Add stream unzip

src/stream/stream/mod.rs

@@ -124,6 +124,7 @@ cfg_unstable! {
 
     use count::CountFuture;
     use partition::PartitionFuture;
+    use unzip::UnzipFuture;
 
     pub use merge::Merge;
     pub use flatten::Flatten;
@@ -138,6 +139,7 @@ cfg_unstable! {
     mod partition;
     mod timeout;
     mod throttle;
+    mod unzip;
 }
 
 extension_trait! {
@@ -1717,6 +1719,44 @@ extension_trait! {
             Zip::new(self, other)
         }
 
+        #[doc = r#"
+            Converts an stream of pairs into a pair of containers.
+
+            unzip() consumes an entire stream of pairs, producing two collections: one from the left elements of the pairs, and one from the right elements.
+
+            This function is, in some sense, the opposite of [`zip`].
+
+            [`zip`]: trait.Stream.html#method.zip
+
+            # Example
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::prelude::*;
+            use async_std::stream;
+
+            let s  = stream::from_iter(vec![(1,2), (3,4)]);
+
+            let (left, right): (Vec<_>, Vec<_>) = s.unzip().await;
+
+            assert_eq!(left, [1, 3]);
+            assert_eq!(right, [2, 4]);
+            #
+            # }) }
+            ```
+        "#]
+        #[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 +1,57 @@
+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! {
+    #[derive(Clone, Debug)]
+    #[cfg(all(feature = "default", feature = "unstable"))]
+    #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+    pub struct UnzipFuture<S, FromA, FromB> {
+        #[pin]
+        stream: S,
+        res: Option<(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: Some((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>,
+    FromB: Default + Extend<B>,
+{
+    type Output = (FromA, FromB);
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut this = self.project();
+
+        loop {
+            let next = futures_core::ready!(this.stream.as_mut().poll_next(cx));
+
+            match next {
+                Some((a, b)) => {
+                    let res = this.res.as_mut().unwrap();
+                    res.0.extend(Some(a));
+                    res.1.extend(Some(b));
+                }
+                None => return Poll::Ready(this.res.take().unwrap()),
+            }
+        }
+    }
+}