(amt as u64, self.limit) as usize;
        self.limit -= amt as u64;
        self.inner.consume(amt);
    }
}

impl<T> SizeHint for Take<T> {
    #[inline]
    fn lower_bound(&self) -> usize {
        cmp::min(SizeHint::lower_bound(&self.inner) as u64, self.limit) as usize
    }

    #[inline]
    fn upper_bound(&self) -> Option<usize> {
        match SizeHint::upper_bound(&self.inner) {
            Some(upper_bound) => Some(cmp::min(upper_bound as u64, self.limit) as usize),
            None => self.limit.try_into().ok(),
        }
    }
}

/// An iterator over `u8` values of a reader.
///
/// This struct is generally created by calling [`bytes`] on a reader.
/// Please see the documentation of [`bytes`] for more details.
///
/// [`bytes`]: Read::bytes
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct Bytes<R> {
    inner: R,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<R: Read> Iterator for Bytes<R> {
    type Item = Result<u8>;

    // Not `#[inline]`. This function gets inlined even without it, but having
    // the inline annotation can result in worse code generation. See #116785.
    fn next(&mut self) -> Option<Result<u8>> {
        SpecReadByte::spec_read_byte(&mut self.inner)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        SizeHint::size_hint(&self.inner)
    }
}

/// For the specialization of `Bytes::next`.
trait SpecReadByte {
    fn spec_read_byte(&mut self) -> Option<Result<u8>>;
}

impl<R> SpecReadByte for R
where
    Self: Read,
{
    #[inline]
    default fn spec_read_byte(&mut self) -> Option<Result<u8>> {
        inlined_slow_read_byte(self)
    }
}

/// Read a single byte in a slow, generic way. This is used by the default
/// `spec_read_byte`.
#[inline]
fn inlined_slow_read_byte<R: Read>(reader: &mut R) -> Option<Result<u8>> {
    let mut byte = 0;
    loop {
        return match reader.read(slice::from_mut(&mut byte)) {
            Ok(0) => None,
            Ok(..) => Some(Ok(byte)),
            Err(ref e) if e.is_interrupted() => continue,
            Err(e) => Some(Err(e)),
        };
    }
}

// Used by `BufReader::spec_read_byte`, for which the `inline(ever)` is
// important.
#[inline(never)]
fn uninlined_slow_read_byte<R: Read>(reader: &mut R) -> Option<Result<u8>> {
    inlined_slow_read_byte(reader)
}

trait SizeHint {
    fn lower_bound(&self) -> usize;

    fn upper_bound(&self) -> Option<usize>;

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.lower_bound(), self.upper_bound())
    }
}

impl<T: ?Sized> SizeHint for T {
    #[inline]
    default fn lower_bound(&self) -> usize {
        0
    }

    #[inline]
    default fn upper_bound(&self) -> Option<usize> {
        None
    }
}

impl<T> SizeHint for &mut T {
    #[inline]
    fn lower_bound(&self) -> usize {
        SizeHint::lower_bound(*self)
    }

    #[inline]
    fn upper_bound(&self) -> Option<usize> {
        SizeHint::upper_bound(*self)
    }
}

impl<T> SizeHint for Box<T> {
    #[inline]
    fn lower_bound(&self) -> usize {
        SizeHint::lower_bound(&**self)
    }

    #[inline]
    fn upper_bound(&self) -> Option<usize> {
        SizeHint::upper_bound(&**self)
    }
}

impl SizeHint for &[u8] {
    #[inline]
    fn lower_bound(&self) -> usize {
        self.len()
    }

    #[inline]
    fn upper_bound(&self) -> Option<usize> {
        Some(self.len())
    }
}

/// An iterator over the contents of an instance of `BufRead` split on a
/// particular byte.
///
/// This struct is generally created by calling [`split`] on a `BufRead`.
/// Please see the documentation of [`split`] for more details.
///
/// [`split`]: BufRead::split
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct Split<B> {
    buf: B,
    delim: u8,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B: BufRead> Iterator for Split<B> {
    type Item = Result<Vec<u8>>;

    fn next(&mut self) -> Option<Result<Vec<u8>>> {
        let mut buf = Vec::new();
        match self.buf.read_until(self.delim, &mut buf) {
            Ok(0) => None,
            Ok(_n) => {
                if buf[buf.len() - 1] == self.delim {
                    buf.pop();
                }
                Some(Ok(buf))
            }
            Err(e) => Some(Err(e)),
        }
    }
}

/// An iterator over the lines of an instance of `BufRead`.
///
/// This struct is generally created by calling [`lines`] on a `BufRead`.
/// Please see the documentation of [`lines`] for more details.
///
/// [`lines`]: BufRead::lines
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
#[cfg_attr(not(test), rustc_diagnostic_item = "IoLines")]
pub struct Lines<B> {
    buf: B,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B: BufRead> Iterator for Lines<B> {
    type Item = Result<String>;

    fn next(&mut self) -> Option<Result<String>> {
        let mut buf = String::new();
        match self.buf.read_line(&mut buf) {
            Ok(0) => None,
            Ok(_n) => {
                if buf.ends_with('\n') {
                    buf.pop();
                    if buf.ends_with('\r') {
                        buf.pop();
                    }
                }
                Some(Ok(buf))
            }
            Err(e) => Some(Err(e)),
        }
    }
}

//! The `Box<T>` type for heap allocation.
//!
//! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
//! heap allocation in Rust. Boxes provide ownership for this allocation, and
//! drop their contents when they go out of scope. Boxes also ensure that they
//! never allocate more than `isize::MAX` bytes.
//!
//! # Examples
//!
//! Move a value from the stack to the heap by creating a [`Box`]:
//!
//! ```
//! let val: u8 = 5;
//! let boxed: Box<u8> = Box::new(val);
//! ```
//!
//! Move a value from a [`Box`] back to the stack by [dereferencing]:
//!
//! ```
//! let boxed: Box<u8> = Box::new(5);
//! let val: u8 = *boxed;
//! ```
//!
//! Creating a recursive data structure:
//!
//! ```
//! ##[allow(dead_code)]
//! #[derive(Debug)]
//! enum List<T> {
//!     Cons(T, Box<List<T>>),
//!     Nil,
//! }
//!
//! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
//! println!("{list:?}");
//! ```
//!
//! This will print `Cons(1, Cons(2, Nil))`.
//!
//! Recursive structures must be boxed, because if the definition of `Cons`
//! looked like this:
//!
//! ```compile_fail,E0072
//! # enum List<T> {
//! Cons(T, List<T>),
//! # }
//! ```
//!
//! It wouldn't work. This is because the size of a `List` depends on how many
//! elements are in the list, and so we don't know how much memory to allocate
//! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
//! big `Cons` needs to be.
//!
//! # Memory layout
//!
//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
//! its allocation. It is valid to convert both ways between a [`Box`] and a
//! raw pointer allocated with the [`Global`] allocator, given that the
//! [`Layout`] used with the allocator is correct for the type. More precisely,
//! a `value: *mut T` that has been allocated with the [`Global`] allocator
//! with `Layout::for_value(&*value)` may be converted into a box using
//! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
//! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
//! [`Global`] allocator with [`Layout::for_value(&*value)`].
//!
//! For zero-sized values, the `Box` pointer still has to be [valid] for reads
//! and writes and sufficiently aligned. In particular, casting any aligned
//! non-zero integer literal to a raw pointer produces a valid pointer, but a
//! pointer pointing into previously allocated memory that since got freed is
//! not valid. The recommended way to build a Box to a ZST if `Box::new` cannot
//! be used is to use [`ptr::NonNull::dangling`].
//!
//! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
//! as a single pointer and is also ABI-compatible with C pointers
//! (i.e. the C type `T*`). This means that if you have extern "C"
//! Rust functions that will be called from C, you can define those
//! Rust functions using `Box<T>` types, and use `T*` as corresponding
//! type on the C side. As an example, consider this C header which
//! declares functions that create and destroy some kind of `Foo`
//! value:
//!
//! ```c
//! /* C header */
//!
//! /* Returns ownership to the caller */
//! struct Foo* foo_new(void);
//!
//! /* Takes ownership from the caller; no-op when invoked with null */
//! void foo_delete(struct Foo*);
//! ```
//!
//! These two functions might be implemented in Rust as follows. Here, the
//! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
//! the ownership constraints. Note also that the nullable argument to
//! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
//! cannot be null.
//!
//! ```
//! #[repr(C)]
//! pub struct Foo;
//!
//! #[no_mangle]
//! pub extern "C-unwind" fn foo_new() -> Box<Foo> {
//!     Box::new(Foo)
//! }
//!
//! #[no_mangle]
//! pub extern "C-unwind" fn foo_delete(_: Option<Box<Foo>>) {}
//! ```
//!
//! Even though `Box<T>` has the same representation and C ABI as a C pointer,
//! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
//! and expect things to work. `Box<T>` values will always be fully aligned,
//! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
//! free the value with the global allocator. In general, the best practice
//! is to only use `Box<T>` for pointers that originated from the global
//! allocator.
//!
//! **Important.** At least at present, you should avoid using
//! `Box<T>` types for functions that are defined in C but invoked
//! from Rust. In those cases, you should directly mirror the C types
//! as closely as possible. Using types like `Box<T>` where the C
//! definition is just using `T*` can lead to undefined behavior, as
//! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
//!
//! # Considerations for unsafe code
//!
//! **Warning: This section is not normative and is subject to change, possibly
//! being relaxed in the future! It is a simplified summary of the rules
//! currently implemented in the compiler.**
//!
//! The aliasing rules for `Box<T>` are the same as for `&mut T`. `Box<T>`
//! asserts uniqueness over its content. Using raw pointers derived from a box
//! after that box has been mutated through, moved or borrowed as `&mut T`
//! is not allowed. For more guidance on working with box from unsafe code, see
//! [rust-lang/unsafe-code-guidelines#326][ucg#326].
//!
//!
//! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
//! [ucg#326]: https://github.com/rust-lang/unsafe-code-guidelines/issues/326
//! [dereferencing]: core::ops::Deref
//! [`Box::<T>::from_raw(value)`]: Box::from_raw
//! [`Global`]: crate::alloc::Global
//! [`Layout`]: crate::alloc::Layout
//! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
//! [valid]: ptr#safety

#![stable(feature = "rust1", since = "1.0.0")]

use core::any::Any;
use core::async_iter::AsyncIterator;
use core::borrow;
use core::cmp::Ordering;
use core::error::Error;
use core::fmt;
use core::future::Future;
use core::hash::{Hash, Hasher};
use core::iter::FusedIterator;
use core::marker::Tuple;
use core::marker::Unsize;
use core::mem::{self, SizedTypeProperties};
use core::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
use core::ops::{
    CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut, DerefPure, DispatchFromDyn, Receiver,
};
use core::pin::Pin;
use core::ptr::{self, addr_of_mut, NonNull, Unique};
use core::task::{Context, Poll};

#[cfg(not(no_global_oom_handling))]
use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
use crate::alloc::{AllocError, Allocator, Global, Layout};
#[cfg(not(no_global_oom_handling))]
use crate::borrow::Cow;
use crate::raw_vec::RawVec;
#[cfg(not(no_global_oom_handling))]
use crate::str::from_boxed_utf8_unchecked;
#[cfg(not(no_global_oom_handling))]
use crate::string::String;
#[cfg(not(no_global_oom_handling))]
use crate::vec::Vec;

#[unstable(feature = "thin_box", issue = "92791")]
pub use thin::ThinBox;

mod thin;

/// A pointer type that uniquely owns a heap allocation of type `T`.
///
/// See the [module-level documentation](../../std/boxed/index.html) for more.
#[lang = "owned_box"]
#[fundamental]
#[stable(feature = "rust1", since = "1.0.0")]
// The declaration of the `Box` struct must be kept in sync with the
// compiler or ICEs will happen.
pub struct Box<
    T: ?Sized,
    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
>(Unique<T>, A);

impl<T> Box<T> {
    /// Allocates memory on the heap and then places `x` into it.
    ///
    /// This doesn't actually allocate if `T` is zero-sized.
    ///
    /// # Examples
    ///
    /// ```
    /// let five = Box::new(5);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline(always)]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    #[rustc_diagnostic_item = "box_new"]
    pub fn new(x: T) -> Self {
        #[rustc_box]
        Box::new(x)
    }

    /// Constructs a new box with uninitialized contents.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let mut five = Box::<u32>::new_uninit();
    ///
    /// let five = unsafe {
    ///     // Deferred initialization:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5)
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    #[inline]
    pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
        Self::new_uninit_in(Global)
    }

    /// Constructs a new `Box` with uninitialized contents, with the memory
    /// being filled with `0` bytes.
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let zero = Box::<u32>::new_zeroed();
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0)
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
        Self::new_zeroed_in(Global)
    }

    /// Constructs a new `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
    /// `x` will be pinned in memory and unable to be moved.
    ///
    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
    /// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
    /// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
    /// construct a (pinned) `Box` in a different way than with [`Box::new`].
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "pin", since = "1.33.0")]
    #[must_use]
    #[inline(always)]
    pub fn pin(x: T) -> Pin<Box<T>> {
        Box::new(x).into()
    }

    /// Allocates memory on the heap then places `x` into it,
    /// returning an error if the allocation fails
    ///
    /// This doesn't actually allocate if `T` is zero-sized.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// let five = Box::try_new(5)?;
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn try_new(x: T) -> Result<Self, AllocError> {
        Self::try_new_in(x, Global)
    }

    /// Constructs a new box with uninitialized contents on the heap,
    /// returning an error if the allocation fails
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// let mut five = Box::<u32>::try_new_uninit()?;
    ///
    /// let five = unsafe {
    ///     // Deferred initialization:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
        Box::try_new_uninit_in(Global)
    }

    /// Constructs a new `Box` with uninitialized contents, with the memory
    /// being filled with `0` bytes on the heap
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// let zero = Box::<u32>::try_new_zeroed()?;
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
        Box::try_new_zeroed_in(Global)
    }
}

impl<T, A: Allocator> Box<T, A> {
    /// Allocates memory in the given allocator then places `x` into it.
    ///
    /// This doesn't actually allocate if `T` is zero-sized.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let five = Box::new_in(5, System);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[must_use]
    #[inline]
    pub fn new_in(x: T, alloc: A) -> Self
    where
        A: Allocator,
    {
        let mut boxed = Self::new_uninit_in(alloc);
        unsafe {
            boxed.as_mut_ptr().write(x);
            boxed.assume_init()
        }
    }

    /// Allocates memory in the given allocator then places `x` into it,
    /// returning an error if the allocation fails
    ///
    /// This doesn't actually allocate if `T` is zero-sized.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let five = Box::try_new_in(5, System)?;
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
    where
        A: Allocator,
    {
        let mut boxed = Self::try_new_uninit_in(alloc)?;
        unsafe {
            boxed.as_mut_ptr().write(x);
            Ok(boxed.assume_init())
        }
    }

    /// Constructs a new box with uninitialized contents in the provided allocator.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let mut five = Box::<u32, _>::new_uninit_in(System);
    ///
    /// let five = unsafe {
    ///     // Deferred initialization:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5)
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[cfg(not(no_global_oom_handling))]
    #[must_use]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
    where
        A: Allocator,
    {
        let layout = Layout::new::<mem::MaybeUninit<T>>();
        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
        // That would make code size bigger.
        match Box::try_new_uninit_in(alloc) {
            Ok(m) => m,
            Err(_) => handle_alloc_error(layout),
        }
    }

    /// Constructs a new box with uninitialized contents in the provided allocator,
    /// returning an error if the allocation fails
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
    ///
    /// let five = unsafe {
    ///     // Deferred initialization:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
    where
        A: Allocator,
    {
        let ptr = if T::IS_ZST {
            NonNull::dangling()
        } else {
            let layout = Layout::new::<mem::MaybeUninit<T>>();
            alloc.allocate(layout)?.cast()
        };
        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
    }

    /// Constructs a new `Box` with uninitialized contents, with the memory
    /// being filled with `0` bytes in the provided allocator.
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let zero = Box::<u32, _>::new_zeroed_in(System);
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0)
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[cfg(not(no_global_oom_handling))]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
    where
        A: Allocator,
    {
        let layout = Layout::new::<mem::MaybeUninit<T>>();
        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
        // That would make code size bigger.
        match Box::try_new_zeroed_in(alloc) {
            Ok(m) => m,
            Err(_) => handle_alloc_error(layout),
        }
    }

    /// Constructs a new `Box` with uninitialized contents, with the memory
    /// being filled with `0` bytes in the provided allocator,
    /// returning an error if the allocation fails,
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
    /// let zero = unsafe { zero.assume_init() };
    ///
    /// assert_eq!(*zero, 0);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
    where
        A: Allocator,
    {
        let ptr = if T::IS_ZST {
            NonNull::dangling()
        } else {
            let layout = Layout::new::<mem::MaybeUninit<T>>();
            alloc.allocate_zeroed(layout)?.cast()
        };
        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
    }

    /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
    /// `x` will be pinned in memory and unable to be moved.
    ///
    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
    /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
    /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
    /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[must_use]
    #[inline(always)]
    pub fn pin_in(x: T, alloc: A) -> Pin<Self>
    where
        A: 'static + Allocator,
    {
        Self::into_pin(Self::new_in(x, alloc))
    }

    /// Converts a `Box<T>` into a `Box<[T]>`
    ///
    /// This conversion does not allocate on the heap and happens in place.
    #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
    pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
        let (raw, alloc) = Box::into_raw_with_allocator(boxed);
        unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
    }

    /// Consumes the `Box`, returning the wrapped value.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(box_into_inner)]
    ///
    /// let c = Box::new(5);
    ///
    /// assert_eq!(Box::into_inner(c), 5);
    /// ```
    #[unstable(feature = "box_into_inner", issue = "80437")]
    #[inline]
    pub fn into_inner(boxed: Self) -> T {
        *boxed
    }
}

impl<T> Box<[T]> {
    /// Constructs a new boxed slice with uninitialized contents.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
    ///
    /// let values = unsafe {
    ///     // Deferred initialization:
    ///     values[0].as_mut_ptr().write(1);
    ///     values[1].as_mut_ptr().write(2);
    ///     values[2].as_mut_ptr().write(3);
    ///
    ///     values.assume_init()
    /// };
    ///
    /// assert_eq!(*values, [1, 2, 3])
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
        unsafe { RawVec::with_capacity(len).into_box(len) }
    }

