Stream::size_hint for mpsc channels (#2660)

futures-channel/src/mpsc/mod.rs

@@ -1078,6 +1078,14 @@ impl<T> Stream for Receiver<T> {
  }
  }
  }
+
+ fn size_hint(&self) -> (usize, Option<usize>) {
+ if let Some(inner) = &self.inner {
+ decode_state(inner.state.load(SeqCst)).size_hint()
+ } else {
+ (0, Some(0))
+ }
+ }
 }
 
 impl<T> Drop for Receiver<T> {
@@ -1222,6 +1230,14 @@ impl<T> Stream for UnboundedReceiver<T> {
  }
  }
  }
+
+ fn size_hint(&self) -> (usize, Option<usize>) {
+ if let Some(inner) = &self.inner {
+ decode_state(inner.state.load(SeqCst)).size_hint()
+ } else {
+ (0, Some(0))
+ }
+ }
 }
 
 impl<T> Drop for UnboundedReceiver<T> {
@@ -1312,6 +1328,14 @@ impl State {
  fn is_closed(&self) -> bool {
  !self.is_open && self.num_messages == 0
  }
+
+ fn size_hint(&self) -> (usize, Option<usize>) {
+ if self.is_open {
+ (self.num_messages, None)
+ } else {
+ (self.num_messages, Some(self.num_messages))
+ }
+ }
 }
 
 /*

futures-channel/tests/mpsc-size_hint.rs

@@ -0,0 +1,40 @@
+use futures::channel::mpsc;
+use futures::stream::Stream;
+
+#[test]
+fn unbounded_size_hint() {
+ let (tx, mut rx) = mpsc::unbounded::<u32>();
+ assert_eq!((0, None), rx.size_hint());
+ tx.unbounded_send(1).unwrap();
+ assert_eq!((1, None), rx.size_hint());
+ rx.try_next().unwrap().unwrap();
+ assert_eq!((0, None), rx.size_hint());
+ tx.unbounded_send(2).unwrap();
+ tx.unbounded_send(3).unwrap();
+ assert_eq!((2, None), rx.size_hint());
+ drop(tx);
+ assert_eq!((2, Some(2)), rx.size_hint());
+ rx.try_next().unwrap().unwrap();
+ assert_eq!((1, Some(1)), rx.size_hint());
+ rx.try_next().unwrap().unwrap();
+ assert_eq!((0, Some(0)), rx.size_hint());
+}
+
+#[test]
+fn channel_size_hint() {
+ let (mut tx, mut rx) = mpsc::channel::<u32>(10);
+ assert_eq!((0, None), rx.size_hint());
+ tx.try_send(1).unwrap();
+ assert_eq!((1, None), rx.size_hint());
+ rx.try_next().unwrap().unwrap();
+ assert_eq!((0, None), rx.size_hint());
+ tx.try_send(2).unwrap();
+ tx.try_send(3).unwrap();
+ assert_eq!((2, None), rx.size_hint());
+ drop(tx);
+ assert_eq!((2, Some(2)), rx.size_hint());
+ rx.try_next().unwrap().unwrap();
+ assert_eq!((1, Some(1)), rx.size_hint());
+ rx.try_next().unwrap().unwrap();
+ assert_eq!((0, Some(0)), rx.size_hint());
+}