stream: add `StreamExt::fuse` (#2085)

2 files changed, 121 insertions, 0 deletions

diff --git a/tokio/src/stream/fuse.rs b/tokio/src/stream/fuse.rs
new file mode 100644
index 00000000..6c9e02d6
--- /dev/null
+++ b/tokio/src/stream/fuse.rs
@@ -0,0 +1,53 @@
+use crate::stream::Stream;
+
+use pin_project_lite::pin_project;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pin_project! {
+    /// Stream returned by [`fuse()`][super::StreamExt::fuse].
+    #[derive(Debug)]
+    pub struct Fuse<T> {
+        #[pin]
+        stream: Option<T>,
+    }
+}
+
+impl<T> Fuse<T>
+where
+    T: Stream,
+{
+    pub(crate) fn new(stream: T) -> Fuse<T> {
+        Fuse {
+            stream: Some(stream),
+        }
+    }
+}
+
+impl<T> Stream for Fuse<T>
+where
+    T: Stream,
+{
+    type Item = T::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> {
+        let res = match Option::as_pin_mut(self.as_mut().project().stream) {
+            Some(stream) => ready!(stream.poll_next(cx)),
+            None => return Poll::Ready(None),
+        };
+
+        if res.is_none() {
+            // Do not poll the stream anymore
+            self.as_mut().project().stream.set(None);
+        }
+
+        Poll::Ready(res)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self.stream {
+            Some(ref stream) => stream.size_hint(),
+            None => (0, Some(0)),
+        }
+    }
+}
diff --git a/tokio/src/stream/mod.rs b/tokio/src/stream/mod.rs
index b427e3c4..a204af3c 100644
--- a/tokio/src/stream/mod.rs
+++ b/tokio/src/stream/mod.rs
@@ -16,6 +16,9 @@ use filter::Filter;
 mod filter_map;
 use filter_map::FilterMap;
 
+mod fuse;
+use fuse::Fuse;
+
 mod iter;
 pub use iter::{iter, Iter};
 
@@ -222,6 +225,71 @@ pub trait StreamExt: Stream {
         FilterMap::new(self, f)
     }
 
+    /// Creates a stream which ends after the first `None`.
+    ///
+    /// After a stream returns `None`, behavior is undefined. Future calls to
+    /// `poll_next` may or may not return `Some(T)` again or they may panic.
+    /// `fuse()` adapts a stream, ensuring that after `None` is given, it will
+    /// return `None` forever.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::stream::{Stream, StreamExt};
+    ///
+    /// use std::pin::Pin;
+    /// use std::task::{Context, Poll};
+    ///
+    /// // a stream which alternates between Some and None
+    /// struct Alternate {
+    ///     state: i32,
+    /// }
+    ///
+    /// impl Stream for Alternate {
+    ///     type Item = i32;
+    ///
+    ///     fn poll_next(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<i32>> {
+    ///         let val = self.state;
+    ///         self.state = self.state + 1;
+    ///
+    ///         // if it's even, Some(i32), else None
+    ///         if val % 2 == 0 {
+    ///             Poll::Ready(Some(val))
+    ///         } else {
+    ///             Poll::Ready(None)
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let mut stream = Alternate { state: 0 };
+    ///
+    ///     // the stream goes back and forth
+    ///     assert_eq!(stream.next().await, Some(0));
+    ///     assert_eq!(stream.next().await, None);
+    ///     assert_eq!(stream.next().await, Some(2));
+    ///     assert_eq!(stream.next().await, None);
+    ///
+    ///     // however, once it is fused
+    ///     let mut stream = stream.fuse();
+    ///
+    ///     assert_eq!(stream.next().await, Some(4));
+    ///     assert_eq!(stream.next().await, None);
+    ///
+    ///     // it will always return `None` after the first time.
+    ///     assert_eq!(stream.next().await, None);
+    ///     assert_eq!(stream.next().await, None);
+    ///     assert_eq!(stream.next().await, None);
+    /// }
+    /// ```
+    fn fuse(self) -> Fuse<Self>
+    where
+        Self: Sized,
+    {
+        Fuse::new(self)
+    }
+
     /// Creates a new stream of at most `n` items of the underlying stream.
     ///
     /// Once `n` items have been yielded from this stream then it will always