    /// Constructs a new boxed slice with uninitialized contents, with the memory
    /// being filled with `0` bytes.
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let values = Box::<[u32]>::new_zeroed_slice(3);
    /// let values = unsafe { values.assume_init() };
    ///
    /// assert_eq!(*values, [0, 0, 0])
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
        unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
    }

    /// Constructs a new boxed slice with uninitialized contents. Returns an error if
    /// the allocation fails
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
    /// let values = unsafe {
    ///     // Deferred initialization:
    ///     values[0].as_mut_ptr().write(1);
    ///     values[1].as_mut_ptr().write(2);
    ///     values[2].as_mut_ptr().write(3);
    ///     values.assume_init()
    /// };
    ///
    /// assert_eq!(*values, [1, 2, 3]);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
        let ptr = if T::IS_ZST || len == 0 {
            NonNull::dangling()
        } else {
            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
                Ok(l) => l,
                Err(_) => return Err(AllocError),
            };
            Global.allocate(layout)?.cast()
        };
        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
    }

    /// Constructs a new boxed slice with uninitialized contents, with the memory
    /// being filled with `0` bytes. Returns an error if the allocation fails
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
    /// let values = unsafe { values.assume_init() };
    ///
    /// assert_eq!(*values, [0, 0, 0]);
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
        let ptr = if T::IS_ZST || len == 0 {
            NonNull::dangling()
        } else {
            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
                Ok(l) => l,
                Err(_) => return Err(AllocError),
            };
            Global.allocate_zeroed(layout)?.cast()
        };
        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
    }
}

impl<T, A: Allocator> Box<[T], A> {
    /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
    ///
    /// let values = unsafe {
    ///     // Deferred initialization:
    ///     values[0].as_mut_ptr().write(1);
    ///     values[1].as_mut_ptr().write(2);
    ///     values[2].as_mut_ptr().write(3);
    ///
    ///     values.assume_init()
    /// };
    ///
    /// assert_eq!(*values, [1, 2, 3])
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
        unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
    }

    /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
    /// with the memory being filled with `0` bytes.
    ///
    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
    /// of this method.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(allocator_api, new_uninit)]
    ///
    /// use std::alloc::System;
    ///
    /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
    /// let values = unsafe { values.assume_init() };
    ///
    /// assert_eq!(*values, [0, 0, 0])
    /// ```
    ///
    /// [zeroed]: mem::MaybeUninit::zeroed
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "allocator_api", issue = "32838")]
    // #[unstable(feature = "new_uninit", issue = "63291")]
    #[must_use]
    pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
        unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
    }
}

impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
    /// Converts to `Box<T, A>`.
    ///
    /// # Safety
    ///
    /// As with [`MaybeUninit::assume_init`],
    /// it is up to the caller to guarantee that the value
    /// really is in an initialized state.
    /// Calling this when the content is not yet fully initialized
    /// causes immediate undefined behavior.
    ///
    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let mut five = Box::<u32>::new_uninit();
    ///
    /// let five: Box<u32> = unsafe {
    ///     // Deferred initialization:
    ///     five.as_mut_ptr().write(5);
    ///
    ///     five.assume_init()
    /// };
    ///
    /// assert_eq!(*five, 5)
    /// ```
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub unsafe fn assume_init(self) -> Box<T, A> {
        let (raw, alloc) = Box::into_raw_with_allocator(self);
        unsafe { Box::from_raw_in(raw as *mut T, alloc) }
    }

    /// Writes the value and converts to `Box<T, A>`.
    ///
    /// This method converts the box similarly to [`Box::assume_init`] but
    /// writes `value` into it before conversion thus guaranteeing safety.
    /// In some scenarios use of this method may improve performance because
    /// the compiler may be able to optimize copying from stack.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let big_box = Box::<[usize; 1024]>::new_uninit();
    ///
    /// let mut array = [0; 1024];
    /// for (i, place) in array.iter_mut().enumerate() {
    ///     *place = i;
    /// }
    ///
    /// // The optimizer may be able to elide this copy, so previous code writes
    /// // to heap directly.
    /// let big_box = Box::write(big_box, array);
    ///
    /// for (i, x) in big_box.iter().enumerate() {
    ///     assert_eq!(*x, i);
    /// }
    /// ```
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
        unsafe {
            (*boxed).write(value);
            boxed.assume_init()
        }
    }
}

impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
    /// Converts to `Box<[T], A>`.
    ///
    /// # Safety
    ///
    /// As with [`MaybeUninit::assume_init`],
    /// it is up to the caller to guarantee that the values
    /// really are in an initialized state.
    /// Calling this when the content is not yet fully initialized
    /// causes immediate undefined behavior.
    ///
    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(new_uninit)]
    ///
    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
    ///
    /// let values = unsafe {
    ///     // Deferred initialization:
    ///     values[0].as_mut_ptr().write(1);
    ///     values[1].as_mut_ptr().write(2);
    ///     values[2].as_mut_ptr().write(3);
    ///
    ///     values.assume_init()
    /// };
    ///
    /// assert_eq!(*values, [1, 2, 3])
    /// ```
    #[unstable(feature = "new_uninit", issue = "63291")]
    #[inline]
    pub unsafe fn assume_init(self) -> Box<[T], A> {
        let (raw, alloc) = Box::into_raw_with_allocator(self);
        unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
    }
}

impl<T: ?Sized> Box<T> {
    /// Constructs a box from a raw pointer.
    ///
    /// After calling this function, the raw pointer is owned by the
    /// resulting `Box`. Specifically, the `Box` destructor will call
    /// the destructor of `T` and free the allocated memory. For this
    /// to be safe, the memory must have been allocated in accordance
    /// with the [memory layout] used by `Box` .
    ///
    /// # Safety
    ///
    /// This function is unsafe because improper use may lead to
    /// memory problems. For example, a double-free may occur if the
    /// function is called twice on the same raw pointer.
    ///
    /// The safety conditions are described in the [memory layout] section.
    ///
    /// # Examples
    ///
    /// Recreate a `Box` which was previously converted to a raw pointer
    /// using [`Box::into_raw`]:
    /// ```
    /// let x = Box::new(5);
    /// let ptr = Box::into_raw(x);
    /// let x = unsafe { Box::from_raw(ptr) };
    /// ```
    /// Manually create a `Box` from scratch by using the global allocator:
    /// ```
    /// use std::alloc::{alloc, Layout};
    ///
    /// unsafe {
    ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
    ///     // In general .write is required to avoid attempting to destruct
    ///     // the (uninitialized) previous contents of `ptr`, though for this
    ///     // simple example `*ptr = 5` would have worked as well.
    ///     ptr.write(5);
    ///     let x = Box::from_raw(ptr);
    /// }
    /// ```
    ///
    /// [memory layout]: self#memory-layout
    /// [`Layout`]: crate::Layout
    #[stable(feature = "box_raw", since = "1.4.0")]
    #[inline]
    #[must_use = "call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`"]
    pub unsafe fn from_raw(raw: *mut T) -> Self {
        unsafe { Self::from_raw_in(raw, Global) }
    }
}

impl<T: ?Sized, A: Allocator> Box<T, A> {
    /// Constructs a box from a raw pointer in the given allocator.
    ///
    /// After calling this function, the raw pointer is owned by the
    /// resulting `Box`. Specifically, the `Box` destructor will call
    /// the destructor of `T` and free the allocated memory. For this
    /// to be safe, the memory must have been allocated in accordance
    /// with the [memory layout] used by `Box` .
    ///
    /// # Safety
    ///
    /// This function is unsafe because improper use may lead to
    /// memory problems. For example, a double-free may occur if the
    /// function is called twice on the same raw pointer.
    ///
    ///
    /// # Examples
    ///
    /// Recreate a `Box` which was previously converted to a raw pointer
    /// using [`Box::into_raw_with_allocator`]:
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let x = Box::new_in(5, System);
    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
    /// ```
    /// Manually create a `Box` from scratch by using the system allocator:
    /// ```
    /// #![feature(allocator_api, slice_ptr_get)]
    ///
    /// use std::alloc::{Allocator, Layout, System};
    ///
    /// unsafe {
    ///     let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
    ///     // In general .write is required to avoid attempting to destruct
    ///     // the (uninitialized) previous contents of `ptr`, though for this
    ///     // simple example `*ptr = 5` would have worked as well.
    ///     ptr.write(5);
    ///     let x = Box::from_raw_in(ptr, System);
    /// }
    /// # Ok::<(), std::alloc::AllocError>(())
    /// ```
    ///
    /// [memory layout]: self#memory-layout
    /// [`Layout`]: crate::Layout
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
    #[inline]
    pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
        Box(unsafe { Unique::new_unchecked(raw) }, alloc)
    }

    /// Consumes the `Box`, returning a wrapped raw pointer.
    ///
    /// The pointer will be properly aligned and non-null.
    ///
    /// After calling this function, the caller is responsible for the
    /// memory previously managed by the `Box`. In particular, the
    /// caller should properly destroy `T` and release the memory, taking
    /// into account the [memory layout] used by `Box`. The easiest way to
    /// do this is to convert the raw pointer back into a `Box` with the
    /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
    /// the cleanup.
    ///
    /// Note: this is an associated function, which means that you have
    /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
    /// is so that there is no conflict with a method on the inner type.
    ///
    /// # Examples
    /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
    /// for automatic cleanup:
    /// ```
    /// let x = Box::new(String::from("Hello"));
    /// let ptr = Box::into_raw(x);
    /// let x = unsafe { Box::from_raw(ptr) };
    /// ```
    /// Manual cleanup by explicitly running the destructor and deallocating
    /// the memory:
    /// ```
    /// use std::alloc::{dealloc, Layout};
    /// use std::ptr;
    ///
    /// let x = Box::new(String::from("Hello"));
    /// let ptr = Box::into_raw(x);
    /// unsafe {
    ///     ptr::drop_in_place(ptr);
    ///     dealloc(ptr as *mut u8, Layout::new::<String>());
    /// }
    /// ```
    /// Note: This is equivalent to the following:
    /// ```
    /// let x = Box::new(String::from("Hello"));
    /// let ptr = Box::into_raw(x);
    /// unsafe {
    ///     drop(Box::from_raw(ptr));
    /// }
    /// ```
    ///
    /// [memory layout]: self#memory-layout
    #[stable(feature = "box_raw", since = "1.4.0")]
    #[inline]
    pub fn into_raw(b: Self) -> *mut T {
        // Make sure Miri realizes that we transition from a noalias pointer to a raw pointer here.
        unsafe { addr_of_mut!(*&mut *Self::into_raw_with_allocator(b).0) }
    }

    /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
    ///
    /// The pointer will be properly aligned and non-null.
    ///
    /// After calling this function, the caller is responsible for the
    /// memory previously managed by the `Box`. In particular, the
    /// caller should properly destroy `T` and release the memory, taking
    /// into account the [memory layout] used by `Box`. The easiest way to
    /// do this is to convert the raw pointer back into a `Box` with the
    /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
    /// the cleanup.
    ///
    /// Note: this is an associated function, which means that you have
    /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
    /// is so that there is no conflict with a method on the inner type.
    ///
    /// # Examples
    /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
    /// for automatic cleanup:
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::System;
    ///
    /// let x = Box::new_in(String::from("Hello"), System);
    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
    /// ```
    /// Manual cleanup by explicitly running the destructor and deallocating
    /// the memory:
    /// ```
    /// #![feature(allocator_api)]
    ///
    /// use std::alloc::{Allocator, Layout, System};
    /// use std::ptr::{self, NonNull};
    ///
    /// let x = Box::new_in(String::from("Hello"), System);
    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
    /// unsafe {
    ///     ptr::drop_in_place(ptr);
    ///     let non_null = NonNull::new_unchecked(ptr);
    ///     alloc.deallocate(non_null.cast(), Layout::new::<String>());
    /// }
    /// ```
    ///
    /// [memory layout]: self#memory-layout
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[inline]
    pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
        let mut b = mem::ManuallyDrop::new(b);
        // We carefully get the raw pointer out in a way that Miri's aliasing model understands what
        // is happening: using the primitive "deref" of `Box`. In case `A` is *not* `Global`, we
        // want *no* aliasing requirements here!
        // In case `A` *is* `Global`, this does not quite have the right behavior; `into_raw`
        // works around that.
        let ptr = addr_of_mut!(**b);
        let alloc = unsafe { ptr::read(&b.1) };
        (ptr, alloc)
    }

    #[unstable(
        feature = "ptr_internals",
        issue = "none",
        reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
    )]
    #[inline]
    #[doc(hidden)]
    pub fn into_unique(b: Self) -> (Unique<T>, A) {
        let (ptr, alloc) = Box::into_raw_with_allocator(b);
        unsafe { (Unique::from(&mut *ptr), alloc) }
    }

    /// Returns a reference to the underlying allocator.
    ///
    /// Note: this is an associated function, which means that you have
    /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
    /// is so that there is no conflict with a method on the inner type.
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
    #[inline]
    pub const fn allocator(b: &Self) -> &A {
        &b.1
    }

    /// Consumes and leaks the `Box`, returning a mutable reference,
    /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
    /// `'a`. If the type has only static references, or none at all, then this
    /// may be chosen to be `'static`.
    ///
    /// This function is mainly useful for data that lives for the remainder of
    /// the program's life. Dropping the returned reference will cause a memory
    /// leak. If this is not acceptable, the reference should first be wrapped
    /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
    /// then be dropped which will properly destroy `T` and release the
    /// allocated memory.
    ///
    /// Note: this is an associated function, which means that you have
    /// to call it as `Box::leak(b)` instead of `b.leak()`. This
    /// is so that there is no conflict with a method on the inner type.
    ///
    /// # Examples
    ///
    /// Simple usage:
    ///
    /// ```
    /// let x = Box::new(41);
    /// let static_ref: &'static mut usize = Box::leak(x);
    /// *static_ref += 1;
    /// assert_eq!(*static_ref, 42);
    /// ```
    ///
    /// Unsized data:
    ///
    /// ```
    /// let x = vec![1, 2, 3].into_boxed_slice();
    /// let static_ref = Box::leak(x);
    /// static_ref[0] = 4;
    /// assert_eq!(*static_ref, [4, 2, 3]);
    /// ```
    #[stable(feature = "box_leak", since = "1.26.0")]
    #[inline]
    pub fn leak<'a>(b: Self) -> &'a mut T
    where
        A: 'a,
    {
        unsafe { &mut *Box::into_raw(b) }
    }

    /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
    /// `*boxed` will be pinned in memory and unable to be moved.
    ///
    /// This conversion does not allocate on the heap and happens in place.
    ///
    /// This is also available via [`From`].
    ///
    /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
    /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
    /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
    /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
    ///
    /// # Notes
    ///
    /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
    /// as it'll introduce an ambiguity when calling `Pin::from`.
    /// A demonstration of such a poor impl is shown below.
    ///
    /// ```compile_fail
    /// # use std::pin::Pin;
    /// struct Foo; // A type defined in this crate.
    /// impl From<Box<()>> for Pin<Foo> {
    ///     fn from(_: Box<()>) -> Pin<Foo> {
    ///         Pin::new(Foo)
    ///     }
    /// }
    ///
    /// let foo = Box::new(());
    /// let bar = Pin::from(foo);
    /// ```
    #[stable(feature = "box_into_pin", since = "1.63.0")]
    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
    pub const fn into_pin(boxed: Self) -> Pin<Self>
    where
        A: 'static,
    {
        // It's not possible to move or replace the insides of a `Pin<Box<T>>`
        // when `T: !Unpin`, so it's safe to pin it directly without any
        // additional requirements.
        unsafe { Pin::new_unchecked(boxed) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
    #[inline]
    fn drop(&mut self) {
        // the T in the Box is dropped by the compiler before the destructor is run

        let ptr = self.0;

        unsafe {
            let layout = Layout::for_value_raw(ptr.as_ptr());
            if layout.size() != 0 {
                self.1.deallocate(From::from(ptr.cast()), layout);
            }
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Box<T> {
    /// Creates a `Box<T>`, with the `Default` value for T.
    #[inline]
    fn default() -> Self {
        Box::new(T::default())
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Default for Box<[T]> {
    #[inline]
    fn default() -> Self {
        let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
        Box(ptr, Global)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "default_box_extra", since = "1.17.0")]
impl Default for Box<str> {
    #[inline]
    fn default() -> Self {
        // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
        let ptr: Unique<str> = unsafe {
            let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
            Unique::new_unchecked(bytes.as_ptr() as *mut str)
        };
        Box(ptr, Global)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
    /// Returns a new box with a `clone()` of this box's contents.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Box::new(5);
    /// let y = x.clone();
    ///
    /// // The value is the same
    /// assert_eq!(x, y);
    ///
    /// // But they are unique objects
    /// assert_ne!(&*x as *const i32, &*y as *const i32);
    /// ```
    #[inline]
    fn clone(&self) -> Self {
        // Pre-allocate memory to allow writing the cloned value directly.
        let mut boxed = Self::new_uninit_in(self.1.clone());
        unsafe {
            (**self).write_clone_into_raw(boxed.as_mut_ptr());
            boxed.assume_init()
        }
    }

    /// Copies `source`'s contents into `self` without creating a new allocation.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Box::new(5);
    /// let mut y = Box::new(10);
    /// let yp: *const i32 = &*y;
    ///
    /// y.clone_from(&x);
    ///
    /// // The value is the same
    /// assert_eq!(x, y);
    ///
    /// // And no allocation occurred
    /// assert_eq!(yp, &*y);
    /// ```
    #[inline]
    fn clone_from(&mut self, source: &Self) {
        (**self).clone_from(&(**source));
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_slice_clone", since = "1.3.0")]
impl Clone for Box<str> {
    fn clone(&self) -> Self {
        // this makes a copy of the data
        let buf: Box<[u8]> = self.as_bytes().into();
        unsafe { from_boxed_utf8_unchecked(buf) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        PartialEq::eq(&**self, &**other)
    }
    #[inline]
    fn ne(&self, other: &Self) -> bool {
        PartialEq::ne(&**self, &**other)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
    #[inline]
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        PartialOrd::partial_cmp(&**self, &**other)
    }
    #[inline]
    fn lt(&self, other: &Self) -> bool {
        PartialOrd::lt(&**self, &**other)
    }
    #[inline]
    fn le(&self, other: &Self) -> bool {
        PartialOrd::le(&**self, &**other)
    }
    #[inline]
    fn ge(&self, other: &Self) -> bool {
        PartialOrd::ge(&**self, &**other)
    }
    #[inline]
    fn gt(&self, other: &Self) -> bool {
        PartialOrd::gt(&**self, &**other)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
    #[inline]
    fn cmp(&self, other: &Self) -> Ordering {
        Ord::cmp(&**self, &**other)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        (**self).hash(state);
    }
}

