Merge pull request #615 from lqf96/pending-stream

Add an implementation of pending stream

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 +1,68 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+use crate::stream::{DoubleEndedStream, ExactSizeStream, FusedStream, Stream};
+
+/// 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.
+/// # Examples
+///
+/// ```
+/// # async_std::task::block_on(async {
+/// #
+/// use std::time::Duration;
+///
+/// use async_std::prelude::*;
+/// use async_std::stream;
+///
+/// let dur = Duration::from_millis(100);
+/// let mut s = stream::pending::<()>().timeout(dur);
+///
+/// let item = s.next().await;
+///
+/// assert!(item.is_some());
+/// assert!(item.unwrap().is_err());
+///
+/// #
+/// # })
+/// ```
+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
+    }
+}
+
+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
+    }
+}