feat: backport of StreamExt::interleave

futures-util/src/stream/mod.rs

@@ -19,9 +19,9 @@ pub use futures_core::stream::{FusedStream, Stream, TryStream};
 mod stream;
 pub use self::stream::{
     All, Any, Chain, Collect, Concat, Count, Cycle, Enumerate, Filter, FilterMap, FlatMap, Flatten,
-    Fold, ForEach, Fuse, Inspect, Map, Next, NextIf, NextIfEq, Peek, PeekMut, Peekable, Scan,
-    SelectNextSome, Skip, SkipWhile, StreamExt, StreamFuture, Take, TakeUntil, TakeWhile, Then,
-    Unzip, Zip,
+    Fold, ForEach, Fuse, Inspect, Interleave, Map, Next, NextIf, NextIfEq, Peek, PeekMut, Peekable,
+    Scan, SelectNextSome, Skip, SkipWhile, StreamExt, StreamFuture, Take, TakeUntil, TakeWhile,
+    Then, Unzip, Zip,
 };
 
 #[cfg(feature = "std")]

futures-util/src/stream/stream/interleave.rs

@@ -0,0 +1,90 @@
+use core::{
+    pin::Pin,
+    task::{ready, Context, Poll},
+};
+
+use futures_core::{FusedStream, Stream};
+use pin_project_lite::pin_project;
+
+use crate::stream::Fuse;
+
+pin_project! {
+    /// Stream for the [`interleave`](super::StreamExt::interleave) method.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Interleave<I, J> {
+        #[pin] i: Fuse<I>,
+        #[pin] j: Fuse<J>,
+        next_coming_from_j: bool,
+    }
+}
+
+impl<I, J> Interleave<I, J> {
+    pub(super) fn new(i: I, j: J) -> Self {
+        Self { i: Fuse::new(i), j: Fuse::new(j), next_coming_from_j: false }
+    }
+    /// Acquires a reference to the underlying streams that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> (&I, &J) {
+        (self.i.get_ref(), self.j.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 I, &mut J) {
+        (self.i.get_mut(), self.j.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 I>, Pin<&mut J>) {
+        let this = self.project();
+        (this.i.get_pin_mut(), this.j.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) -> (I, J) {
+        (self.i.into_inner(), self.j.into_inner())
+    }
+}
+
+impl<I: Stream, J: Stream<Item = I::Item>> Stream for Interleave<I, J> {
+    type Item = I::Item;
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let this = self.project();
+        *this.next_coming_from_j = !*this.next_coming_from_j;
+        match this.next_coming_from_j {
+            true => match ready!(this.i.poll_next(cx)) {
+                Some(it) => Poll::Ready(Some(it)),
+                None => this.j.poll_next(cx),
+            },
+            false => match ready!(this.j.poll_next(cx)) {
+                Some(it) => Poll::Ready(Some(it)),
+                None => this.i.poll_next(cx),
+            },
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (ilo, ihi) = self.i.size_hint();
+        let (jlo, jhi) = self.j.size_hint();
+        let lo = ilo.saturating_add(jlo);
+        let hi = ihi.and_then(|it| it.checked_add(jhi?));
+        (lo, hi)
+    }
+}
+
+impl<I: FusedStream, J: FusedStream<Item = I::Item>> FusedStream for Interleave<I, J> {
+    fn is_terminated(&self) -> bool {
+        self.i.is_terminated() && self.j.is_terminated()
+    }
+}

futures-util/src/stream/stream/mod.rs

@@ -103,6 +103,9 @@ mod fuse;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::fuse::Fuse;
 
+mod interleave;
+pub use self::interleave::Interleave;
+
 mod into_future;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::into_future::StreamFuture;
@@ -1694,4 +1697,31 @@ pub trait StreamExt: Stream {
     {
         assert_future::<Self::Item, _>(SelectNextSome::new(self))
     }
+    /// Creates a stream which interleaves items between `self` and `other`,
+    /// in turn.
+    ///
+    /// When one stream is exhausted, only items from the other are yielded.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::{stream, StreamExt as _};
+    ///
+    /// let stream = stream::iter(['a', 'b'])
+    ///     .interleave(stream::iter(['A', 'B', 'C', 'D']));
+    ///
+    /// assert_eq!(
+    ///     "aAbBCD",
+    ///     stream.collect::<String>().await
+    /// );
+    /// # })
+    /// ```
+    fn interleave<St>(self, other: St) -> Interleave<Self, St>
+    where
+        Self: Sized,
+        St: Stream<Item = Self::Item>,
+    {
+        assert_stream(Interleave::new(self, other))
+    }
 }