#[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
    fn finish(&self) -> u64 {
        (**self).finish()
    }
    fn write(&mut self, bytes: &[u8]) {
        (**self).write(bytes)
    }
    fn write_u8(&mut self, i: u8) {
        (**self).write_u8(i)
    }
    fn write_u16(&mut self, i: u16) {
        (**self).write_u16(i)
    }
    fn write_u32(&mut self, i: u32) {
        (**self).write_u32(i)
    }
    fn write_u64(&mut self, i: u64) {
        (**self).write_u64(i)
    }
    fn write_u128(&mut self, i: u128) {
        (**self).write_u128(i)
    }
    fn write_usize(&mut self, i: usize) {
        (**self).write_usize(i)
    }
    fn write_i8(&mut self, i: i8) {
        (**self).write_i8(i)
    }
    fn write_i16(&mut self, i: i16) {
        (**self).write_i16(i)
    }
    fn write_i32(&mut self, i: i32) {
        (**self).write_i32(i)
    }
    fn write_i64(&mut self, i: i64) {
        (**self).write_i64(i)
    }
    fn write_i128(&mut self, i: i128) {
        (**self).write_i128(i)
    }
    fn write_isize(&mut self, i: isize) {
        (**self).write_isize(i)
    }
    fn write_length_prefix(&mut self, len: usize) {
        (**self).write_length_prefix(len)
    }
    fn write_str(&mut self, s: &str) {
        (**self).write_str(s)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "from_for_ptrs", since = "1.6.0")]
impl<T> From<T> for Box<T> {
    /// Converts a `T` into a `Box<T>`
    ///
    /// The conversion allocates on the heap and moves `t`
    /// from the stack into it.
    ///
    /// # Examples
    ///
    /// ```rust
    /// let x = 5;
    /// let boxed = Box::new(5);
    ///
    /// assert_eq!(Box::from(x), boxed);
    /// ```
    fn from(t: T) -> Self {
        Box::new(t)
    }
}

#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
where
    A: 'static,
{
    /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
    /// `*boxed` will be pinned in memory and unable to be moved.
    ///
    /// This conversion does not allocate on the heap and happens in place.
    ///
    /// This is also available via [`Box::into_pin`].
    ///
    /// Constructing and pinning a `Box` with <code><Pin<Box\<T>>>::from([Box::new]\(x))</code>
    /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
    /// This `From` implementation is useful if you already have a `Box<T>`, or you are
    /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
    fn from(boxed: Box<T, A>) -> Self {
        Box::into_pin(boxed)
    }
}

/// Specialization trait used for `From<&[T]>`.
#[cfg(not(no_global_oom_handling))]
trait BoxFromSlice<T> {
    fn from_slice(slice: &[T]) -> Self;
}

#[cfg(not(no_global_oom_handling))]
impl<T: Clone> BoxFromSlice<T> for Box<[T]> {
    #[inline]
    default fn from_slice(slice: &[T]) -> Self {
        slice.to_vec().into_boxed_slice()
    }
}

#[cfg(not(no_global_oom_handling))]
impl<T: Copy> BoxFromSlice<T> for Box<[T]> {
    #[inline]
    fn from_slice(slice: &[T]) -> Self {
        let len = slice.len();
        let buf = RawVec::with_capacity(len);
        unsafe {
            ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
            buf.into_box(slice.len()).assume_init()
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_slice", since = "1.17.0")]
impl<T: Clone> From<&[T]> for Box<[T]> {
    /// Converts a `&[T]` into a `Box<[T]>`
    ///
    /// This conversion allocates on the heap
    /// and performs a copy of `slice` and its contents.
    ///
    /// # Examples
    /// ```rust
    /// // create a &[u8] which will be used to create a Box<[u8]>
    /// let slice: &[u8] = &[104, 101, 108, 108, 111];
    /// let boxed_slice: Box<[u8]> = Box::from(slice);
    ///
    /// println!("{boxed_slice:?}");
    /// ```
    #[inline]
    fn from(slice: &[T]) -> Box<[T]> {
        <Self as BoxFromSlice<T>>::from_slice(slice)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_cow", since = "1.45.0")]
impl<T: Clone> From<Cow<'_, [T]>> for Box<[T]> {
    /// Converts a `Cow<'_, [T]>` into a `Box<[T]>`
    ///
    /// When `cow` is the `Cow::Borrowed` variant, this
    /// conversion allocates on the heap and copies the
    /// underlying slice. Otherwise, it will try to reuse the owned
    /// `Vec`'s allocation.
    #[inline]
    fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
        match cow {
            Cow::Borrowed(slice) => Box::from(slice),
            Cow::Owned(slice) => Box::from(slice),
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_slice", since = "1.17.0")]
impl From<&str> for Box<str> {
    /// Converts a `&str` into a `Box<str>`
    ///
    /// This conversion allocates on the heap
    /// and performs a copy of `s`.
    ///
    /// # Examples
    ///
    /// ```rust
    /// let boxed: Box<str> = Box::from("hello");
    /// println!("{boxed}");
    /// ```
    #[inline]
    fn from(s: &str) -> Box<str> {
        unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_cow", since = "1.45.0")]
impl From<Cow<'_, str>> for Box<str> {
    /// Converts a `Cow<'_, str>` into a `Box<str>`
    ///
    /// When `cow` is the `Cow::Borrowed` variant, this
    /// conversion allocates on the heap and copies the
    /// underlying `str`. Otherwise, it will try to reuse the owned
    /// `String`'s allocation.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::borrow::Cow;
    ///
    /// let unboxed = Cow::Borrowed("hello");
    /// let boxed: Box<str> = Box::from(unboxed);
    /// println!("{boxed}");
    /// ```
    ///
    /// ```rust
    /// # use std::borrow::Cow;
    /// let unboxed = Cow::Owned("hello".to_string());
    /// let boxed: Box<str> = Box::from(unboxed);
    /// println!("{boxed}");
    /// ```
    #[inline]
    fn from(cow: Cow<'_, str>) -> Box<str> {
        match cow {
            Cow::Borrowed(s) => Box::from(s),
            Cow::Owned(s) => Box::from(s),
        }
    }
}

#[stable(feature = "boxed_str_conv", since = "1.19.0")]
impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
    /// Converts a `Box<str>` into a `Box<[u8]>`
    ///
    /// This conversion does not allocate on the heap and happens in place.
    ///
    /// # Examples
    /// ```rust
    /// // create a Box<str> which will be used to create a Box<[u8]>
    /// let boxed: Box<str> = Box::from("hello");
    /// let boxed_str: Box<[u8]> = Box::from(boxed);
    ///
    /// // create a &[u8] which will be used to create a Box<[u8]>
    /// let slice: &[u8] = &[104, 101, 108, 108, 111];
    /// let boxed_slice = Box::from(slice);
    ///
    /// assert_eq!(boxed_slice, boxed_str);
    /// ```
    #[inline]
    fn from(s: Box<str, A>) -> Self {
        let (raw, alloc) = Box::into_raw_with_allocator(s);
        unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_array", since = "1.45.0")]
impl<T, const N: usize> From<[T; N]> for Box<[T]> {
    /// Converts a `[T; N]` into a `Box<[T]>`
    ///
    /// This conversion moves the array to newly heap-allocated memory.
    ///
    /// # Examples
    ///
    /// ```rust
    /// let boxed: Box<[u8]> = Box::from([4, 2]);
    /// println!("{boxed:?}");
    /// ```
    fn from(array: [T; N]) -> Box<[T]> {
        Box::new(array)
    }
}

/// Casts a boxed slice to a boxed array.
///
/// # Safety
///
/// `boxed_slice.len()` must be exactly `N`.
unsafe fn boxed_slice_as_array_unchecked<T, A: Allocator, const N: usize>(
    boxed_slice: Box<[T], A>,
) -> Box<[T; N], A> {
    debug_assert_eq!(boxed_slice.len(), N);

    let (ptr, alloc) = Box::into_raw_with_allocator(boxed_slice);
    // SAFETY: Pointer and allocator came from an existing box,
    // and our safety condition requires that the length is exactly `N`
    unsafe { Box::from_raw_in(ptr as *mut [T; N], alloc) }
}

#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
    type Error = Box<[T]>;

    /// Attempts to convert a `Box<[T]>` into a `Box<[T; N]>`.
    ///
    /// The conversion occurs in-place and does not require a
    /// new memory allocation.
    ///
    /// # Errors
    ///
    /// Returns the old `Box<[T]>` in the `Err` variant if
    /// `boxed_slice.len()` does not equal `N`.
    fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
        if boxed_slice.len() == N {
            Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
        } else {
            Err(boxed_slice)
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "boxed_array_try_from_vec", since = "1.66.0")]
impl<T, const N: usize> TryFrom<Vec<T>> for Box<[T; N]> {
    type Error = Vec<T>;

    /// Attempts to convert a `Vec<T>` into a `Box<[T; N]>`.
    ///
    /// Like [`Vec::into_boxed_slice`], this is in-place if `vec.capacity() == N`,
    /// but will require a reallocation otherwise.
    ///
    /// # Errors
    ///
    /// Returns the original `Vec<T>` in the `Err` variant if
    /// `boxed_slice.len()` does not equal `N`.
    ///
    /// # Examples
    ///
    /// This can be used with [`vec!`] to create an array on the heap:
    ///
    /// ```
    /// let state: Box<[f32; 100]> = vec![1.0; 100].try_into().unwrap();
    /// assert_eq!(state.len(), 100);
    /// ```
    fn try_from(vec: Vec<T>) -> Result<Self, Self::Error> {
        if vec.len() == N {
            let boxed_slice = vec.into_boxed_slice();
            Ok(unsafe { boxed_slice_as_array_unchecked(boxed_slice) })
        } else {
            Err(vec)
        }
    }
}

impl<A: Allocator> Box<dyn Any, A> {
    /// Attempt to downcast the box to a concrete type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::any::Any;
    ///
    /// fn print_if_string(value: Box<dyn Any>) {
    ///     if let Ok(string) = value.downcast::<String>() {
    ///         println!("String ({}): {}", string.len(), string);
    ///     }
    /// }
    ///
    /// let my_string = "Hello World".to_string();
    /// print_if_string(Box::new(my_string));
    /// print_if_string(Box::new(0i8));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
    }

    /// Downcasts the box to a concrete type.
    ///
    /// For a safe alternative see [`downcast`].
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(downcast_unchecked)]
    ///
    /// use std::any::Any;
    ///
    /// let x: Box<dyn Any> = Box::new(1_usize);
    ///
    /// unsafe {
    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
    /// }
    /// ```
    ///
    /// # Safety
    ///
    /// The contained value must be of type `T`. Calling this method
    /// with the incorrect type is *undefined behavior*.
    ///
    /// [`downcast`]: Self::downcast
    #[inline]
    #[unstable(feature = "downcast_unchecked", issue = "90850")]
    pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
        debug_assert!(self.is::<T>());
        unsafe {
            let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
            Box::from_raw_in(raw as *mut T, alloc)
        }
    }
}

impl<A: Allocator> Box<dyn Any + Send, A> {
    /// Attempt to downcast the box to a concrete type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::any::Any;
    ///
    /// fn print_if_string(value: Box<dyn Any + Send>) {
    ///     if let Ok(string) = value.downcast::<String>() {
    ///         println!("String ({}): {}", string.len(), string);
    ///     }
    /// }
    ///
    /// let my_string = "Hello World".to_string();
    /// print_if_string(Box::new(my_string));
    /// print_if_string(Box::new(0i8));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
    }

    /// Downcasts the box to a concrete type.
    ///
    /// For a safe alternative see [`downcast`].
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(downcast_unchecked)]
    ///
    /// use std::any::Any;
    ///
    /// let x: Box<dyn Any + Send> = Box::new(1_usize);
    ///
    /// unsafe {
    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
    /// }
    /// ```
    ///
    /// # Safety
    ///
    /// The contained value must be of type `T`. Calling this method
    /// with the incorrect type is *undefined behavior*.
    ///
    /// [`downcast`]: Self::downcast
    #[inline]
    #[unstable(feature = "downcast_unchecked", issue = "90850")]
    pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
        debug_assert!(self.is::<T>());
        unsafe {
            let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
            Box::from_raw_in(raw as *mut T, alloc)
        }
    }
}

impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
    /// Attempt to downcast the box to a concrete type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::any::Any;
    ///
    /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
    ///     if let Ok(string) = value.downcast::<String>() {
    ///         println!("String ({}): {}", string.len(), string);
    ///     }
    /// }
    ///
    /// let my_string = "Hello World".to_string();
    /// print_if_string(Box::new(my_string));
    /// print_if_string(Box::new(0i8));
    /// ```
    #[inline]
    #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
    pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
        if self.is::<T>() { unsafe { Ok(self.downcast_unchecked::<T>()) } } else { Err(self) }
    }

    /// Downcasts the box to a concrete type.
    ///
    /// For a safe alternative see [`downcast`].
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(downcast_unchecked)]
    ///
    /// use std::any::Any;
    ///
    /// let x: Box<dyn Any + Send + Sync> = Box::new(1_usize);
    ///
    /// unsafe {
    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
    /// }
    /// ```
    ///
    /// # Safety
    ///
    /// The contained value must be of type `T`. Calling this method
    /// with the incorrect type is *undefined behavior*.
    ///
    /// [`downcast`]: Self::downcast
    #[inline]
    #[unstable(feature = "downcast_unchecked", issue = "90850")]
    pub unsafe fn downcast_unchecked<T: Any>(self) -> Box<T, A> {
        debug_assert!(self.is::<T>());
        unsafe {
            let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
                Box::into_raw_with_allocator(self);
            Box::from_raw_in(raw as *mut T, alloc)
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&**self, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // It's not possible to extract the inner Uniq directly from the Box,
        // instead we cast it to a *const which aliases the Unique
        let ptr: *const T = &**self;
        fmt::Pointer::fmt(&ptr, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
    type Target = T;

    fn deref(&self) -> &T {
        &**self
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
    fn deref_mut(&mut self) -> &mut T {
        &mut **self
    }
}

#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized, A: Allocator> DerefPure for Box<T, A> {}

#[unstable(feature = "receiver_trait", issue = "none")]
impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
    type Item = I::Item;
    fn next(&mut self) -> Option<I::Item> {
        (**self).next()
    }
    fn size_hint(&self) -> (usize, Option<usize>) {
        (**self).size_hint()
    }
    fn nth(&mut self, n: usize) -> Option<I::Item> {
        (**self).nth(n)
    }
    fn last(self) -> Option<I::Item> {
        BoxIter::last(self)
    }
}

trait BoxIter {
    type Item;
    fn last(self) -> Option<Self::Item>;
}

impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
    type Item = I::Item;
    default fn last(self) -> Option<I::Item> {
        #[inline]
        fn some<T>(_: Option<T>, x: T) -> Option<T> {
            Some(x)
        }

        self.fold(None, some)
    }
}

/// Specialization for sized `I`s that uses `I`s implementation of `last()`
/// instead of the default.
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
    fn last(self) -> Option<I::Item> {
        (*self).last()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
    fn next_back(&mut self) -> Option<I::Item> {
        (**self).next_back()
    }
    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
        (**self).nth_back(n)
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
    fn len(&self) -> usize {
        (**self).len()
    }
    fn is_empty(&self) -> bool {
        (**self).is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}

#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
    type Output = <F as FnOnce<Args>>::Output;

    extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
        <F as FnOnce<Args>>::call_once(*self, args)
    }
}

#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
        <F as FnMut<Args>>::call_mut(self, args)
    }
}

#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
    extern "rust-call" fn call(&self, args: Args) -> Self::Output {
        <F as Fn<Args>>::call(self, args)
    }
}

#[unstable(feature = "async_fn_traits", issue = "none")]
impl<Args: Tuple, F: AsyncFnOnce<Args> + ?Sized, A: Allocator> AsyncFnOnce<Args> for Box<F, A> {
    type Output = F::Output;
    type CallOnceFuture = F::CallOnceFuture;

    extern "rust-call" fn async_call_once(self, args: Args) -> Self::CallOnceFuture {
        F::async_call_once(*self, args)
    }
}

#[unstable(feature = "async_fn_traits", issue = "none")]
impl<Args: Tuple, F: AsyncFnMut<Args> + ?Sized, A: Allocator> AsyncFnMut<Args> for Box<F, A> {
    type CallRefFuture<'a> = F::CallRefFuture<'a> where Self: 'a;

    extern "rust-call" fn async_call_mut(&mut self, args: Args) -> Self::CallRefFuture<'_> {
        F::async_call_mut(self, args)
    }
}

#[unstable(feature = "async_fn_traits", issue = "none")]
impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box<F, A> {
    extern "rust-call" fn async_call(&self, args: Args) -> Self::CallRefFuture<'_> {
        F::async_call(self, args)
    }
}

#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}

// It is quite crucial that we only allow the `Global` allocator here.
// Handling arbitrary custom allocators (which can affect the `Box` layout heavily!)
// would need a lot of codegen and interpreter adjustments.
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
impl<I> FromIterator<I> for Box<[I]> {
    fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
        iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_slice_clone", since = "1.3.0")]
impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
    fn clone(&self) -> Self {
        let alloc = Box::allocator(self).clone();
        self.to_vec_in(alloc).into_boxed_slice()
    }

    /// Copies `source`'s contents into `self` without creating a new allocation,
    /// so long as the two are of the same length.
    ///
    /// # Examples
    ///
    /// ```
    /// let x = Box::new([5, 6, 7]);
    /// let mut y = Box::new([8, 9, 10]);
    /// let yp: *const [i32] = &*y;
    ///
    /// y.clone_from(&x);
    ///
    /// // The value is the same
    /// assert_eq!(x, y);
    ///
    /// // And no allocation occurred
    /// assert_eq!(yp, &*y);
    /// ```
    fn clone_from(&mut self, source: &Self) {
        if self.len() == source.len() {
            self.clone_from_slice(&source);
        } else {
            *self = source.clone();
        }
    }
}

#[stable(feature = "box_borrow", since = "1.1.0")]
impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
    fn borrow(&self) -> &T {
        &**self
    }
}

#[stable(feature = "box_borrow", since = "1.1.0")]
impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
    fn borrow_mut(&mut self) -> &mut T {
        &mut **self
    }
}

#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
    fn as_ref(&self) -> &T {
        &**self
    }
}

#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
    fn as_mut(&mut self) -> &mut T {
        &mut **self
    }
}

