1 files changed, 117 insertions, 0 deletions
diff --git a/third_party/rust/futures-util/src/stream/select.rs b/third_party/rust/futures-util/src/stream/select.rs
new file mode 100644
index 0000000000..0c1e3af782
--- /dev/null
+++ b/third_party/rust/futures-util/src/stream/select.rs
@@ -0,0 +1,117 @@
+use super::assert_stream;
+use crate::stream::{select_with_strategy, PollNext, SelectWithStrategy};
+use core::pin::Pin;
+use futures_core::stream::{FusedStream, Stream};
+use futures_core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [`select()`] function.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Select<St1, St2> {
+        #[pin]
+        inner: SelectWithStrategy<St1, St2, fn(&mut PollNext)-> PollNext, PollNext>,
+    }
+}
+
+/// This function will attempt to pull items from both streams. Each
+/// stream will be polled in a round-robin fashion, and whenever a stream is
+/// ready to yield an item that item is yielded.
+///
+/// After one of the two input streams completes, the remaining one will be
+/// polled exclusively. The returned stream completes when both input
+/// streams have completed.
+///
+/// Note that this function consumes both streams and returns a wrapped
+/// version of them.
+///
+/// ## Examples
+///
+/// ```rust
+/// # futures::executor::block_on(async {
+/// use futures::stream::{ repeat, select, StreamExt };
+///
+/// let left = repeat(1);
+/// let right = repeat(2);
+///
+/// let mut out = select(left, right);
+///
+/// for _ in 0..100 {
+///     // We should be alternating.
+///     assert_eq!(1, out.select_next_some().await);
+///     assert_eq!(2, out.select_next_some().await);
+/// }
+/// # });
+/// ```
+pub fn select<St1, St2>(stream1: St1, stream2: St2) -> Select<St1, St2>
+where
+    St1: Stream,
+    St2: Stream<Item = St1::Item>,
+{
+    fn round_robin(last: &mut PollNext) -> PollNext {
+        last.toggle()
+    }
+
+    assert_stream::<St1::Item, _>(Select {
+        inner: select_with_strategy(stream1, stream2, round_robin),
+    })
+}
+
+impl<St1, St2> Select<St1, St2> {
+    /// Acquires a reference to the underlying streams that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> (&St1, &St2) {
+        self.inner.get_ref()
+    }
+
+    /// Acquires a mutable reference to the underlying streams that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_mut(&mut self) -> (&mut St1, &mut St2) {
+        self.inner.get_mut()
+    }
+
+    /// Acquires a pinned mutable reference to the underlying streams that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut St1>, Pin<&mut St2>) {
+        let this = self.project();
+        this.inner.get_pin_mut()
+    }
+
+    /// Consumes this combinator, returning the underlying streams.
+    ///
+    /// Note that this may discard intermediate state of this combinator, so
+    /// care should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> (St1, St2) {
+        self.inner.into_inner()
+    }
+}
+
+impl<St1, St2> FusedStream for Select<St1, St2>
+where
+    St1: Stream,
+    St2: Stream<Item = St1::Item>,
+{
+    fn is_terminated(&self) -> bool {
+        self.inner.is_terminated()
+    }
+}
+
+impl<St1, St2> Stream for Select<St1, St2>
+where
+    St1: Stream,
+    St2: Stream<Item = St1::Item>,
+{
+    type Item = St1::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<St1::Item>> {
+        let this = self.project();
+        this.inner.poll_next(cx)
+    }
+}