Only implement the DoubleEndedStream for once when the flag is on

src/stream/once.rs

@@ -4,7 +4,9 @@ use pin_project_lite::pin_project;
 
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
-use crate::stream::double_ended_stream::DoubleEndedStream;
+
+#[cfg(feature = "unstable")]
+use crate::stream::DoubleEndedStream;
 
 /// Creates a stream that yields a single item.
 ///
@@ -48,6 +50,7 @@ impl<T> Stream for Once<T> {
     }
 }
 
+#[cfg(feature = "unstable")]
 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())