/* Nota bene
 *
 *  We could have chosen not to add this impl, and instead have written a
 *  function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
 *  because Box<T> implements Unpin even when T does not, as a result of
 *  this impl.
 *
 *  We chose this API instead of the alternative for a few reasons:
 *      - Logically, it is helpful to understand pinning in regard to the
 *        memory region being pointed to. For this reason none of the
 *        standard library pointer types support projecting through a pin
 *        (Box<T> is the only pointer type in std for which this would be
 *        safe.)
 *      - It is in practice very useful to have Box<T> be unconditionally
 *        Unpin because of trait objects, for which the structural auto
 *        trait functionality does not apply (e.g., Box<dyn Foo> would
 *        otherwise not be Unpin).
 *
 *  Another type with the same semantics as Box but only a conditional
 *  implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
 *  could have a method to project a Pin<T> from it.
 */
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> {}

#[unstable(feature = "coroutine_trait", issue = "43122")]
impl<G: ?Sized + Coroutine<R> + Unpin, R, A: Allocator> Coroutine<R> for Box<G, A> {
    type Yield = G::Yield;
    type Return = G::Return;

    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
        G::resume(Pin::new(&mut *self), arg)
    }
}

#[unstable(feature = "coroutine_trait", issue = "43122")]
impl<G: ?Sized + Coroutine<R>, R, A: Allocator> Coroutine<R> for Pin<Box<G, A>>
where
    A: 'static,
{
    type Yield = G::Yield;
    type Return = G::Return;

    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
        G::resume((*self).as_mut(), arg)
    }
}

#[stable(feature = "futures_api", since = "1.36.0")]
impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A> {
    type Output = F::Output;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        F::poll(Pin::new(&mut *self), cx)
    }
}

#[unstable(feature = "async_iterator", issue = "79024")]
impl<S: ?Sized + AsyncIterator + Unpin> AsyncIterator for Box<S> {
    type Item = S::Item;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        Pin::new(&mut **self).poll_next(cx)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (**self).size_hint()
    }
}

impl dyn Error {
    #[inline]
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[rustc_allow_incoherent_impl]
    /// Attempts to downcast the box to a concrete type.
    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error>> {
        if self.is::<T>() {
            unsafe {
                let raw: *mut dyn Error = Box::into_raw(self);
                Ok(Box::from_raw(raw as *mut T))
            }
        } else {
            Err(self)
        }
    }
}

impl dyn Error + Send {
    #[inline]
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[rustc_allow_incoherent_impl]
    /// Attempts to downcast the box to a concrete type.
    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<dyn Error + Send>> {
        let err: Box<dyn Error> = self;
        <dyn Error>::downcast(err).map_err(|s| unsafe {
            // Reapply the `Send` marker.
            Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send))
        })
    }
}

impl dyn Error + Send + Sync {
    #[inline]
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[rustc_allow_incoherent_impl]
    /// Attempts to downcast the box to a concrete type.
    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
        let err: Box<dyn Error> = self;
        <dyn Error>::downcast(err).map_err(|s| unsafe {
            // Reapply the `Send + Sync` marker.
            Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send + Sync))
        })
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, E: Error + 'a> From<E> for Box<dyn Error + 'a> {
    /// Converts a type of [`Error`] into a box of dyn [`Error`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::fmt;
    /// use std::mem;
    ///
    /// #[derive(Debug)]
    /// struct AnError;
    ///
    /// impl fmt::Display for AnError {
    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    ///         write!(f, "An error")
    ///     }
    /// }
    ///
    /// impl Error for AnError {}
    ///
    /// let an_error = AnError;
    /// assert!(0 == mem::size_of_val(&an_error));
    /// let a_boxed_error = Box::<dyn Error>::from(an_error);
    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
    /// ```
    fn from(err: E) -> Box<dyn Error + 'a> {
        Box::new(err)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<dyn Error + Send + Sync + 'a> {
    /// Converts a type of [`Error`] + [`Send`] + [`Sync`] into a box of
    /// dyn [`Error`] + [`Send`] + [`Sync`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::fmt;
    /// use std::mem;
    ///
    /// #[derive(Debug)]
    /// struct AnError;
    ///
    /// impl fmt::Display for AnError {
    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    ///         write!(f, "An error")
    ///     }
    /// }
    ///
    /// impl Error for AnError {}
    ///
    /// unsafe impl Send for AnError {}
    ///
    /// unsafe impl Sync for AnError {}
    ///
    /// let an_error = AnError;
    /// assert!(0 == mem::size_of_val(&an_error));
    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
    /// assert!(
    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
    /// ```
    fn from(err: E) -> Box<dyn Error + Send + Sync + 'a> {
        Box::new(err)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> From<String> for Box<dyn Error + Send + Sync + 'a> {
    /// Converts a [`String`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::mem;
    ///
    /// let a_string_error = "a string error".to_string();
    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_string_error);
    /// assert!(
    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
    /// ```
    #[inline]
    fn from(err: String) -> Box<dyn Error + Send + Sync + 'a> {
        struct StringError(String);

        impl Error for StringError {
            #[allow(deprecated)]
            fn description(&self) -> &str {
                &self.0
            }
        }

        impl fmt::Display for StringError {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                fmt::Display::fmt(&self.0, f)
            }
        }

        // Purposefully skip printing "StringError(..)"
        impl fmt::Debug for StringError {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                fmt::Debug::fmt(&self.0, f)
            }
        }

        Box::new(StringError(err))
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "string_box_error", since = "1.6.0")]
impl<'a> From<String> for Box<dyn Error + 'a> {
    /// Converts a [`String`] into a box of dyn [`Error`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::mem;
    ///
    /// let a_string_error = "a string error".to_string();
    /// let a_boxed_error = Box::<dyn Error>::from(a_string_error);
    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
    /// ```
    fn from(str_err: String) -> Box<dyn Error + 'a> {
        let err1: Box<dyn Error + Send + Sync> = From::from(str_err);
        let err2: Box<dyn Error> = err1;
        err2
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
    /// Converts a [`str`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
    ///
    /// [`str`]: prim@str
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::mem;
    ///
    /// let a_str_error = "a str error";
    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_str_error);
    /// assert!(
    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
    /// ```
    #[inline]
    fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
        From::from(String::from(err))
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "string_box_error", since = "1.6.0")]
impl<'a> From<&str> for Box<dyn Error + 'a> {
    /// Converts a [`str`] into a box of dyn [`Error`].
    ///
    /// [`str`]: prim@str
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::mem;
    ///
    /// let a_str_error = "a str error";
    /// let a_boxed_error = Box::<dyn Error>::from(a_str_error);
    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
    /// ```
    fn from(err: &str) -> Box<dyn Error + 'a> {
        From::from(String::from(err))
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_box_error", since = "1.22.0")]
impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a> {
    /// Converts a [`Cow`] into a box of dyn [`Error`] + [`Send`] + [`Sync`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::mem;
    /// use std::borrow::Cow;
    ///
    /// let a_cow_str_error = Cow::from("a str error");
    /// let a_boxed_error = Box::<dyn Error + Send + Sync>::from(a_cow_str_error);
    /// assert!(
    ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
    /// ```
    fn from(err: Cow<'b, str>) -> Box<dyn Error + Send + Sync + 'a> {
        From::from(String::from(err))
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_box_error", since = "1.22.0")]
impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + 'a> {
    /// Converts a [`Cow`] into a box of dyn [`Error`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::mem;
    /// use std::borrow::Cow;
    ///
    /// let a_cow_str_error = Cow::from("a str error");
    /// let a_boxed_error = Box::<dyn Error>::from(a_cow_str_error);
    /// assert!(mem::size_of::<Box<dyn Error>>() == mem::size_of_val(&a_boxed_error))
    /// ```
    fn from(err: Cow<'b, str>) -> Box<dyn Error + 'a> {
        From::from(String::from(err))
    }
}

#[stable(feature = "box_error", since = "1.8.0")]
impl<T: core::error::Error> core::error::Error for Box<T> {
    #[allow(deprecated, deprecated_in_future)]
    fn description(&self) -> &str {
        core::error::Error::description(&**self)
    }

    #[allow(deprecated)]
    fn cause(&self) -> Option<&dyn core::error::Error> {
        core::error::Error::cause(&**self)
    }

    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
        core::error::Error::source(&**self)
    }

    fn provide<'b>(&'b self, request: &mut core::error::Request<'b>) {
        core::error::Error::provide(&**self, request);
    }
}

//! A UTF-8–encoded, growable string.
//!
//! This module contains the [`String`] type, the [`ToString`] trait for
//! converting to strings, and several error types that may result from
//! working with [`String`]s.
//!
//! # Examples
//!
//! There are multiple ways to create a new [`String`] from a string literal:
//!
//! ```
//! let s = "Hello".to_string();
//!
//! let s = String::from("world");
//! let s: String = "also this".into();
//! ```
//!
//! You can create a new [`String`] from an existing one by concatenating with
//! `+`:
//!
//! ```
//! let s = "Hello".to_string();
//!
//! let message = s + " world!";
//! ```
//!
//! If you have a vector of valid UTF-8 bytes, you can make a [`String`] out of
//! it. You can do the reverse too.
//!
//! ```
//! let sparkle_heart = vec![240, 159, 146, 150];
//!
//! // We know these bytes are valid, so we'll use `unwrap()`.
//! let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
//!
//! assert_eq!("💖", sparkle_heart);
//!
//! let bytes = sparkle_heart.into_bytes();
//!
//! assert_eq!(bytes, [240, 159, 146, 150]);
//! ```

#![stable(feature = "rust1", since = "1.0.0")]

use core::hash;
#[cfg(not(no_global_oom_handling))]
use core::iter::from_fn;
#[cfg(not(no_global_oom_handling))]
use core::ops::Add;
#[cfg(not(no_global_oom_handling))]
use core::ops::AddAssign;
#[cfg(not(no_global_oom_handling))]
use core::ops::Bound::{Excluded, Included, Unbounded};
use core::ops::{self, Range, RangeBounds};
use core::ptr;
use core::slice;
use core::str::pattern::Pattern;

#[cfg(not(no_global_oom_handling))]
use crate::borrow::{Cow, ToOwned};
use crate::boxed::Box;
use crate::collections::TryReserveError;
use crate::str::{self, from_utf8_unchecked_mut, Chars, Utf8Error};
#[cfg(not(no_global_oom_handling))]
use crate::str::{from_boxed_utf8_unchecked, FromStr};

/// A UTF-8–encoded, growable string.
///
/// `String` is the most common string type. It has ownership over the contents
/// of the string, stored in a heap-allocated buffer (see [Representation](#representation)).
/// It is closely related to its borrowed counterpart, the primitive [`str`].
///
/// # Examples
///
/// You can create a `String` from [a literal string][`&str`] with [`String::from`]:
///
/// [`String::from`]: From::from
///
/// ```
/// let hello = String::from("Hello, world!");
/// ```
///
/// You can append a [`char`] to a `String` with the [`push`] method, and
/// append a [`&str`] with the [`push_str`] method:
///
/// ```
/// let mut hello = String::from("Hello, ");
///
/// hello.push('w');
/// hello.push_str("orld!");
/// ```
///
/// [`push`]: String::push
/// [`push_str`]: String::push_str
///
/// If you have a vector of UTF-8 bytes, you can create a `String` from it with
/// the [`from_utf8`] method:
///
/// ```
/// // some bytes, in a vector
/// let sparkle_heart = vec![240, 159, 146, 150];
///
/// // We know these bytes are valid, so we'll use `unwrap()`.
/// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
///
/// assert_eq!("💖", sparkle_heart);
/// ```
///
/// [`from_utf8`]: String::from_utf8
///
/// # UTF-8
///
/// `String`s are always valid UTF-8. If you need a non-UTF-8 string, consider
/// [`OsString`]. It is similar, but without the UTF-8 constraint. Because UTF-8
/// is a variable width encoding, `String`s are typically smaller than an array of
/// the same `chars`:
///
/// ```
/// use std::mem;
///
/// // `s` is ASCII which represents each `char` as one byte
/// let s = "hello";
/// assert_eq!(s.len(), 5);
///
/// // A `char` array with the same contents would be longer because
/// // every `char` is four bytes
/// let s = ['h', 'e', 'l', 'l', 'o'];
/// let size: usize = s.into_iter().map(|c| mem::size_of_val(&c)).sum();
/// assert_eq!(size, 20);
///
/// // However, for non-ASCII strings, the difference will be smaller
/// // and sometimes they are the same
/// let s = "💖💖💖💖💖";
/// assert_eq!(s.len(), 20);
///
/// let s = ['💖', '💖', '💖', '💖', '💖'];
/// let size: usize = s.into_iter().map(|c| mem::size_of_val(&c)).sum();
/// assert_eq!(size, 20);
/// ```
///
/// This raises interesting questions as to how `s[i]` should work.
/// What should `i` be here? Several options include byte indices and
/// `char` indices but, because of UTF-8 encoding, only byte indices
/// would provide constant time indexing. Getting the `i`th `char`, for
/// example, is available using [`chars`]:
///
/// ```
/// let s = "hello";
/// let third_character = s.chars().nth(2);
/// assert_eq!(third_character, Some('l'));
///
/// let s = "💖💖💖💖💖";
/// let third_character = s.chars().nth(2);
/// assert_eq!(third_character, Some('💖'));
/// ```
///
/// Next, what should `s[i]` return? Because indexing returns a reference
/// to underlying data it could be `&u8`, `&[u8]`, or something else similar.
/// Since we're only providing one index, `&u8` makes the most sense but that
/// might not be what the user expects and can be explicitly achieved with
/// [`as_bytes()`]:
///
/// ```
/// // The first byte is 104 - the byte value of `'h'`
/// let s = "hello";
/// assert_eq!(s.as_bytes()[0], 104);
/// // or
/// assert_eq!(s.as_bytes()[0], b'h');
///
/// // The first byte is 240 which isn't obviously useful
/// let s = "💖💖💖💖💖";
/// assert_eq!(s.as_bytes()[0], 240);
/// ```
///
/// Due to these ambiguities/restrictions, indexing with a `usize` is simply
/// forbidden:
///
/// ```compile_fail,E0277
/// let s = "hello";
///
/// // The following will not compile!
/// println!("The first letter of s is {}", s[0]);
/// ```
///
/// It is more clear, however, how `&s[i..j]` should work (that is,
/// indexing with a range). It should accept byte indices (to be constant-time)
/// and return a `&str` which is UTF-8 encoded. This is also called "string slicing".
/// Note this will panic if the byte indices provided are not character
/// boundaries - see [`is_char_boundary`] for more details. See the implementations
/// for [`SliceIndex<str>`] for more details on string slicing. For a non-panicking
/// version of string slicing, see [`get`].
///
/// [`OsString`]: ../../std/ffi/struct.OsString.html "ffi::OsString"
/// [`SliceIndex<str>`]: core::slice::SliceIndex
/// [`as_bytes()`]: str::as_bytes
/// [`get`]: str::get
/// [`is_char_boundary`]: str::is_char_boundary
///
/// The [`bytes`] and [`chars`] methods return iterators over the bytes and
/// codepoints of the string, respectively. To iterate over codepoints along
/// with byte indices, use [`char_indices`].
///
/// [`bytes`]: str::bytes
/// [`chars`]: str::chars
/// [`char_indices`]: str::char_indices
///
/// # Deref
///
/// `String` implements <code>[Deref]<Target = [str]></code>, and so inherits all of [`str`]'s
/// methods. In addition, this means that you can pass a `String` to a
/// function which takes a [`&str`] by using an ampersand (`&`):
///
/// ```
/// fn takes_str(s: &str) { }
///
/// let s = String::from("Hello");
///
/// takes_str(&s);
/// ```
///
/// This will create a [`&str`] from the `String` and pass it in. This
/// conversion is very inexpensive, and so generally, functions will accept
/// [`&str`]s as arguments unless they need a `String` for some specific
/// reason.
///
/// In certain cases Rust doesn't have enough information to make this
/// conversion, known as [`Deref`] coercion. In the following example a string
/// slice [`&'a str`][`&str`] implements the trait `TraitExample`, and the function
/// `example_func` takes anything that implements the trait. In this case Rust
/// would need to make two implicit conversions, which Rust doesn't have the
/// means to do. For that reason, the following example will not compile.
///
/// ```compile_fail,E0277
/// trait TraitExample {}
///
/// impl<'a> TraitExample for &'a str {}
///
/// fn example_func<A: TraitExample>(example_arg: A) {}
///
/// let example_string = String::from("example_string");
/// example_func(&example_string);
/// ```
///
/// There are two options that would work instead. The first would be to
/// change the line `example_func(&example_string);` to
/// `example_func(example_string.as_str());`, using the method [`as_str()`]
/// to explicitly extract the string slice containing the string. The second
/// way changes `example_func(&example_string);` to
/// `example_func(&*example_string);`. In this case we are dereferencing a
/// `String` to a [`str`], then referencing the [`str`] back to
/// [`&str`]. The second way is more idiomatic, however both work to do the
/// conversion explicitly rather than relying on the implicit conversion.
///
/// # Representation
///
/// A `String` is made up of three components: a pointer to some bytes, a
/// length, and a capacity. The pointer points to the internal buffer which `String`
/// uses to store its data. The length is the number of bytes currently stored
/// in the buffer, and the capacity is the size of the buffer in bytes. As such,
/// the length will always be less than or equal to the capacity.
///
/// This buffer is always stored on the heap.
///
/// You can look at these with the [`as_ptr`], [`len`], and [`capacity`]
/// methods:
///
/// ```
/// use std::mem;
///
/// let story = String::from("Once upon a time...");
///
// FIXME Update this when vec_into_raw_parts is stabilized
/// // Prevent automatically dropping the String's data
/// let mut story = mem::ManuallyDrop::new(story);
///
/// let ptr = story.as_mut_ptr();
/// let len = story.len();
/// let capacity = story.capacity();
///
/// // story has nineteen bytes
/// assert_eq!(19, len);
///
/// // We can re-build a String out of ptr, len, and capacity. This is all
/// // unsafe because we are responsible for making sure the components are
/// // valid:
/// let s = unsafe { String::from_raw_parts(ptr, len, capacity) } ;
///
/// assert_eq!(String::from("Once upon a time..."), s);
/// ```
///
/// [`as_ptr`]: str::as_ptr
/// [`len`]: String::len
/// [`capacity`]: String::capacity
///
/// If a `String` has enough capacity, adding elements to it will not
/// re-allocate. For example, consider this program:
///
/// ```
/// let mut s = String::new();
///
/// println!("{}", s.capacity());
///
/// for _ in 0..5 {
///     s.push_str("hello");
///     println!("{}", s.capacity());
/// }
/// ```
///
/// This will output the following:
///
/// ```text
/// 0
/// 8
/// 16
/// 16
/// 32
/// 32
/// ```
///
/// At first, we have no memory allocated at all, but as we append to the
/// string, it increases its capacity appropriately. If we instead use the
/// [`with_capacity`] method to allocate the correct capacity initially:
///
/// ```
/// let mut s = String::with_capacity(25);
///
/// println!("{}", s.capacity());
///
/// for _ in 0..5 {
///     s.push_str("hello");
///     println!("{}", s.capacity());
/// }
/// ```
///
/// [`with_capacity`]: String::with_capacity
///
/// We end up with a different output:
///
/// ```text
/// 25
/// 25
/// 25
/// 25
/// 25
/// 25
/// ```
///
/// Here, there's no need to allocate more memory inside the loop.
///
/// [str]: prim@str "str"
/// [`str`]: prim@str "str"
/// [`&str`]: prim@str "&str"
/// [Deref]: core::ops::Deref "ops::Deref"
/// [`Deref`]: core::ops::Deref "ops::Deref"
/// [`as_str()`]: String::as_str
#[derive(PartialEq, PartialOrd, Eq, Ord)]
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), lang = "String")]
pub struct String {
    vec: Vec<u8>,
}

