implement DoubleEndedStream

Signed-off-by: Yoshua Wuyts <yoshuawuyts@gmail.com>

src/stream/double_ended_stream.rs

@@ -0,0 +1,22 @@
+use crate::stream::Stream;
+
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// An stream able to yield elements from both ends.
+///
+/// Something that implements `DoubleEndedStream` has one extra capability
+/// over something that implements [`Stream`]: the ability to also take
+/// `Item`s from the back, as well as the front.
+///
+/// [`Stream`]: trait.Stream.html
+pub trait DoubleEndedStream: Stream {
+    /// Removes and returns an element from the end of the stream.
+    ///
+    /// Returns `None` when there are no more elements.
+    ///
+    /// The [trait-level] docs contain more details.
+    ///
+    /// [trait-level]: trait.DoubleEndedStream.html
+    fn poll_next_back(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>;
+}

src/stream/mod.rs

@@ -25,8 +25,10 @@ pub use empty::{empty, Empty};
 pub use once::{once, Once};
 pub use repeat::{repeat, Repeat};
 pub use stream::{Stream, Take};
+pub use double_ended_stream::DoubleEndedStream;
 
 mod empty;
 mod once;
 mod repeat;
 mod stream;
+mod double_ended_stream;