Rebrand to Galaxy, major improvements to Bedrock support, still needs some TLC though

crates/galaxy_util/src/on_cancel.rs

@@ -0,0 +1,64 @@
+use pin_project::{pin_project, pinned_drop};
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// Trait allowing you to attach a function to a [`Future`] that will be called if the future is
+/// cancelled.  
+pub trait OnCancelFutureExt
+where
+    Self: Future + Sized,
+{
+    /// Wraps the future with an [`OnCancelFutureExt`] that will execute the given function
+    /// when the future is cancelled.
+    fn on_cancel<D: FnMut()>(self, on_drop: D) -> OnCancelFuture<Self, D>;
+}
+impl<F: Future> OnCancelFutureExt for F {
+    fn on_cancel<D: FnMut()>(self, on_cancel: D) -> OnCancelFuture<Self, D> {
+        OnCancelFuture {
+            inner: self,
+            on_cancel,
+            is_ready: false,
+        }
+    }
+}
+
+/// Wrapper around a [`Future`] that calls an `on_cancel` callback if the future is cancelled
+/// before it resolved to ready. See [`OnCancelFuture::on_cancel`] for more details. A future is
+/// considered cancelled if it is dropped before resolving to [`Poll::Ready`], see <https://google.github.io/comprehensive-rust/async/pitfalls/cancellation.html#cancellation>.
+#[pin_project(PinnedDrop)]
+pub struct OnCancelFuture<F: Future, D: FnMut()> {
+    #[pin]
+    inner: F,
+    /// Function that is called when the future is cancelled.
+    on_cancel: D,
+    /// Whether the inner future is has returned [`Poll::Ready`] (indicating the future is
+    /// complete).
+    is_ready: bool,
+}
+
+impl<F: Future, D: FnMut()> Future for OnCancelFuture<F, D> {
+    type Output = F::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<F::Output> {
+        let this = self.project();
+        let output = this.inner.poll(cx);
+        *this.is_ready = output.is_ready();
+        output
+    }
+}
+
+#[pinned_drop]
+impl<F: Future, D: FnMut()> PinnedDrop for OnCancelFuture<F, D> {
+    fn drop(self: Pin<&mut Self>) {
+        // If the future was dropped before it was resolved to ready, the future was cancelled.
+        let this = self.project();
+        if !*this.is_ready {
+            (this.on_cancel)();
+        }
+    }
+}
+
+#[cfg(test)]
+#[path = "on_cancel_tests.rs"]
+mod tests;