/// A possible error value when converting a `String` from a UTF-8 byte vector.
///
/// This type is the error type for the [`from_utf8`] method on [`String`]. It
/// is designed in such a way to carefully avoid reallocations: the
/// [`into_bytes`] method will give back the byte vector that was used in the
/// conversion attempt.
///
/// [`from_utf8`]: String::from_utf8
/// [`into_bytes`]: FromUtf8Error::into_bytes
///
/// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
/// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
/// an analogue to `FromUtf8Error`, and you can get one from a `FromUtf8Error`
/// through the [`utf8_error`] method.
///
/// [`Utf8Error`]: str::Utf8Error "std::str::Utf8Error"
/// [`std::str`]: core::str "std::str"
/// [`&str`]: prim@str "&str"
/// [`utf8_error`]: FromUtf8Error::utf8_error
///
/// # Examples
///
/// ```
/// // some invalid bytes, in a vector
/// let bytes = vec![0, 159];
///
/// let value = String::from_utf8(bytes);
///
/// assert!(value.is_err());
/// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(no_global_oom_handling), derive(Clone))]
#[derive(Debug, PartialEq, Eq)]
pub struct FromUtf8Error {
    bytes: Vec<u8>,
    error: Utf8Error,
}

/// A possible error value when converting a `String` from a UTF-16 byte slice.
///
/// This type is the error type for the [`from_utf16`] method on [`String`].
///
/// [`from_utf16`]: String::from_utf16
///
/// # Examples
///
/// ```
/// // 𝄞mu<invalid>ic
/// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
///           0xD800, 0x0069, 0x0063];
///
/// assert!(String::from_utf16(v).is_err());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug)]
pub struct FromUtf16Error(());

impl String {
    /// Creates a new empty `String`.
    ///
    /// Given that the `String` is empty, this will not allocate any initial
    /// buffer. While that means that this initial operation is very
    /// inexpensive, it may cause excessive allocation later when you add
    /// data. If you have an idea of how much data the `String` will hold,
    /// consider the [`with_capacity`] method to prevent excessive
    /// re-allocation.
    ///
    /// [`with_capacity`]: String::with_capacity
    ///
    /// # Examples
    ///
    /// ```
    /// let s = String::new();
    /// ```
    #[inline]
    #[rustc_const_stable(feature = "const_string_new", since = "1.39.0")]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub const fn new() -> String {
        String { vec: Vec::new() }
    }

    /// Creates a new empty `String` with at least the specified capacity.
    ///
    /// `String`s have an internal buffer to hold their data. The capacity is
    /// the length of that buffer, and can be queried with the [`capacity`]
    /// method. This method creates an empty `String`, but one with an initial
    /// buffer that can hold at least `capacity` bytes. This is useful when you
    /// may be appending a bunch of data to the `String`, reducing the number of
    /// reallocations it needs to do.
    ///
    /// [`capacity`]: String::capacity
    ///
    /// If the given capacity is `0`, no allocation will occur, and this method
    /// is identical to the [`new`] method.
    ///
    /// [`new`]: String::new
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::with_capacity(10);
    ///
    /// // The String contains no chars, even though it has capacity for more
    /// assert_eq!(s.len(), 0);
    ///
    /// // These are all done without reallocating...
    /// let cap = s.capacity();
    /// for _ in 0..10 {
    ///     s.push('a');
    /// }
    ///
    /// assert_eq!(s.capacity(), cap);
    ///
    /// // ...but this may make the string reallocate
    /// s.push('a');
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    pub fn with_capacity(capacity: usize) -> String {
        String { vec: Vec::with_capacity(capacity) }
    }

    /// Creates a new empty `String` with at least the specified capacity.
    ///
    /// # Errors
    ///
    /// Returns [`Err`] if the capacity exceeds `isize::MAX` bytes,
    /// or if the memory allocator reports failure.
    ///
    #[inline]
    #[unstable(feature = "try_with_capacity", issue = "91913")]
    pub fn try_with_capacity(capacity: usize) -> Result<String, TryReserveError> {
        Ok(String { vec: Vec::try_with_capacity(capacity)? })
    }

    // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
    // required for this method definition, is not available. Since we don't
    // require this method for testing purposes, I'll just stub it
    // NB see the slice::hack module in slice.rs for more information
    #[inline]
    #[cfg(test)]
    pub fn from_str(_: &str) -> String {
        panic!("not available with cfg(test)");
    }

    /// Converts a vector of bytes to a `String`.
    ///
    /// A string ([`String`]) is made of bytes ([`u8`]), and a vector of bytes
    /// ([`Vec<u8>`]) is made of bytes, so this function converts between the
    /// two. Not all byte slices are valid `String`s, however: `String`
    /// requires that it is valid UTF-8. `from_utf8()` checks to ensure that
    /// the bytes are valid UTF-8, and then does the conversion.
    ///
    /// If you are sure that the byte slice is valid UTF-8, and you don't want
    /// to incur the overhead of the validity check, there is an unsafe version
    /// of this function, [`from_utf8_unchecked`], which has the same behavior
    /// but skips the check.
    ///
    /// This method will take care to not copy the vector, for efficiency's
    /// sake.
    ///
    /// If you need a [`&str`] instead of a `String`, consider
    /// [`str::from_utf8`].
    ///
    /// The inverse of this method is [`into_bytes`].
    ///
    /// # Errors
    ///
    /// Returns [`Err`] if the slice is not UTF-8 with a description as to why the
    /// provided bytes are not UTF-8. The vector you moved in is also included.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // some bytes, in a vector
    /// let sparkle_heart = vec![240, 159, 146, 150];
    ///
    /// // We know these bytes are valid, so we'll use `unwrap()`.
    /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
    ///
    /// assert_eq!("💖", sparkle_heart);
    /// ```
    ///
    /// Incorrect bytes:
    ///
    /// ```
    /// // some invalid bytes, in a vector
    /// let sparkle_heart = vec![0, 159, 146, 150];
    ///
    /// assert!(String::from_utf8(sparkle_heart).is_err());
    /// ```
    ///
    /// See the docs for [`FromUtf8Error`] for more details on what you can do
    /// with this error.
    ///
    /// [`from_utf8_unchecked`]: String::from_utf8_unchecked
    /// [`Vec<u8>`]: crate::vec::Vec "Vec"
    /// [`&str`]: prim@str "&str"
    /// [`into_bytes`]: String::into_bytes
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error> {
        match str::from_utf8(&vec) {
            Ok(..) => Ok(String { vec }),
            Err(e) => Err(FromUtf8Error { bytes: vec, error: e }),
        }
    }

    /// Converts a slice of bytes to a string, including invalid characters.
    ///
    /// Strings are made of bytes ([`u8`]), and a slice of bytes
    /// ([`&[u8]`][byteslice]) is made of bytes, so this function converts
    /// between the two. Not all byte slices are valid strings, however: strings
    /// are required to be valid UTF-8. During this conversion,
    /// `from_utf8_lossy()` will replace any invalid UTF-8 sequences with
    /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD], which looks like this: �
    ///
    /// [byteslice]: prim@slice
    /// [U+FFFD]: core::char::REPLACEMENT_CHARACTER
    ///
    /// If you are sure that the byte slice is valid UTF-8, and you don't want
    /// to incur the overhead of the conversion, there is an unsafe version
    /// of this function, [`from_utf8_unchecked`], which has the same behavior
    /// but skips the checks.
    ///
    /// [`from_utf8_unchecked`]: String::from_utf8_unchecked
    ///
    /// This function returns a [`Cow<'a, str>`]. If our byte slice is invalid
    /// UTF-8, then we need to insert the replacement characters, which will
    /// change the size of the string, and hence, require a `String`. But if
    /// it's already valid UTF-8, we don't need a new allocation. This return
    /// type allows us to handle both cases.
    ///
    /// [`Cow<'a, str>`]: crate::borrow::Cow "borrow::Cow"
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// // some bytes, in a vector
    /// let sparkle_heart = vec![240, 159, 146, 150];
    ///
    /// let sparkle_heart = String::from_utf8_lossy(&sparkle_heart);
    ///
    /// assert_eq!("💖", sparkle_heart);
    /// ```
    ///
    /// Incorrect bytes:
    ///
    /// ```
    /// // some invalid bytes
    /// let input = b"Hello \xF0\x90\x80World";
    /// let output = String::from_utf8_lossy(input);
    ///
    /// assert_eq!("Hello �World", output);
    /// ```
    #[must_use]
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str> {
        let mut iter = v.utf8_chunks();

        let first_valid = if let Some(chunk) = iter.next() {
            let valid = chunk.valid();
            if chunk.invalid().is_empty() {
                debug_assert_eq!(valid.len(), v.len());
                return Cow::Borrowed(valid);
            }
            valid
        } else {
            return Cow::Borrowed("");
        };

        const REPLACEMENT: &str = "\u{FFFD}";

        let mut res = String::with_capacity(v.len());
        res.push_str(first_valid);
        res.push_str(REPLACEMENT);

        for chunk in iter {
            res.push_str(chunk.valid());
            if !chunk.invalid().is_empty() {
                res.push_str(REPLACEMENT);
            }
        }

        Cow::Owned(res)
    }

    /// Decode a UTF-16–encoded vector `v` into a `String`, returning [`Err`]
    /// if `v` contains any invalid data.
    ///
    /// # Examples
    ///
    /// ```
    /// // 𝄞music
    /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
    ///           0x0073, 0x0069, 0x0063];
    /// assert_eq!(String::from("𝄞music"),
    ///            String::from_utf16(v).unwrap());
    ///
    /// // 𝄞mu<invalid>ic
    /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
    ///           0xD800, 0x0069, 0x0063];
    /// assert!(String::from_utf16(v).is_err());
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
        // This isn't done via collect::<Result<_, _>>() for performance reasons.
        // FIXME: the function can be simplified again when #48994 is closed.
        let mut ret = String::with_capacity(v.len());
        for c in char::decode_utf16(v.iter().cloned()) {
            if let Ok(c) = c {
                ret.push(c);
            } else {
                return Err(FromUtf16Error(()));
            }
        }
        Ok(ret)
    }

    /// Decode a UTF-16–encoded slice `v` into a `String`, replacing
    /// invalid data with [the replacement character (`U+FFFD`)][U+FFFD].
    ///
    /// Unlike [`from_utf8_lossy`] which returns a [`Cow<'a, str>`],
    /// `from_utf16_lossy` returns a `String` since the UTF-16 to UTF-8
    /// conversion requires a memory allocation.
    ///
    /// [`from_utf8_lossy`]: String::from_utf8_lossy
    /// [`Cow<'a, str>`]: crate::borrow::Cow "borrow::Cow"
    /// [U+FFFD]: core::char::REPLACEMENT_CHARACTER
    ///
    /// # Examples
    ///
    /// ```
    /// // 𝄞mus<invalid>ic<invalid>
    /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
    ///           0x0073, 0xDD1E, 0x0069, 0x0063,
    ///           0xD834];
    ///
    /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
    ///            String::from_utf16_lossy(v));
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[must_use]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn from_utf16_lossy(v: &[u16]) -> String {
        char::decode_utf16(v.iter().cloned())
            .map(|r| r.unwrap_or(char::REPLACEMENT_CHARACTER))
            .collect()
    }

    /// Decode a UTF-16LE–encoded vector `v` into a `String`, returning [`Err`]
    /// if `v` contains any invalid data.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(str_from_utf16_endian)]
    /// // 𝄞music
    /// let v = &[0x34, 0xD8, 0x1E, 0xDD, 0x6d, 0x00, 0x75, 0x00,
    ///           0x73, 0x00, 0x69, 0x00, 0x63, 0x00];
    /// assert_eq!(String::from("𝄞music"),
    ///            String::from_utf16le(v).unwrap());
    ///
    /// // 𝄞mu<invalid>ic
    /// let v = &[0x34, 0xD8, 0x1E, 0xDD, 0x6d, 0x00, 0x75, 0x00,
    ///           0x00, 0xD8, 0x69, 0x00, 0x63, 0x00];
    /// assert!(String::from_utf16le(v).is_err());
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "str_from_utf16_endian", issue = "116258")]
    pub fn from_utf16le(v: &[u8]) -> Result<String, FromUtf16Error> {
        if v.len() % 2 != 0 {
            return Err(FromUtf16Error(()));
        }
        match (cfg!(target_endian = "little"), unsafe { v.align_to::<u16>() }) {
            (true, ([], v, [])) => Self::from_utf16(v),
            _ => char::decode_utf16(v.array_chunks::<2>().copied().map(u16::from_le_bytes))
                .collect::<Result<_, _>>()
                .map_err(|_| FromUtf16Error(())),
        }
    }

    /// Decode a UTF-16LE–encoded slice `v` into a `String`, replacing
    /// invalid data with [the replacement character (`U+FFFD`)][U+FFFD].
    ///
    /// Unlike [`from_utf8_lossy`] which returns a [`Cow<'a, str>`],
    /// `from_utf16le_lossy` returns a `String` since the UTF-16 to UTF-8
    /// conversion requires a memory allocation.
    ///
    /// [`from_utf8_lossy`]: String::from_utf8_lossy
    /// [`Cow<'a, str>`]: crate::borrow::Cow "borrow::Cow"
    /// [U+FFFD]: core::char::REPLACEMENT_CHARACTER
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(str_from_utf16_endian)]
    /// // 𝄞mus<invalid>ic<invalid>
    /// let v = &[0x34, 0xD8, 0x1E, 0xDD, 0x6d, 0x00, 0x75, 0x00,
    ///           0x73, 0x00, 0x1E, 0xDD, 0x69, 0x00, 0x63, 0x00,
    ///           0x34, 0xD8];
    ///
    /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
    ///            String::from_utf16le_lossy(v));
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "str_from_utf16_endian", issue = "116258")]
    pub fn from_utf16le_lossy(v: &[u8]) -> String {
        match (cfg!(target_endian = "little"), unsafe { v.align_to::<u16>() }) {
            (true, ([], v, [])) => Self::from_utf16_lossy(v),
            (true, ([], v, [_remainder])) => Self::from_utf16_lossy(v) + "\u{FFFD}",
            _ => {
                let mut iter = v.array_chunks::<2>();
                let string = char::decode_utf16(iter.by_ref().copied().map(u16::from_le_bytes))
                    .map(|r| r.unwrap_or(char::REPLACEMENT_CHARACTER))
                    .collect();
                if iter.remainder().is_empty() { string } else { string + "\u{FFFD}" }
            }
        }
    }

    /// Decode a UTF-16BE–encoded vector `v` into a `String`, returning [`Err`]
    /// if `v` contains any invalid data.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(str_from_utf16_endian)]
    /// // 𝄞music
    /// let v = &[0xD8, 0x34, 0xDD, 0x1E, 0x00, 0x6d, 0x00, 0x75,
    ///           0x00, 0x73, 0x00, 0x69, 0x00, 0x63];
    /// assert_eq!(String::from("𝄞music"),
    ///            String::from_utf16be(v).unwrap());
    ///
    /// // 𝄞mu<invalid>ic
    /// let v = &[0xD8, 0x34, 0xDD, 0x1E, 0x00, 0x6d, 0x00, 0x75,
    ///           0xD8, 0x00, 0x00, 0x69, 0x00, 0x63];
    /// assert!(String::from_utf16be(v).is_err());
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "str_from_utf16_endian", issue = "116258")]
    pub fn from_utf16be(v: &[u8]) -> Result<String, FromUtf16Error> {
        if v.len() % 2 != 0 {
            return Err(FromUtf16Error(()));
        }
        match (cfg!(target_endian = "big"), unsafe { v.align_to::<u16>() }) {
            (true, ([], v, [])) => Self::from_utf16(v),
            _ => char::decode_utf16(v.array_chunks::<2>().copied().map(u16::from_be_bytes))
                .collect::<Result<_, _>>()
                .map_err(|_| FromUtf16Error(())),
        }
    }

    /// Decode a UTF-16BE–encoded slice `v` into a `String`, replacing
    /// invalid data with [the replacement character (`U+FFFD`)][U+FFFD].
    ///
    /// Unlike [`from_utf8_lossy`] which returns a [`Cow<'a, str>`],
    /// `from_utf16le_lossy` returns a `String` since the UTF-16 to UTF-8
    /// conversion requires a memory allocation.
    ///
    /// [`from_utf8_lossy`]: String::from_utf8_lossy
    /// [`Cow<'a, str>`]: crate::borrow::Cow "borrow::Cow"
    /// [U+FFFD]: core::char::REPLACEMENT_CHARACTER
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(str_from_utf16_endian)]
    /// // 𝄞mus<invalid>ic<invalid>
    /// let v = &[0xD8, 0x34, 0xDD, 0x1E, 0x00, 0x6d, 0x00, 0x75,
    ///           0x00, 0x73, 0xDD, 0x1E, 0x00, 0x69, 0x00, 0x63,
    ///           0xD8, 0x34];
    ///
    /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
    ///            String::from_utf16be_lossy(v));
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "str_from_utf16_endian", issue = "116258")]
    pub fn from_utf16be_lossy(v: &[u8]) -> String {
        match (cfg!(target_endian = "big"), unsafe { v.align_to::<u16>() }) {
            (true, ([], v, [])) => Self::from_utf16_lossy(v),
            (true, ([], v, [_remainder])) => Self::from_utf16_lossy(v) + "\u{FFFD}",
            _ => {
                let mut iter = v.array_chunks::<2>();
                let string = char::decode_utf16(iter.by_ref().copied().map(u16::from_be_bytes))
                    .map(|r| r.unwrap_or(char::REPLACEMENT_CHARACTER))
                    .collect();
                if iter.remainder().is_empty() { string } else { string + "\u{FFFD}" }
            }
        }
    }

