Make Once a DoubleEndedStream

Author: felipesere

src/stream/once.rs

@@ -4,6 +4,7 @@ use pin_project_lite::pin_project;
 
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
+use crate::stream::double_ended_stream::DoubleEndedStream;
 
 /// Creates a stream that yields a single item.
 ///
@@ -46,3 +47,9 @@ impl<T> Stream for Once<T> {
         Poll::Ready(self.project().value.take())
     }
 }
+
+impl <T> DoubleEndedStream for Once<T> {
+    fn poll_next_back(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        Poll::Ready(self.project().value.take())
+    }
+}