feat(futures-util/stream): unzip operator

closes: rust-lang#2234

Author: binier

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

@@ -29,6 +29,10 @@ mod collect;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::collect::Collect;
 
+mod unzip;
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+pub use self::unzip::Unzip;
+
 mod concat;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::concat::Concat;
@@ -477,6 +481,45 @@ pub trait StreamExt: Stream {
         assert_future::<C, _>(Collect::new(self))
     }
 
+    /// Converts a stream of pairs into a future, which
+    /// resolves to pair of containers.
+    ///
+    /// `unzip()` produces a future, which resolves to two
+    /// collections: one from the left elements of the pairs,
+    /// and one from the right elements.
+    ///
+    /// The returned future will be resolved when the stream terminates.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::channel::mpsc;
+    /// use futures::stream::StreamExt;
+    /// use std::thread;
+    ///
+    /// let (tx, rx) = mpsc::unbounded();
+    ///
+    /// thread::spawn(move || {
+    ///     tx.unbounded_send((1, 2)).unwrap();
+    ///     tx.unbounded_send((3, 4)).unwrap();
+    ///     tx.unbounded_send((5, 6)).unwrap();
+    /// });
+    ///
+    /// let (o1, o2): (Vec<_>, Vec<_>) = rx.unzip().await;
+    /// assert_eq!(o1, vec![1, 3, 5]);
+    /// assert_eq!(o2, vec![2, 4, 6]);
+    /// # });
+    /// ```
+    fn unzip<A, B, FromA, FromB>(self) -> Unzip<Self, FromA, FromB>
+    where
+        FromA: Default + Extend<A>,
+        FromB: Default + Extend<B>,
+        Self: Sized + Stream<Item = (A, B)>,
+    {
+        assert_future::<(FromA, FromB), _>(Unzip::new(self))
+    }
+
     /// Concatenate all items of a stream into a single extendable
     /// destination, returning a future representing the end result.
     ///

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

@@ -0,0 +1,66 @@
+use core::mem;
+use core::pin::Pin;
+use futures_core::future::{FusedFuture, Future};
+use futures_core::stream::{FusedStream, Stream};
+use futures_core::task::{Context, Poll};
+use pin_project::pin_project;
+
+/// Future for the [`unzip`](super::StreamExt::unzip) method.
+#[pin_project]
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Unzip<St, FromA, FromB> {
+    #[pin]
+    stream: St,
+    left: FromA,
+    right: FromB,
+}
+
+impl<St: Stream, FromA: Default, FromB: Default> Unzip<St, FromA, FromB> {
+    fn finish(self: Pin<&mut Self>) -> (FromA, FromB) {
+        let this = self.project();
+        (
+            mem::take(this.left),
+            mem::take(this.right),
+        )
+    }
+
+    pub(super) fn new(stream: St) -> Self {
+        Self {
+            stream,
+            left: Default::default(),
+            right: Default::default(),
+        }
+    }
+}
+
+impl<St, A, B, FromA, FromB> FusedFuture for Unzip<St, FromA, FromB>
+where St: FusedStream<Item = (A, B)>,
+      FromA: Default + Extend<A>,
+      FromB: Default + Extend<B>,
+{
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}
+
+impl<St, A, B, FromA, FromB> Future for Unzip<St, FromA, FromB>
+where St: Stream<Item = (A, B)>,
+      FromA: Default + Extend<A>,
+      FromB: Default + Extend<B>,
+{
+    type Output = (FromA, FromB);
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<(FromA, FromB)> {
+        let mut this = self.as_mut().project();
+        loop {
+            match ready!(this.stream.as_mut().poll_next(cx)) {
+                Some(e) => {
+                    this.left.extend(Some(e.0));
+                    this.right.extend(Some(e.1));
+                },
+                None => return Poll::Ready(self.finish()),
+            }
+        }
+    }
+}