wip: Add stream unzip

src/stream/stream/mod.rs

@@ -130,6 +130,7 @@ cfg_unstable! {
     pub use flat_map::FlatMap;
     pub use timeout::{TimeoutError, Timeout};
     pub use throttle::Throttle;
+    pub use unzip::UnzipFuture;
 
     mod count;
     mod merge;
@@ -138,6 +139,7 @@ cfg_unstable! {
     mod partition;
     mod timeout;
     mod throttle;
+    mod unzip;
 }
 
 extension_trait! {
@@ -1717,6 +1719,17 @@ extension_trait! {
             Zip::new(self, other)
         }
 
+        #[cfg(feature = "unstable")]
+        #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+        fn unzip<A, B, FromA, FromB>(self) -> impl Future<Output = (FromA, FromB)> [UnzipFuture<Self, FromA, FromB>]
+        where
+        FromA: Default + Extend<A>,
+        FromB: Default + Extend<B>,
+        Self: Stream<Item = (A, B)> + Sized,
+        {
+            UnzipFuture::new(self)
+        }
+
         #[doc = r#"
             Transforms a stream into a collection.
 

src/stream/stream/unzip.rs

@@ -0,0 +1,53 @@
+use std::future::Future;
+use std::pin::Pin;
+
+use pin_project_lite::pin_project;
+
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+pin_project! {
+    #[cfg(all(feature = "default", feature = "unstable"))]
+    #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+    pub struct UnzipFuture<S: Stream, FromA, FromB> {
+        #[pin]
+        stream: S,
+        res: (FromA, FromB),
+    }
+}
+
+impl<S: Stream, FromA, FromB> UnzipFuture<S, FromA, FromB>
+where
+    FromA: Default,
+    FromB: Default,
+{
+    pub(super) fn new(stream: S) -> Self {
+        UnzipFuture {
+            stream,
+            res: (FromA::default(), FromB::default()),
+        }
+    }
+}
+
+impl<S, A, B, FromA, FromB> Future for UnzipFuture<S, FromA, FromB>
+where
+    S: Stream<Item = (A, B)>,
+    FromA: Default + Extend<A> + Copy,
+    FromB: Default + Extend<B> + Copy,
+{
+    type Output = (FromA, FromB);
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
+        let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
+
+        match next {
+            Some((a, b)) => {
+                this.res.0.extend(Some(a));
+                this.res.1.extend(Some(b));
+                Poll::Pending
+            }
+            None => Poll::Ready(*this.res),
+        }
+    }
+}