1 files changed, 67 insertions, 0 deletions
diff --git a/third_party/rust/futures-util/src/stream/once.rs b/third_party/rust/futures-util/src/stream/once.rs
new file mode 100644
index 0000000000..ee21c8b594
--- /dev/null
+++ b/third_party/rust/futures-util/src/stream/once.rs
@@ -0,0 +1,67 @@
+use super::assert_stream;
+use core::pin::Pin;
+use futures_core::future::Future;
+use futures_core::ready;
+use futures_core::stream::{FusedStream, Stream};
+use futures_core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+/// Creates a stream of a single element.
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::stream::{self, StreamExt};
+///
+/// let stream = stream::once(async { 17 });
+/// let collected = stream.collect::<Vec<i32>>().await;
+/// assert_eq!(collected, vec![17]);
+/// # });
+/// ```
+pub fn once<Fut: Future>(future: Fut) -> Once<Fut> {
+    assert_stream::<Fut::Output, _>(Once::new(future))
+}
+
+pin_project! {
+    /// A stream which emits single element and then EOF.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Once<Fut> {
+        #[pin]
+        future: Option<Fut>
+    }
+}
+
+impl<Fut> Once<Fut> {
+    pub(crate) fn new(future: Fut) -> Self {
+        Self { future: Some(future) }
+    }
+}
+
+impl<Fut: Future> Stream for Once<Fut> {
+    type Item = Fut::Output;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.project();
+        let v = match this.future.as_mut().as_pin_mut() {
+            Some(fut) => ready!(fut.poll(cx)),
+            None => return Poll::Ready(None),
+        };
+
+        this.future.set(None);
+        Poll::Ready(Some(v))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.future.is_some() {
+            (1, Some(1))
+        } else {
+            (0, Some(0))
+        }
+    }
+}
+
+impl<Fut: Future> FusedStream for Once<Fut> {
+    fn is_terminated(&self) -> bool {
+        self.future.is_none()
+    }
+}