1 files changed, 80 insertions, 0 deletions
diff --git a/third_party/rust/futures-util/src/stream/poll_immediate.rs b/third_party/rust/futures-util/src/stream/poll_immediate.rs
new file mode 100644
index 0000000000..c7e8a5b3c6
--- /dev/null
+++ b/third_party/rust/futures-util/src/stream/poll_immediate.rs
@@ -0,0 +1,80 @@
+use core::pin::Pin;
+use futures_core::task::{Context, Poll};
+use futures_core::Stream;
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [poll_immediate](poll_immediate()) function.
+    ///
+    /// It will never return [Poll::Pending](core::task::Poll::Pending)
+    #[derive(Debug, Clone)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct PollImmediate<S> {
+        #[pin]
+        stream: Option<S>
+    }
+}
+
+impl<T, S> Stream for PollImmediate<S>
+where
+    S: Stream<Item = T>,
+{
+    type Item = Poll<T>;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.project();
+        let stream = match this.stream.as_mut().as_pin_mut() {
+            // inner is gone, so we can continue to signal that the stream is closed.
+            None => return Poll::Ready(None),
+            Some(inner) => inner,
+        };
+
+        match stream.poll_next(cx) {
+            Poll::Ready(Some(t)) => Poll::Ready(Some(Poll::Ready(t))),
+            Poll::Ready(None) => {
+                this.stream.set(None);
+                Poll::Ready(None)
+            }
+            Poll::Pending => Poll::Ready(Some(Poll::Pending)),
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.stream.as_ref().map_or((0, Some(0)), Stream::size_hint)
+    }
+}
+
+impl<S: Stream> super::FusedStream for PollImmediate<S> {
+    fn is_terminated(&self) -> bool {
+        self.stream.is_none()
+    }
+}
+
+/// Creates a new stream that always immediately returns [Poll::Ready](core::task::Poll::Ready) when awaiting it.
+///
+/// This is useful when immediacy is more important than waiting for the next item to be ready.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::stream::{self, StreamExt};
+/// use futures::task::Poll;
+///
+/// let mut r = stream::poll_immediate(Box::pin(stream::iter(1_u32..3)));
+/// assert_eq!(r.next().await, Some(Poll::Ready(1)));
+/// assert_eq!(r.next().await, Some(Poll::Ready(2)));
+/// assert_eq!(r.next().await, None);
+///
+/// let mut p = stream::poll_immediate(Box::pin(stream::once(async {
+///     futures::pending!();
+///     42_u8
+/// })));
+/// assert_eq!(p.next().await, Some(Poll::Pending));
+/// assert_eq!(p.next().await, Some(Poll::Ready(42)));
+/// assert_eq!(p.next().await, None);
+/// # });
+/// ```
+pub fn poll_immediate<S: Stream>(s: S) -> PollImmediate<S> {
+    super::assert_stream::<Poll<S::Item>, PollImmediate<S>>(PollImmediate { stream: Some(s) })
+}