Join stream

src/join_stream.rs

@@ -0,0 +1,76 @@
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+use futures_core::Stream;
+
+/// A stream joining two or more streams.
+///
+/// This stream is returned by `join!`.
+#[derive(Debug)]
+pub struct JoinStream<L, R> {
+    left: L,
+    right: R,
+}
+
+impl<L, R> Unpin for JoinStream<L, R> {}
+
+impl<L, R> JoinStream<L, R> {
+    #[doc(hidden)]
+    pub fn new(left: L, right: R) -> Self {
+        Self { left, right }
+    }
+}
+
+impl<L, R, T> Stream for JoinStream<L, R>
+where
+    L: Stream<Item = T> + Unpin,
+    R: Stream<Item = T> + Unpin,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        if let Poll::Ready(Some(item)) = Pin::new(&mut self.left).poll_next(cx) {
+            // The first stream made progress. The JoinStream needs to be polled
+            // again to check the progress of the second stream.
+            cx.waker().wake_by_ref();
+            Poll::Ready(Some(item))
+        } else {
+            Pin::new(&mut self.right).poll_next(cx)
+        }
+    }
+}
+
+/// Combines multiple streams into a single stream of all their outputs.
+///
+/// This macro is only usable inside of async functions, closures, and blocks.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use async_macros::join_stream as join;
+/// use futures::stream::{self, StreamExt};
+/// use futures::future::ready;
+///
+/// let a = stream::once(ready(1u8));
+/// let b = stream::once(ready(2u8));
+/// let c = stream::once(ready(3u8));
+///
+/// let mut s = join!(a, b, c);
+///
+/// assert_eq!(s.next().await, Some(1u8));
+/// assert_eq!(s.next().await, Some(2u8));
+/// assert_eq!(s.next().await, Some(3u8));
+/// assert_eq!(s.next().await, None);
+/// # });
+/// ```
+#[macro_export]
+macro_rules! join_stream {
+    ($stream1:ident, $stream2:ident, $($stream:ident),* $(,)?) => {{
+        let joined = $crate::JoinStream::new($stream1, $stream2);
+        $(
+            let joined = $crate::JoinStream::new(joined, $stream);
+        )*
+        joined
+    }};
+}

src/lib.rs

@@ -3,7 +3,6 @@
 //! # Examples
 //!
 //! ```
-//! #![feature(async_await)]
 //! # futures::executor::block_on(async {
 //! use async_macros::join;
 //! use futures::future;
@@ -21,13 +20,15 @@
 #![cfg_attr(test, deny(warnings))]
 
 mod join;
+mod join_stream;
 mod maybe_done;
 mod poll_fn;
 mod ready;
 mod select;
 mod try_join;
 mod try_select;
 
+pub use join_stream::JoinStream;
 pub use maybe_done::MaybeDone;
 
 /// Helper re-exports for use in macros.