Add stream::pending::{pending, Pending}

Author: lqf96

src/stream/mod.rs

@@ -325,6 +325,7 @@ cfg_unstable! {
     mod fused_stream;
     mod interval;
     mod into_stream;
+    mod pending;
     mod product;
     mod successors;
     mod sum;
@@ -336,6 +337,7 @@ cfg_unstable! {
     pub use fused_stream::FusedStream;
     pub use interval::{interval, Interval};
     pub use into_stream::IntoStream;
+    pub use pending::{pending, Pending};
     pub use product::Product;
     pub use stream::Merge;
     pub use successors::{successors, Successors};

src/stream/pending.rs

@@ -0,0 +1,49 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+use crate::stream::{Stream, DoubleEndedStream, ExactSizeStream, FusedStream};
+
+/// A stream that never returns any items.
+/// 
+/// This stream is created by the [`pending`] function. See its
+/// documentation for more.
+/// 
+/// [`pending`]: fn.pending.html
+#[derive(Debug)]
+pub struct Pending<T> {
+    _marker: PhantomData<T>
+}
+
+/// Creates a stream that never returns any items.
+/// 
+/// The returned stream will always return `Pending` when polled.
+pub fn pending<T>() -> Pending<T> {
+    Pending { _marker: PhantomData }
+}
+
+impl<T> Stream for Pending<T> {
+    type Item = T;
+
+    fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<T>> {
+        Poll::Pending
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
+}
+
+impl<T> DoubleEndedStream for Pending<T> {
+    fn poll_next_back(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<T>> {
+        Poll::Pending
+    }
+}
+
+impl<T> FusedStream for Pending<T> {}
+
+impl<T> ExactSizeStream for Pending<T> {
+    fn len(&self) -> usize {
+        0
+    }
+}