    /// Decomposes a `String` into its raw components: `(pointer, length, capacity)`.
    ///
    /// Returns the raw pointer to the underlying data, the length of
    /// the string (in bytes), and the allocated capacity of the data
    /// (in bytes). These are the same arguments in the same order as
    /// the arguments to [`from_raw_parts`].
    ///
    /// After calling this function, the caller is responsible for the
    /// memory previously managed by the `String`. The only way to do
    /// this is to convert the raw pointer, length, and capacity back
    /// into a `String` with the [`from_raw_parts`] function, allowing
    /// the destructor to perform the cleanup.
    ///
    /// [`from_raw_parts`]: String::from_raw_parts
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(vec_into_raw_parts)]
    /// let s = String::from("hello");
    ///
    /// let (ptr, len, cap) = s.into_raw_parts();
    ///
    /// let rebuilt = unsafe { String::from_raw_parts(ptr, len, cap) };
    /// assert_eq!(rebuilt, "hello");
    /// ```
    #[must_use = "`self` will be dropped if the result is not used"]
    #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
    pub fn into_raw_parts(self) -> (*mut u8, usize, usize) {
        self.vec.into_raw_parts()
    }

    /// Creates a new `String` from a pointer, a length and a capacity.
    ///
    /// # Safety
    ///
    /// This is highly unsafe, due to the number of invariants that aren't
    /// checked:
    ///
    /// * The memory at `buf` needs to have been previously allocated by the
    ///   same allocator the standard library uses, with a required alignment of exactly 1.
    /// * `length` needs to be less than or equal to `capacity`.
    /// * `capacity` needs to be the correct value.
    /// * The first `length` bytes at `buf` need to be valid UTF-8.
    ///
    /// Violating these may cause problems like corrupting the allocator's
    /// internal data structures. For example, it is normally **not** safe to
    /// build a `String` from a pointer to a C `char` array containing UTF-8
    /// _unless_ you are certain that array was originally allocated by the
    /// Rust standard library's allocator.
    ///
    /// The ownership of `buf` is effectively transferred to the
    /// `String` which may then deallocate, reallocate or change the
    /// contents of memory pointed to by the pointer at will. Ensure
    /// that nothing else uses the pointer after calling this
    /// function.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// unsafe {
    ///     let s = String::from("hello");
    ///
    // FIXME Update this when vec_into_raw_parts is stabilized
    ///     // Prevent automatically dropping the String's data
    ///     let mut s = mem::ManuallyDrop::new(s);
    ///
    ///     let ptr = s.as_mut_ptr();
    ///     let len = s.len();
    ///     let capacity = s.capacity();
    ///
    ///     let s = String::from_raw_parts(ptr, len, capacity);
    ///
    ///     assert_eq!(String::from("hello"), s);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn from_raw_parts(buf: *mut u8, length: usize, capacity: usize) -> String {
        unsafe { String { vec: Vec::from_raw_parts(buf, length, capacity) } }
    }

    /// Converts a vector of bytes to a `String` without checking that the
    /// string contains valid UTF-8.
    ///
    /// See the safe version, [`from_utf8`], for more details.
    ///
    /// [`from_utf8`]: String::from_utf8
    ///
    /// # Safety
    ///
    /// This function is unsafe because it does not check that the bytes passed
    /// to it are valid UTF-8. If this constraint is violated, it may cause
    /// memory unsafety issues with future users of the `String`, as the rest of
    /// the standard library assumes that `String`s are valid UTF-8.
    ///
    /// # Examples
    ///
    /// ```
    /// // some bytes, in a vector
    /// let sparkle_heart = vec![240, 159, 146, 150];
    ///
    /// let sparkle_heart = unsafe {
    ///     String::from_utf8_unchecked(sparkle_heart)
    /// };
    ///
    /// assert_eq!("💖", sparkle_heart);
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
        String { vec: bytes }
    }

    /// Converts a `String` into a byte vector.
    ///
    /// This consumes the `String`, so we do not need to copy its contents.
    ///
    /// # Examples
    ///
    /// ```
    /// let s = String::from("hello");
    /// let bytes = s.into_bytes();
    ///
    /// assert_eq!(&[104, 101, 108, 108, 111][..], &bytes[..]);
    /// ```
    #[inline]
    #[must_use = "`self` will be dropped if the result is not used"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn into_bytes(self) -> Vec<u8> {
        self.vec
    }

    /// Extracts a string slice containing the entire `String`.
    ///
    /// # Examples
    ///
    /// ```
    /// let s = String::from("foo");
    ///
    /// assert_eq!("foo", s.as_str());
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "string_as_str", since = "1.7.0")]
    pub fn as_str(&self) -> &str {
        self
    }

    /// Converts a `String` into a mutable string slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("foobar");
    /// let s_mut_str = s.as_mut_str();
    ///
    /// s_mut_str.make_ascii_uppercase();
    ///
    /// assert_eq!("FOOBAR", s_mut_str);
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "string_as_str", since = "1.7.0")]
    pub fn as_mut_str(&mut self) -> &mut str {
        self
    }

    /// Appends a given string slice onto the end of this `String`.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("foo");
    ///
    /// s.push_str("bar");
    ///
    /// assert_eq!("foobar", s);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_confusables("append", "push")]
    pub fn push_str(&mut self, string: &str) {
        self.vec.extend_from_slice(string.as_bytes())
    }

    /// Copies elements from `src` range to the end of the string.
    ///
    /// # Panics
    ///
    /// Panics if the starting point or end point do not lie on a [`char`]
    /// boundary, or if they're out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(string_extend_from_within)]
    /// let mut string = String::from("abcde");
    ///
    /// string.extend_from_within(2..);
    /// assert_eq!(string, "abcdecde");
    ///
    /// string.extend_from_within(..2);
    /// assert_eq!(string, "abcdecdeab");
    ///
    /// string.extend_from_within(4..8);
    /// assert_eq!(string, "abcdecdeabecde");
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "string_extend_from_within", issue = "103806")]
    pub fn extend_from_within<R>(&mut self, src: R)
    where
        R: RangeBounds<usize>,
    {
        let src @ Range { start, end } = slice::range(src, ..self.len());

        assert!(self.is_char_boundary(start));
        assert!(self.is_char_boundary(end));

        self.vec.extend_from_within(src);
    }

    /// Returns this `String`'s capacity, in bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// let s = String::with_capacity(10);
    ///
    /// assert!(s.capacity() >= 10);
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn capacity(&self) -> usize {
        self.vec.capacity()
    }

    /// Reserves capacity for at least `additional` bytes more than the
    /// current length. The allocator may reserve more space to speculatively
    /// avoid frequent allocations. After calling `reserve`,
    /// capacity will be greater than or equal to `self.len() + additional`.
    /// Does nothing if capacity is already sufficient.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity overflows [`usize`].
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut s = String::new();
    ///
    /// s.reserve(10);
    ///
    /// assert!(s.capacity() >= 10);
    /// ```
    ///
    /// This might not actually increase the capacity:
    ///
    /// ```
    /// let mut s = String::with_capacity(10);
    /// s.push('a');
    /// s.push('b');
    ///
    /// // s now has a length of 2 and a capacity of at least 10
    /// let capacity = s.capacity();
    /// assert_eq!(2, s.len());
    /// assert!(capacity >= 10);
    ///
    /// // Since we already have at least an extra 8 capacity, calling this...
    /// s.reserve(8);
    ///
    /// // ... doesn't actually increase.
    /// assert_eq!(capacity, s.capacity());
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve(&mut self, additional: usize) {
        self.vec.reserve(additional)
    }

    /// Reserves the minimum capacity for at least `additional` bytes more than
    /// the current length. Unlike [`reserve`], this will not
    /// deliberately over-allocate to speculatively avoid frequent allocations.
    /// After calling `reserve_exact`, capacity will be greater than or equal to
    /// `self.len() + additional`. Does nothing if the capacity is already
    /// sufficient.
    ///
    /// [`reserve`]: String::reserve
    ///
    /// # Panics
    ///
    /// Panics if the new capacity overflows [`usize`].
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut s = String::new();
    ///
    /// s.reserve_exact(10);
    ///
    /// assert!(s.capacity() >= 10);
    /// ```
    ///
    /// This might not actually increase the capacity:
    ///
    /// ```
    /// let mut s = String::with_capacity(10);
    /// s.push('a');
    /// s.push('b');
    ///
    /// // s now has a length of 2 and a capacity of at least 10
    /// let capacity = s.capacity();
    /// assert_eq!(2, s.len());
    /// assert!(capacity >= 10);
    ///
    /// // Since we already have at least an extra 8 capacity, calling this...
    /// s.reserve_exact(8);
    ///
    /// // ... doesn't actually increase.
    /// assert_eq!(capacity, s.capacity());
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn reserve_exact(&mut self, additional: usize) {
        self.vec.reserve_exact(additional)
    }

    /// Tries to reserve capacity for at least `additional` bytes more than the
    /// current length. The allocator may reserve more space to speculatively
    /// avoid frequent allocations. After calling `try_reserve`, capacity will be
    /// greater than or equal to `self.len() + additional` if it returns
    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
    /// preserves the contents even if an error occurs.
    ///
    /// # Errors
    ///
    /// If the capacity overflows, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::TryReserveError;
    ///
    /// fn process_data(data: &str) -> Result<String, TryReserveError> {
    ///     let mut output = String::new();
    ///
    ///     // Pre-reserve the memory, exiting if we can't
    ///     output.try_reserve(data.len())?;
    ///
    ///     // Now we know this can't OOM in the middle of our complex work
    ///     output.push_str(data);
    ///
    ///     Ok(output)
    /// }
    /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
    /// ```
    #[stable(feature = "try_reserve", since = "1.57.0")]
    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.vec.try_reserve(additional)
    }

    /// Tries to reserve the minimum capacity for at least `additional` bytes
    /// more than the current length. Unlike [`try_reserve`], this will not
    /// deliberately over-allocate to speculatively avoid frequent allocations.
    /// After calling `try_reserve_exact`, capacity will be greater than or
    /// equal to `self.len() + additional` if it returns `Ok(())`.
    /// Does nothing if the capacity is already sufficient.
    ///
    /// Note that the allocator may give the collection more space than it
    /// requests. Therefore, capacity can not be relied upon to be precisely
    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
    ///
    /// [`try_reserve`]: String::try_reserve
    ///
    /// # Errors
    ///
    /// If the capacity overflows, or the allocator reports a failure, then an error
    /// is returned.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::TryReserveError;
    ///
    /// fn process_data(data: &str) -> Result<String, TryReserveError> {
    ///     let mut output = String::new();
    ///
    ///     // Pre-reserve the memory, exiting if we can't
    ///     output.try_reserve_exact(data.len())?;
    ///
    ///     // Now we know this can't OOM in the middle of our complex work
    ///     output.push_str(data);
    ///
    ///     Ok(output)
    /// }
    /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
    /// ```
    #[stable(feature = "try_reserve", since = "1.57.0")]
    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
        self.vec.try_reserve_exact(additional)
    }

    /// Shrinks the capacity of this `String` to match its length.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("foo");
    ///
    /// s.reserve(100);
    /// assert!(s.capacity() >= 100);
    ///
    /// s.shrink_to_fit();
    /// assert_eq!(3, s.capacity());
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn shrink_to_fit(&mut self) {
        self.vec.shrink_to_fit()
    }

    /// Shrinks the capacity of this `String` with a lower bound.
    ///
    /// The capacity will remain at least as large as both the length
    /// and the supplied value.
    ///
    /// If the current capacity is less than the lower limit, this is a no-op.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("foo");
    ///
    /// s.reserve(100);
    /// assert!(s.capacity() >= 100);
    ///
    /// s.shrink_to(10);
    /// assert!(s.capacity() >= 10);
    /// s.shrink_to(0);
    /// assert!(s.capacity() >= 3);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "shrink_to", since = "1.56.0")]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        self.vec.shrink_to(min_capacity)
    }

    /// Appends the given [`char`] to the end of this `String`.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("abc");
    ///
    /// s.push('1');
    /// s.push('2');
    /// s.push('3');
    ///
    /// assert_eq!("abc123", s);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn push(&mut self, ch: char) {
        match ch.len_utf8() {
            1 => self.vec.push(ch as u8),
            _ => self.vec.extend_from_slice(ch.encode_utf8(&mut [0; 4]).as_bytes()),
        }
    }

    /// Returns a byte slice of this `String`'s contents.
    ///
    /// The inverse of this method is [`from_utf8`].
    ///
    /// [`from_utf8`]: String::from_utf8
    ///
    /// # Examples
    ///
    /// ```
    /// let s = String::from("hello");
    ///
    /// assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn as_bytes(&self) -> &[u8] {
        &self.vec
    }

    /// Shortens this `String` to the specified length.
    ///
    /// If `new_len` is greater than or equal to the string's current length, this has no
    /// effect.
    ///
    /// Note that this method has no effect on the allocated capacity
    /// of the string
    ///
    /// # Panics
    ///
    /// Panics if `new_len` does not lie on a [`char`] boundary.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("hello");
    ///
    /// s.truncate(2);
    ///
    /// assert_eq!("he", s);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn truncate(&mut self, new_len: usize) {
        if new_len <= self.len() {
            assert!(self.is_char_boundary(new_len));
            self.vec.truncate(new_len)
        }
    }

    /// Removes the last character from the string buffer and returns it.
    ///
    /// Returns [`None`] if this `String` is empty.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("abč");
    ///
    /// assert_eq!(s.pop(), Some('č'));
    /// assert_eq!(s.pop(), Some('b'));
    /// assert_eq!(s.pop(), Some('a'));
    ///
    /// assert_eq!(s.pop(), None);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn pop(&mut self) -> Option<char> {
        let ch = self.chars().rev().next()?;
        let newlen = self.len() - ch.len_utf8();
        unsafe {
            self.vec.set_len(newlen);
        }
        Some(ch)
    }

    /// Removes a [`char`] from this `String` at a byte position and returns it.
    ///
    /// This is an *O*(*n*) operation, as it requires copying every element in the
    /// buffer.
    ///
    /// # Panics
    ///
    /// Panics if `idx` is larger than or equal to the `String`'s length,
    /// or if it does not lie on a [`char`] boundary.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("abç");
    ///
    /// assert_eq!(s.remove(0), 'a');
    /// assert_eq!(s.remove(1), 'ç');
    /// assert_eq!(s.remove(0), 'b');
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_confusables("delete", "take")]
    pub fn remove(&mut self, idx: usize) -> char {
        let ch = match self[idx..].chars().next() {
            Some(ch) => ch,
            None => panic!("cannot remove a char from the end of a string"),
        };

        let next = idx + ch.len_utf8();
        let len = self.len();
        unsafe {
            ptr::copy(self.vec.as_ptr().add(next), self.vec.as_mut_ptr().add(idx), len - next);
            self.vec.set_len(len - (next - idx));
        }
        ch
    }

    /// Remove all matches of pattern `pat` in the `String`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(string_remove_matches)]
    /// let mut s = String::from("Trees are not green, the sky is not blue.");
    /// s.remove_matches("not ");
    /// assert_eq!("Trees are green, the sky is blue.", s);
    /// ```
    ///
    /// Matches will be detected and removed iteratively, so in cases where
    /// patterns overlap, only the first pattern will be removed:
    ///
    /// ```
    /// #![feature(string_remove_matches)]
    /// let mut s = String::from("banana");
    /// s.remove_matches("ana");
    /// assert_eq!("bna", s);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[unstable(feature = "string_remove_matches", reason = "new API", issue = "72826")]
    pub fn remove_matches<'a, P>(&'a mut self, pat: P)
    where
        P: for<'x> Pattern<'x>,
    {
        use core::str::pattern::Searcher;

        let rejections = {
            let mut searcher = pat.into_searcher(self);
            // Per Searcher::next:
            //
            // A Match result needs to contain the whole matched pattern,
            // however Reject results may be split up into arbitrary many
            // adjacent fragments. Both ranges may have zero length.
            //
            // In practice the implementation of Searcher::next_match tends to
            // be more efficient, so we use it here and do some work to invert
            // matches into rejections since that's what we want to copy below.
            let mut front = 0;
            let rejections: Vec<_> = from_fn(|| {
                let (start, end) = searcher.next_match()?;
                let prev_front = front;
                front = end;
                Some((prev_front, start))
            })
            .collect();
            rejections.into_iter().chain(core::iter::once((front, self.len())))
        };

        let mut len = 0;
        let ptr = self.vec.as_mut_ptr();

        for (start, end) in rejections {
            let count = end - start;
            if start != len {
                // SAFETY: per Searcher::next:
                //
                // The stream of Match and Reject values up to a Done will
                // contain index ranges that are adjacent, non-overlapping,
                // covering the whole haystack, and laying on utf8
                // boundaries.
                unsafe {
                    ptr::copy(ptr.add(start), ptr.add(len), count);
                }
            }
            len += count;
        }

        unsafe {
            self.vec.set_len(len);
        }
    }

    /// Retains only the characters specified by the predicate.
    ///
    /// In other words, remove all characters `c` such that `f(c)` returns `false`.
    /// This method operates in place, visiting each character exactly once in the
    /// original order, and preserves the order of the retained characters.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("f_o_ob_ar");
    ///
    /// s.retain(|c| c != '_');
    ///
    /// assert_eq!(s, "foobar");
    /// ```
    ///
    /// Because the elements are visited exactly once in the original order,
    /// external state may be used to decide which elements to keep.
    ///
    /// ```
    /// let mut s = String::from("abcde");
    /// let keep = [false, true, true, false, true];
    /// let mut iter = keep.iter();
    /// s.retain(|_| *iter.next().unwrap());
    /// assert_eq!(s, "bce");
    /// ```
    #[inline]
    #[stable(feature = "string_retain", since = "1.26.0")]
    pub fn retain<F>(&mut self, mut f: F)
    where
        F: FnMut(char) -> bool,
    {
        struct SetLenOnDrop<'a> {
            s: &'a mut String,
            idx: usize,
            del_bytes: usize,
        }

        impl<'a> Drop for SetLenOnDrop<'a> {
            fn drop(&mut self) {
                let new_len = self.idx - self.del_bytes;
                debug_assert!(new_len <= self.s.len());
                unsafe { self.s.vec.set_len(new_len) };
            }
        }

        let len = self.len();
        let mut guard = SetLenOnDrop { s: self, idx: 0, del_bytes: 0 };

        while guard.idx < len {
            let ch =
                // SAFETY: `guard.idx` is positive-or-zero and less that len so the `get_unchecked`
                // is in bound. `self` is valid UTF-8 like string and the returned slice starts at
                // a unicode code point so the `Chars` always return one character.
                unsafe { guard.s.get_unchecked(guard.idx..len).chars().next().unwrap_unchecked() };
            let ch_len = ch.len_utf8();

            if !f(ch) {
                guard.del_bytes += ch_len;
            } else if guard.del_bytes > 0 {
                // SAFETY: `guard.idx` is in bound and `guard.del_bytes` represent the number of
                // bytes that are erased from the string so the resulting `guard.idx -
                // guard.del_bytes` always represent a valid unicode code point.
                //
                // `guard.del_bytes` >= `ch.len_utf8()`, so taking a slice with `ch.len_utf8()` len
                // is safe.
                ch.encode_utf8(unsafe {
                    crate::slice::from_raw_parts_mut(
                        guard.s.as_mut_ptr().add(guard.idx - guard.del_bytes),
                        ch.len_utf8(),
                    )
                });
            }

