1 files changed, 104 insertions, 0 deletions
diff --git a/third_party/rust/futures-util/src/future/maybe_done.rs b/third_party/rust/futures-util/src/future/maybe_done.rs
new file mode 100644
index 0000000000..26e6c27588
--- /dev/null
+++ b/third_party/rust/futures-util/src/future/maybe_done.rs
@@ -0,0 +1,104 @@
+//! Definition of the MaybeDone combinator
+
+use super::assert_future;
+use core::mem;
+use core::pin::Pin;
+use futures_core::future::{FusedFuture, Future};
+use futures_core::ready;
+use futures_core::task::{Context, Poll};
+
+/// A future that may have completed.
+///
+/// This is created by the [`maybe_done()`] function.
+#[derive(Debug)]
+pub enum MaybeDone<Fut: Future> {
+    /// A not-yet-completed future
+    Future(/* #[pin] */ Fut),
+    /// The output of the completed future
+    Done(Fut::Output),
+    /// The empty variant after the result of a [`MaybeDone`] has been
+    /// taken using the [`take_output`](MaybeDone::take_output) method.
+    Gone,
+}
+
+impl<Fut: Future + Unpin> Unpin for MaybeDone<Fut> {}
+
+/// Wraps a future into a `MaybeDone`
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::future;
+/// use futures::pin_mut;
+///
+/// let future = future::maybe_done(async { 5 });
+/// pin_mut!(future);
+/// assert_eq!(future.as_mut().take_output(), None);
+/// let () = future.as_mut().await;
+/// assert_eq!(future.as_mut().take_output(), Some(5));
+/// assert_eq!(future.as_mut().take_output(), None);
+/// # });
+/// ```
+pub fn maybe_done<Fut: Future>(future: Fut) -> MaybeDone<Fut> {
+    assert_future::<(), _>(MaybeDone::Future(future))
+}
+
+impl<Fut: Future> MaybeDone<Fut> {
+    /// Returns an [`Option`] containing a mutable reference to the output of the future.
+    /// The output of this method will be [`Some`] if and only if the inner
+    /// future has been completed and [`take_output`](MaybeDone::take_output)
+    /// has not yet been called.
+    #[inline]
+    pub fn output_mut(self: Pin<&mut Self>) -> Option<&mut Fut::Output> {
+        unsafe {
+            match self.get_unchecked_mut() {
+                MaybeDone::Done(res) => Some(res),
+                _ => None,
+            }
+        }
+    }
+
+    /// Attempt to take the output of a `MaybeDone` without driving it
+    /// towards completion.
+    #[inline]
+    pub fn take_output(self: Pin<&mut Self>) -> Option<Fut::Output> {
+        match &*self {
+            Self::Done(_) => {}
+            Self::Future(_) | Self::Gone => return None,
+        }
+        unsafe {
+            match mem::replace(self.get_unchecked_mut(), Self::Gone) {
+                MaybeDone::Done(output) => Some(output),
+                _ => unreachable!(),
+            }
+        }
+    }
+}
+
+impl<Fut: Future> FusedFuture for MaybeDone<Fut> {
+    fn is_terminated(&self) -> bool {
+        match self {
+            Self::Future(_) => false,
+            Self::Done(_) | Self::Gone => true,
+        }
+    }
+}
+
+impl<Fut: Future> Future for MaybeDone<Fut> {
+    type Output = ();
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        unsafe {
+            match self.as_mut().get_unchecked_mut() {
+                MaybeDone::Future(f) => {
+                    let res = ready!(Pin::new_unchecked(f).poll(cx));
+                    self.set(Self::Done(res));
+                }
+                MaybeDone::Done(_) => {}
+                MaybeDone::Gone => panic!("MaybeDone polled after value taken"),
+            }
+        }
+        Poll::Ready(())
+    }
+}