            // Point idx to the next char
            guard.idx += ch_len;
        }

        drop(guard);
    }

    /// Inserts a character into this `String` at a byte position.
    ///
    /// This is an *O*(*n*) operation as it requires copying every element in the
    /// buffer.
    ///
    /// # Panics
    ///
    /// Panics if `idx` is larger than the `String`'s length, or if it does not
    /// lie on a [`char`] boundary.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::with_capacity(3);
    ///
    /// s.insert(0, 'f');
    /// s.insert(1, 'o');
    /// s.insert(2, 'o');
    ///
    /// assert_eq!("foo", s);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_confusables("set")]
    pub fn insert(&mut self, idx: usize, ch: char) {
        assert!(self.is_char_boundary(idx));
        let mut bits = [0; 4];
        let bits = ch.encode_utf8(&mut bits).as_bytes();

        unsafe {
            self.insert_bytes(idx, bits);
        }
    }

    #[cfg(not(no_global_oom_handling))]
    unsafe fn insert_bytes(&mut self, idx: usize, bytes: &[u8]) {
        let len = self.len();
        let amt = bytes.len();
        self.vec.reserve(amt);

        unsafe {
            ptr::copy(self.vec.as_ptr().add(idx), self.vec.as_mut_ptr().add(idx + amt), len - idx);
            ptr::copy_nonoverlapping(bytes.as_ptr(), self.vec.as_mut_ptr().add(idx), amt);
            self.vec.set_len(len + amt);
        }
    }

    /// Inserts a string slice into this `String` at a byte position.
    ///
    /// This is an *O*(*n*) operation as it requires copying every element in the
    /// buffer.
    ///
    /// # Panics
    ///
    /// Panics if `idx` is larger than the `String`'s length, or if it does not
    /// lie on a [`char`] boundary.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("bar");
    ///
    /// s.insert_str(0, "foo");
    ///
    /// assert_eq!("foobar", s);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "insert_str", since = "1.16.0")]
    pub fn insert_str(&mut self, idx: usize, string: &str) {
        assert!(self.is_char_boundary(idx));

        unsafe {
            self.insert_bytes(idx, string.as_bytes());
        }
    }

    /// Returns a mutable reference to the contents of this `String`.
    ///
    /// # Safety
    ///
    /// This function is unsafe because the returned `&mut Vec` allows writing
    /// bytes which are not valid UTF-8. If this constraint is violated, using
    /// the original `String` after dropping the `&mut Vec` may violate memory
    /// safety, as the rest of the standard library assumes that `String`s are
    /// valid UTF-8.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("hello");
    ///
    /// unsafe {
    ///     let vec = s.as_mut_vec();
    ///     assert_eq!(&[104, 101, 108, 108, 111][..], &vec[..]);
    ///
    ///     vec.reverse();
    /// }
    /// assert_eq!(s, "olleh");
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
        &mut self.vec
    }

    /// Returns the length of this `String`, in bytes, not [`char`]s or
    /// graphemes. In other words, it might not be what a human considers the
    /// length of the string.
    ///
    /// # Examples
    ///
    /// ```
    /// let a = String::from("foo");
    /// assert_eq!(a.len(), 3);
    ///
    /// let fancy_f = String::from("ƒoo");
    /// assert_eq!(fancy_f.len(), 4);
    /// assert_eq!(fancy_f.chars().count(), 3);
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_confusables("length", "size")]
    pub fn len(&self) -> usize {
        self.vec.len()
    }

    /// Returns `true` if this `String` has a length of zero, and `false` otherwise.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut v = String::new();
    /// assert!(v.is_empty());
    ///
    /// v.push('a');
    /// assert!(!v.is_empty());
    /// ```
    #[inline]
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Splits the string into two at the given byte index.
    ///
    /// Returns a newly allocated `String`. `self` contains bytes `[0, at)`, and
    /// the returned `String` contains bytes `[at, len)`. `at` must be on the
    /// boundary of a UTF-8 code point.
    ///
    /// Note that the capacity of `self` does not change.
    ///
    /// # Panics
    ///
    /// Panics if `at` is not on a `UTF-8` code point boundary, or if it is beyond the last
    /// code point of the string.
    ///
    /// # Examples
    ///
    /// ```
    /// # fn main() {
    /// let mut hello = String::from("Hello, World!");
    /// let world = hello.split_off(7);
    /// assert_eq!(hello, "Hello, ");
    /// assert_eq!(world, "World!");
    /// # }
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    #[stable(feature = "string_split_off", since = "1.16.0")]
    #[must_use = "use `.truncate()` if you don't need the other half"]
    pub fn split_off(&mut self, at: usize) -> String {
        assert!(self.is_char_boundary(at));
        let other = self.vec.split_off(at);
        unsafe { String::from_utf8_unchecked(other) }
    }

    /// Truncates this `String`, removing all contents.
    ///
    /// While this means the `String` will have a length of zero, it does not
    /// touch its capacity.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("foo");
    ///
    /// s.clear();
    ///
    /// assert!(s.is_empty());
    /// assert_eq!(0, s.len());
    /// assert_eq!(3, s.capacity());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn clear(&mut self) {
        self.vec.clear()
    }

    /// Removes the specified range from the string in bulk, returning all
    /// removed characters as an iterator.
    ///
    /// The returned iterator keeps a mutable borrow on the string to optimize
    /// its implementation.
    ///
    /// # Panics
    ///
    /// Panics if the starting point or end point do not lie on a [`char`]
    /// boundary, or if they're out of bounds.
    ///
    /// # Leaking
    ///
    /// If the returned iterator goes out of scope without being dropped (due to
    /// [`core::mem::forget`], for example), the string may still contain a copy
    /// of any drained characters, or may have lost characters arbitrarily,
    /// including characters outside the range.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("α is alpha, β is beta");
    /// let beta_offset = s.find('β').unwrap_or(s.len());
    ///
    /// // Remove the range up until the β from the string
    /// let t: String = s.drain(..beta_offset).collect();
    /// assert_eq!(t, "α is alpha, ");
    /// assert_eq!(s, "β is beta");
    ///
    /// // A full range clears the string, like `clear()` does
    /// s.drain(..);
    /// assert_eq!(s, "");
    /// ```
    #[stable(feature = "drain", since = "1.6.0")]
    pub fn drain<R>(&mut self, range: R) -> Drain<'_>
    where
        R: RangeBounds<usize>,
    {
        // Memory safety
        //
        // The String version of Drain does not have the memory safety issues
        // of the vector version. The data is just plain bytes.
        // Because the range removal happens in Drop, if the Drain iterator is leaked,
        // the removal will not happen.
        let Range { start, end } = slice::range(range, ..self.len());
        assert!(self.is_char_boundary(start));
        assert!(self.is_char_boundary(end));

        // Take out two simultaneous borrows. The &mut String won't be accessed
        // until iteration is over, in Drop.
        let self_ptr = self as *mut _;
        // SAFETY: `slice::range` and `is_char_boundary` do the appropriate bounds checks.
        let chars_iter = unsafe { self.get_unchecked(start..end) }.chars();

        Drain { start, end, iter: chars_iter, string: self_ptr }
    }

    /// Removes the specified range in the string,
    /// and replaces it with the given string.
    /// The given string doesn't need to be the same length as the range.
    ///
    /// # Panics
    ///
    /// Panics if the starting point or end point do not lie on a [`char`]
    /// boundary, or if they're out of bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("α is alpha, β is beta");
    /// let beta_offset = s.find('β').unwrap_or(s.len());
    ///
    /// // Replace the range up until the β from the string
    /// s.replace_range(..beta_offset, "Α is capital alpha; ");
    /// assert_eq!(s, "Α is capital alpha; β is beta");
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "splice", since = "1.27.0")]
    pub fn replace_range<R>(&mut self, range: R, replace_with: &str)
    where
        R: RangeBounds<usize>,
    {
        // Memory safety
        //
        // Replace_range does not have the memory safety issues of a vector Splice.
        // of the vector version. The data is just plain bytes.

        // WARNING: Inlining this variable would be unsound (#81138)
        let start = range.start_bound();
        match start {
            Included(&n) => assert!(self.is_char_boundary(n)),
            Excluded(&n) => assert!(self.is_char_boundary(n + 1)),
            Unbounded => {}
        };
        // WARNING: Inlining this variable would be unsound (#81138)
        let end = range.end_bound();
        match end {
            Included(&n) => assert!(self.is_char_boundary(n + 1)),
            Excluded(&n) => assert!(self.is_char_boundary(n)),
            Unbounded => {}
        };

        // Using `range` again would be unsound (#81138)
        // We assume the bounds reported by `range` remain the same, but
        // an adversarial implementation could change between calls
        unsafe { self.as_mut_vec() }.splice((start, end), replace_with.bytes());
    }

    /// Converts this `String` into a <code>[Box]<[str]></code>.
    ///
    /// This will drop any excess capacity.
    ///
    /// [str]: prim@str "str"
    ///
    /// # Examples
    ///
    /// ```
    /// let s = String::from("hello");
    ///
    /// let b = s.into_boxed_str();
    /// ```
    #[cfg(not(no_global_oom_handling))]
    #[stable(feature = "box_str", since = "1.4.0")]
    #[must_use = "`self` will be dropped if the result is not used"]
    #[inline]
    pub fn into_boxed_str(self) -> Box<str> {
        let slice = self.vec.into_boxed_slice();
        unsafe { from_boxed_utf8_unchecked(slice) }
    }

    /// Consumes and leaks the `String`, returning a mutable reference to the contents,
    /// `&'a mut str`.
    ///
    /// The caller has free choice over the returned lifetime, including `'static`. Indeed,
    /// this function is ideally used for data that lives for the remainder of the program's life,
    /// as dropping the returned reference will cause a memory leak.
    ///
    /// It does not reallocate or shrink the `String`,
    /// so the leaked allocation may include unused capacity that is not part
    /// of the returned slice. If you don't want that, call [`into_boxed_str`],
    /// and then [`Box::leak`].
    ///
    /// [`into_boxed_str`]: Self::into_boxed_str
    ///
    /// # Examples
    ///
    /// ```
    /// let x = String::from("bucket");
    /// let static_ref: &'static mut str = x.leak();
    /// assert_eq!(static_ref, "bucket");
    /// ```
    #[stable(feature = "string_leak", since = "1.72.0")]
    #[inline]
    pub fn leak<'a>(self) -> &'a mut str {
        let slice = self.vec.leak();
        unsafe { from_utf8_unchecked_mut(slice) }
    }
}

impl FromUtf8Error {
    /// Returns a slice of [`u8`]s bytes that were attempted to convert to a `String`.
    ///
    /// # Examples
    ///
    /// ```
    /// // some invalid bytes, in a vector
    /// let bytes = vec![0, 159];
    ///
    /// let value = String::from_utf8(bytes);
    ///
    /// assert_eq!(&[0, 159], value.unwrap_err().as_bytes());
    /// ```
    #[must_use]
    #[stable(feature = "from_utf8_error_as_bytes", since = "1.26.0")]
    pub fn as_bytes(&self) -> &[u8] {
        &self.bytes[..]
    }

    /// Returns the bytes that were attempted to convert to a `String`.
    ///
    /// This method is carefully constructed to avoid allocation. It will
    /// consume the error, moving out the bytes, so that a copy of the bytes
    /// does not need to be made.
    ///
    /// # Examples
    ///
    /// ```
    /// // some invalid bytes, in a vector
    /// let bytes = vec![0, 159];
    ///
    /// let value = String::from_utf8(bytes);
    ///
    /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
    /// ```
    #[must_use = "`self` will be dropped if the result is not used"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn into_bytes(self) -> Vec<u8> {
        self.bytes
    }

    /// Fetch a `Utf8Error` to get more details about the conversion failure.
    ///
    /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
    /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
    /// an analogue to `FromUtf8Error`. See its documentation for more details
    /// on using it.
    ///
    /// [`std::str`]: core::str "std::str"
    /// [`&str`]: prim@str "&str"
    ///
    /// # Examples
    ///
    /// ```
    /// // some invalid bytes, in a vector
    /// let bytes = vec![0, 159];
    ///
    /// let error = String::from_utf8(bytes).unwrap_err().utf8_error();
    ///
    /// // the first byte is invalid here
    /// assert_eq!(1, error.valid_up_to());
    /// ```
    #[must_use]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn utf8_error(&self) -> Utf8Error {
        self.error
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for FromUtf8Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.error, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for FromUtf16Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt("invalid utf-16: lone surrogate found", f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Error for FromUtf8Error {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        "invalid utf-8"
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Error for FromUtf16Error {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        "invalid utf-16"
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl Clone for String {
    fn clone(&self) -> Self {
        String { vec: self.vec.clone() }
    }

    /// Clones the contents of `source` into `self`.
    ///
    /// This method is preferred over simply assigning `source.clone()` to `self`,
    /// as it avoids reallocation if possible.
    fn clone_from(&mut self, source: &Self) {
        self.vec.clone_from(&source.vec);
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl FromIterator<char> for String {
    fn from_iter<I: IntoIterator<Item = char>>(iter: I) -> String {
        let mut buf = String::new();
        buf.extend(iter);
        buf
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "string_from_iter_by_ref", since = "1.17.0")]
impl<'a> FromIterator<&'a char> for String {
    fn from_iter<I: IntoIterator<Item = &'a char>>(iter: I) -> String {
        let mut buf = String::new();
        buf.extend(iter);
        buf
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> FromIterator<&'a str> for String {
    fn from_iter<I: IntoIterator<Item = &'a str>>(iter: I) -> String {
        let mut buf = String::new();
        buf.extend(iter);
        buf
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "extend_string", since = "1.4.0")]
impl FromIterator<String> for String {
    fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
        let mut iterator = iter.into_iter();

        // Because we're iterating over `String`s, we can avoid at least
        // one allocation by getting the first string from the iterator
        // and appending to it all the subsequent strings.
        match iterator.next() {
            None => String::new(),
            Some(mut buf) => {
                buf.extend(iterator);
                buf
            }
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_str2", since = "1.45.0")]
impl FromIterator<Box<str>> for String {
    fn from_iter<I: IntoIterator<Item = Box<str>>>(iter: I) -> String {
        let mut buf = String::new();
        buf.extend(iter);
        buf
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "herd_cows", since = "1.19.0")]
impl<'a> FromIterator<Cow<'a, str>> for String {
    fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
        let mut iterator = iter.into_iter();

        // Because we're iterating over CoWs, we can (potentially) avoid at least
        // one allocation by getting the first item and appending to it all the
        // subsequent items.
        match iterator.next() {
            None => String::new(),
            Some(cow) => {
                let mut buf = cow.into_owned();
                buf.extend(iterator);
                buf
            }
        }
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl Extend<char> for String {
    fn extend<I: IntoIterator<Item = char>>(&mut self, iter: I) {
        let iterator = iter.into_iter();
        let (lower_bound, _) = iterator.size_hint();
        self.reserve(lower_bound);
        iterator.for_each(move |c| self.push(c));
    }

    #[inline]
    fn extend_one(&mut self, c: char) {
        self.push(c);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.reserve(additional);
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "extend_ref", since = "1.2.0")]
impl<'a> Extend<&'a char> for String {
    fn extend<I: IntoIterator<Item = &'a char>>(&mut self, iter: I) {
        self.extend(iter.into_iter().cloned());
    }

    #[inline]
    fn extend_one(&mut self, &c: &'a char) {
        self.push(c);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.reserve(additional);
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> Extend<&'a str> for String {
    fn extend<I: IntoIterator<Item = &'a str>>(&mut self, iter: I) {
        iter.into_iter().for_each(move |s| self.push_str(s));
    }

    #[inline]
    fn extend_one(&mut self, s: &'a str) {
        self.push_str(s);
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_str2", since = "1.45.0")]
impl Extend<Box<str>> for String {
    fn extend<I: IntoIterator<Item = Box<str>>>(&mut self, iter: I) {
        iter.into_iter().for_each(move |s| self.push_str(&s));
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "extend_string", since = "1.4.0")]
impl Extend<String> for String {
    fn extend<I: IntoIterator<Item = String>>(&mut self, iter: I) {
        iter.into_iter().for_each(move |s| self.push_str(&s));
    }

    #[inline]
    fn extend_one(&mut self, s: String) {
        self.push_str(&s);
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "herd_cows", since = "1.19.0")]
impl<'a> Extend<Cow<'a, str>> for String {
    fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I) {
        iter.into_iter().for_each(move |s| self.push_str(&s));
    }

    #[inline]
    fn extend_one(&mut self, s: Cow<'a, str>) {
        self.push_str(&s);
    }
}

/// A convenience impl that delegates to the impl for `&str`.
///
/// # Examples
///
/// ```
/// assert_eq!(String::from("Hello world").find("world"), Some(6));
/// ```
#[unstable(
    feature = "pattern",
    reason = "API not fully fleshed out and ready to be stabilized",
    issue = "27721"
)]
impl<'a, 'b> Pattern<'a> for &'b String {
    type Searcher = <&'b str as Pattern<'a>>::Searcher;

    fn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher {
        self[..].into_searcher(haystack)
    }

    #[inline]
    fn is_contained_in(self, haystack: &'a str) -> bool {
        self[..].is_contained_in(haystack)
    }

    #[inline]
    fn is_prefix_of(self, haystack: &'a str) -> bool {
        self[..].is_prefix_of(haystack)
    }

    #[inline]
    fn strip_prefix_of(self, haystack: &'a str) -> Option<&'a str> {
        self[..].strip_prefix_of(haystack)
    }

    #[inline]
    fn is_suffix_of(self, haystack: &'a str) -> bool {
        self[..].is_suffix_of(haystack)
    }

    #[inline]
    fn strip_suffix_of(self, haystack: &'a str) -> Option<&'a str> {
        self[..].strip_suffix_of(haystack)
    }
}

macro_rules! impl_eq {
    ($lhs:ty, $rhs: ty) => {
        #[stable(feature = "rust1", since = "1.0.0")]
        #[allow(unused_lifetimes)]
        impl<'a, 'b> PartialEq<$rhs> for $lhs {
            #[inline]
            fn eq(&self, other: &$rhs) -> bool {
                PartialEq::eq(&self[..], &other[..])
            }
            #[inline]
            fn ne(&self, other: &$rhs) -> bool {
                PartialEq::ne(&self[..], &other[..])
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        #[allow(unused_lifetimes)]
        impl<'a, 'b> PartialEq<$lhs> for $rhs {
            #[inline]
            fn eq(&self, other: &$lhs) -> bool {
                PartialEq::eq(&self[..], &other[..])
            }
            #[inline]
            fn ne(&self, other: &$lhs) -> bool {
                PartialEq::ne(&self[..], &other[..])
            }
        }
    };
}

impl_eq! { String, str }
impl_eq! { String, &'a str }
#[cfg(not(no_global_oom_handling))]
impl_eq! { Cow<'a, str>, str }
#[cfg(not(no_global_oom_handling))]
impl_eq! { Cow<'a, str>, &'b str }
#[cfg(not(no_global_oom_handling))]
impl_eq! { Cow<'a, str>, String }

#[stable(feature = "rust1", since = "1.0.0")]
impl Default for String {
    /// Creates an empty `String`.
    #[inline]
    fn default() -> String {
        String::new()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for String {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&**self, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Debug for String {
    #[inline]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl hash::Hash for String {
    #[inline]
    fn hash<H: hash::Hasher>(&self, hasher: &mut H) {
        (**self).hash(hasher)
    }
}

/// Implements the `+` operator for concatenating two strings.
///
/// This consumes the `String` on the left-hand side and re-uses its buffer (growing it if
/// necessary). This is done to avoid allocating a new `String` and copying the entire contents on
/// every operation, which would lead to *O*(*n*^2) running time when building an *n*-byte string by
/// repeated concatenation.
///
/// The string on the right-hand side is only borrowed; its contents are copied into the returned
/// `String`.
///
/// # Examples
///
/// Concatenating two `String`s takes the first by value and borrows the second:
///
/// ```
/// let a = String::from("hello");
/// let b = String::from(" world");
/// let c = a + &b;
/// // `a` is moved and can no longer be used here.
/// ```
///
/// If you want to keep using the first `String`, you can clone it and append to the clone instead:
///
/// ```
/// let a = String::from("hello");
/// let b = String::from(" world");
/// let c = a.clone() + &b;
/// // `a` is still valid here.
/// ```
///
/// Concatenating `&str` slices can be done by converting the first to a `String`:
///
/// ```
/// let a = "hello";
/// let b = " world";
/// let c = a.to_string() + b;
/// ```
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl Add<&str> for String {
    type Output = String;

    #[inline]
    fn add(mut self, other: &str) -> String {
        self.push_str(other);
        self
    }
}

/// Implements the `+=` operator for appending to a `String`.
///
/// This has the same behavior as the [`push_str`][String::push_str] method.
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "stringaddassign", since = "1.12.0")]
impl AddAssign<&str> for String {
    #[inline]
    fn add_assign(&mut self, other: &str) {
        self.push_str(other);
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ops::Index<I> for String
where
    I: slice::SliceIndex<str>,
{
    type Output = I::Output;

    #[inline]
    fn index(&self, index: I) -> &I::Output {
        index.index(self.as_str())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<I> ops::IndexMut<I> for String
where
    I: slice::SliceIndex<str>,
{
    #[inline]
    fn index_mut(&mut self, index: I) -> &mut I::Output {
        index.index_mut(self.as_mut_str())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl ops::Deref for String {
    type Target = str;

    #[inline]
    fn deref(&self) -> &str {
        unsafe { str::from_utf8_unchecked(&self.vec) }
    }
}

#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl ops::DerefPure for String {}

#[stable(feature = "derefmut_for_string", since = "1.3.0")]
impl ops::DerefMut for String {
    #[inline]
    fn deref_mut(&mut self) -> &mut str {
        unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
    }
}

/// A type alias for [`Infallible`].
///
/// This alias exists for backwards compatibility, and may be eventually deprecated.
///
/// [`Infallible`]: core::convert::Infallible "convert::Infallible"
#[stable(feature = "str_parse_error", since = "1.5.0")]
pub type ParseError = core::convert::Infallible;

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl FromStr for String {
    type Err = core::convert::Infallible;
    #[inline]
    fn from_str(s: &str) -> Result<String, Self::Err> {
        Ok(String::from(s))
    }
}

/// A trait for converting a value to a `String`.
///
/// This trait is automatically implemented for any type which implements the
/// [`Display`] trait. As such, `ToString` shouldn't be implemented directly:
/// [`Display`] should be implemented instead, and you get the `ToString`
/// implementation for free.
///
/// [`Display`]: fmt::Display
#[cfg_attr(not(test), rustc_diagnostic_item = "ToString")]
#[stable(feature = "rust1", since = "1.0.0")]
pub trait ToString {
    /// Converts the given value to a `String`.
    ///
    /// # Examples
    ///
    /// ```
    /// let i = 5;
    /// let five = String::from("5");
    ///
    /// assert_eq!(five, i.to_string());
    /// ```
    #[rustc_conversion_suggestion]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[cfg_attr(not(test), rustc_diagnostic_item = "to_string_method")]
    fn to_string(&self) -> String;
}

/// # Panics
///
/// In this implementation, the `to_string` method panics
/// if the `Display` implementation returns an error.
/// This indicates an incorrect `Display` implementation
/// since `fmt::Write for String` never returns an error itself.
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Display + ?Sized> ToString for T {
    // A common guideline is to not inline generic functions. However,
    // removing `#[inline]` from this method causes non-negligible regressions.
    // See <https://github.com/rust-lang/rust/pull/74852>, the last attempt
    // to try to remove it.
    #[inline]
    default fn to_string(&self) -> String {
        let mut buf = String::new();
        let mut formatter = core::fmt::Formatter::new(&mut buf);
        // Bypass format_args!() to avoid write_str with zero-length strs
        fmt::Display::fmt(self, &mut formatter)
            .expect("a Display implementation returned an error unexpectedly");
        buf
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "ascii_char", issue = "110998")]
impl ToString for core::ascii::Char {
    #[inline]
    fn to_string(&self) -> String {
        self.as_str().to_owned()
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "char_to_string_specialization", since = "1.46.0")]
impl ToString for char {
    #[inline]
    fn to_string(&self) -> String {
        String::from(self.encode_utf8(&mut [0; 4]))
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "bool_to_string_specialization", since = "1.68.0")]
impl ToString for bool {
    #[inline]
    fn to_string(&self) -> String {
        String::from(if *self { "true" } else { "false" })
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "u8_to_string_specialization", since = "1.54.0")]
impl ToString for u8 {
    #[inline]
    fn to_string(&self) -> String {
        let mut buf = String::with_capacity(3);
        let mut n = *self;
        if n >= 10 {
            if n >= 100 {
                buf.push((b'0' + n / 100) as char);
                n %= 100;
            }
            buf.push((b'0' + n / 10) as char);
            n %= 10;
        }
        buf.push((b'0' + n) as char);
        buf
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "i8_to_string_specialization", since = "1.54.0")]
impl ToString for i8 {
    #[inline]
    fn to_string(&self) -> String {
        let mut buf = String::with_capacity(4);
        if self.is_negative() {
            buf.push('-');
        }
        let mut n = self.unsigned_abs();
        if n >= 10 {
            if n >= 100 {
                buf.push('1');
                n -= 100;
            }
            buf.push((b'0' + n / 10) as char);
            n %= 10;
        }
        buf.push((b'0' + n) as char);
        buf
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "str_to_string_specialization", since = "1.9.0")]
impl ToString for str {
    #[inline]
    fn to_string(&self) -> String {
        String::from(self)
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_str_to_string_specialization", since = "1.17.0")]
impl ToString for Cow<'_, str> {
    #[inline]
    fn to_string(&self) -> String {
        self[..].to_owned()
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "string_to_string_specialization", since = "1.17.0")]
impl ToString for String {
    #[inline]
    fn to_string(&self) -> String {
        self.to_owned()
    }
}

#[doc(hidden)]
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "fmt_arguments_to_string_specialization", since = "1.71.0")]
impl ToString for fmt::Arguments<'_> {
    #[inline]
    fn to_string(&self) -> String {
        crate::fmt::format(*self)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl AsRef<str> for String {
    #[inline]
    fn as_ref(&self) -> &str {
        self
    }
}

#[stable(feature = "string_as_mut", since = "1.43.0")]
impl AsMut<str> for String {
    #[inline]
    fn as_mut(&mut self) -> &mut str {
        self
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl AsRef<[u8]> for String {
    #[inline]
    fn as_ref(&self) -> &[u8] {
        self.as_bytes()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl From<&str> for String {
    /// Converts a `&str` into a [`String`].
    ///
    /// The result is allocated on the heap.
    #[inline]
    fn from(s: &str) -> String {
        s.to_owned()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "from_mut_str_for_string", since = "1.44.0")]
impl From<&mut str> for String {
    /// Converts a `&mut str` into a [`String`].
    ///
    /// The result is allocated on the heap.
    #[inline]
    fn from(s: &mut str) -> String {
        s.to_owned()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "from_ref_string", since = "1.35.0")]
impl From<&String> for String {
    /// Converts a `&String` into a [`String`].
    ///
    /// This clones `s` and returns the clone.
    #[inline]
    fn from(s: &String) -> String {
        s.clone()
    }
}

// note: test pulls in std, which causes errors here
#[cfg(not(test))]
#[stable(feature = "string_from_box", since = "1.18.0")]
impl From<Box<str>> for String {
    /// Converts the given boxed `str` slice to a [`String`].
    /// It is notable that the `str` slice is owned.
    ///
    /// # Examples
    ///
    /// ```
    /// let s1: String = String::from("hello world");
    /// let s2: Box<str> = s1.into_boxed_str();
    /// let s3: String = String::from(s2);
    ///
    /// assert_eq!("hello world", s3)
    /// ```
    fn from(s: Box<str>) -> String {
        s.into_string()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_from_str", since = "1.20.0")]
impl From<String> for Box<str> {
    /// Converts the given [`String`] to a boxed `str` slice that is owned.
    ///
    /// # Examples
    ///
    /// ```
    /// let s1: String = String::from("hello world");
    /// let s2: Box<str> = Box::from(s1);
    /// let s3: String = String::from(s2);
    ///
    /// assert_eq!("hello world", s3)
    /// ```
    fn from(s: String) -> Box<str> {
        s.into_boxed_str()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "string_from_cow_str", since = "1.14.0")]
impl<'a> From<Cow<'a, str>> for String {
    /// Converts a clone-on-write string to an owned
    /// instance of [`String`].
    ///
    /// This extracts the owned string,
    /// clones the string if it is not already owned.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::borrow::Cow;
    /// // If the string is not owned...
    /// let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
    /// // It will allocate on the heap and copy the string.
    /// let owned: String = String::from(cow);
    /// assert_eq!(&owned[..], "eggplant");
    /// ```
    fn from(s: Cow<'a, str>) -> String {
        s.into_owned()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> From<&'a str> for Cow<'a, str> {
    /// Converts a string slice into a [`Borrowed`] variant.
    /// No heap allocation is performed, and the string
    /// is not copied.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::borrow::Cow;
    /// assert_eq!(Cow::from("eggplant"), Cow::Borrowed("eggplant"));
    /// ```
    ///
    /// [`Borrowed`]: crate::borrow::Cow::Borrowed "borrow::Cow::Borrowed"
    #[inline]
    fn from(s: &'a str) -> Cow<'a, str> {
        Cow::Borrowed(s)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a> From<String> for Cow<'a, str> {
    /// Converts a [`String`] into an [`Owned`] variant.
    /// No heap allocation is performed, and the string
    /// is not copied.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::borrow::Cow;
    /// let s = "eggplant".to_string();
    /// let s2 = "eggplant".to_string();
    /// assert_eq!(Cow::from(s), Cow::<'static, str>::Owned(s2));
    /// ```
    ///
    /// [`Owned`]: crate::borrow::Cow::Owned "borrow::Cow::Owned"
    #[inline]
    fn from(s: String) -> Cow<'a, str> {
        Cow::Owned(s)
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_from_string_ref", since = "1.28.0")]
impl<'a> From<&'a String> for Cow<'a, str> {
    /// Converts a [`String`] reference into a [`Borrowed`] variant.
    /// No heap allocation is performed, and the string
    /// is not copied.
    ///
    /// # Example
    ///
    /// ```
    /// # use std::borrow::Cow;
    /// let s = "eggplant".to_string();
    /// assert_eq!(Cow::from(&s), Cow::Borrowed("eggplant"));
    /// ```
    ///
    /// [`Borrowed`]: crate::borrow::Cow::Borrowed "borrow::Cow::Borrowed"
    #[inline]
    fn from(s: &'a String) -> Cow<'a, str> {
        Cow::Borrowed(s.as_str())
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_str_from_iter", since = "1.12.0")]
impl<'a> FromIterator<char> for Cow<'a, str> {
    fn from_iter<I: IntoIterator<Item = char>>(it: I) -> Cow<'a, str> {
        Cow::Owned(FromIterator::from_iter(it))
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_str_from_iter", since = "1.12.0")]
impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str> {
    fn from_iter<I: IntoIterator<Item = &'b str>>(it: I) -> Cow<'a, str> {
        Cow::Owned(FromIterator::from_iter(it))
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_str_from_iter", since = "1.12.0")]
impl<'a> FromIterator<String> for Cow<'a, str> {
    fn from_iter<I: IntoIterator<Item = String>>(it: I) -> Cow<'a, str> {
        Cow::Owned(FromIterator::from_iter(it))
    }
}

#[stable(feature = "from_string_for_vec_u8", since = "1.14.0")]
impl From<String> for Vec<u8> {
    /// Converts the given [`String`] to a vector [`Vec`] that holds values of type [`u8`].
    ///
    /// # Examples
    ///
    /// ```
    /// let s1 = String::from("hello world");
    /// let v1 = Vec::from(s1);
    ///
    /// for b in v1 {
    ///     println!("{b}");
    /// }
    /// ```
    fn from(string: String) -> Vec<u8> {
        string.into_bytes()
    }
}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Write for String {
    #[inline]
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.push_str(s);
        Ok(())
    }

    #[inline]
    fn write_char(&mut self, c: char) -> fmt::Result {
        self.push(c);
        Ok(())
    }
}

/// A draining iterator for `String`.
///
/// This struct is created by the [`drain`] method on [`String`]. See its
/// documentation for more.
///
/// [`drain`]: String::drain
#[stable(feature = "drain", since = "1.6.0")]
pub struct Drain<'a> {
    /// Will be used as &'a mut String in the destructor
    string: *mut String,
    /// Start of part to remove
    start: usize,
    /// End of part to remove
    end: usize,
    /// Current remaining range to remove
    iter: Chars<'a>,
}

#[stable(feature = "collection_debug", since = "1.17.0")]
impl fmt::Debug for Drain<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("Drain").field(&self.as_str()).finish()
    }
}

#[stable(feature = "drain", since = "1.6.0")]
unsafe impl Sync for Drain<'_> {}
#[stable(feature = "drain", since = "1.6.0")]
unsafe impl Send for Drain<'_> {}

#[stable(feature = "drain", since = "1.6.0")]
impl Drop for Drain<'_> {
    fn drop(&mut self) {
        unsafe {
            // Use Vec::drain. "Reaffirm" the bounds checks to avoid
            // panic code being inserted again.
            let self_vec = (*self.string).as_mut_vec();
            if self.start <= self.end && self.end <= self_vec.len() {
                self_vec.drain(self.start..self.end);
            }
        }
    }
}

impl<'a> Drain<'a> {
    /// Returns the remaining (sub)string of this iterator as a slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut s = String::from("abc");
    /// let mut drain = s.drain(..);
    /// assert_eq!(drain.as_str(), "abc");
    /// let _ = drain.next().unwrap();
    /// assert_eq!(drain.as_str(), "bc");
    /// ```
    #[must_use]
    #[stable(feature = "string_drain_as_str", since = "1.55.0")]
    pub fn as_str(&self) -> &str {
        self.iter.as_str()
    }
}

#[stable(feature = "string_drain_as_str", since = "1.55.0")]
impl<'a> AsRef<str> for Drain<'a> {
    fn as_ref(&self) -> &str {
        self.as_str()
    }
}

#[stable(feature = "string_drain_as_str", since = "1.55.0")]
impl<'a> AsRef<[u8]> for Drain<'a> {
    fn as_ref(&self) -> &[u8] {
        self.as_str().as_bytes()
    }
}

#[stable(feature = "drain", since = "1.6.0")]
impl Iterator for Drain<'_> {
    type Item = char;

    #[inline]
    fn next(&mut self) -> Option<char> {
        self.iter.next()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }

    #[inline]
    fn last(mut self) -> Option<char> {
        self.next_back()
    }
}

#[stable(feature = "drain", since = "1.6.0")]
impl DoubleEndedIterator for Drain<'_> {
    #[inline]
    fn next_back(&mut self) -> Option<char> {
        self.iter.next_back()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl FusedIterator for Drain<'_> {}

#[cfg(not(no_global_oom_handling))]
#[stable(feature = "from_char_for_string", since = "1.46.0")]
impl From<char> for String {
    /// Allocates an owned [`String`] from a single character.
    ///
    /// # Example
    /// ```rust
    /// let c: char = 'a';
    /// let s: String = String::from(c);
    /// assert_eq!("a", &s[..]);
    /// ```
    #[inline]
    fn from(c: char) -> Self {
        c.to_string()
    }
}