Re-implemented Throttle to keep last value in memory

src/stream/stream/mod.rs

@@ -281,7 +281,7 @@ extension_trait! {
             TakeWhile::new(self, predicate)
         }
 
-        fn throttle(self, d: Duration) -> Throttle<Self>
+        fn throttle(self, d: Duration) -> Throttle<Self, Self::Item>
         where
             Self: Sized,
         {

src/stream/stream/throttle.rs

@@ -10,47 +10,58 @@ use crate::task::{Context, Poll};
 /// A stream that only yields one element once every `duration`, and drops all others.
 /// #[doc(hidden)]
 #[allow(missing_debug_implementations)]
-pub struct Throttle<S> {
+pub struct Throttle<S, T> {
     stream: S,
     duration: Duration,
     delay: Option<Delay>,
+    last: Option<T>,
 }
 
-impl<S: Unpin> Unpin for Throttle<S> {}
+impl<S: Unpin, T> Unpin for Throttle<S, T> {}
 
-impl<S: Stream> Throttle<S> {
+impl<S: Stream> Throttle<S, S::Item> {
     pin_utils::unsafe_pinned!(stream: S);
     pin_utils::unsafe_unpinned!(duration: Duration);
     pin_utils::unsafe_pinned!(delay: Option<Delay>);
+    pin_utils::unsafe_unpinned!(last: Option<S::Item>);
 
     pub(super) fn new(stream: S, duration: Duration) -> Self {
         Throttle {
             stream,
             duration,
             delay: None,
+            last: None,
         }
     }
 }
 
-impl<S: Stream> Stream for Throttle<S> {
+impl<S: Stream> Stream for Throttle<S, S::Item> {
     type Item = S::Item;
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<S::Item>> {
+        if let Some(d) = self.as_mut().delay().as_pin_mut() {
+            if d.poll(cx).is_ready() {
+                if let Some(v) = self.as_mut().last().take() {
+                    // Sets last to None.
+                    *self.as_mut().delay() = Some(Delay::new(self.duration));
+                    return Poll::Ready(Some(v));
+                }
+            }
+        }
+
         match self.as_mut().stream().poll_next(cx) {
-            Poll::Ready(v) => match self.as_mut().delay().as_pin_mut() {
-                None => {
-                    *self.as_mut().delay() = Some(Delay::new(self.duration));
-                    Poll::Ready(v)
-                }
-                Some(d) => match d.poll(cx) {
-                    Poll::Ready(_) => {
-                        *self.as_mut().delay() = Some(Delay::new(self.duration));
-                        Poll::Ready(v)
-                    }
-                    Poll::Pending => Poll::Pending,
-                },
-            },
             Poll::Pending => Poll::Pending,
+            Poll::Ready(None) => return Poll::Ready(None),
+            Poll::Ready(Some(v)) => {
+                if self.as_mut().delay().is_some() {
+                    *self.as_mut().last() = Some(v);
+                    cx.waker().wake_by_ref(); // Continue driving even though emitting Pending
+                    return Poll::Pending;
+                }
+
+                *self.as_mut().delay() = Some(Delay::new(self.duration));
+                Poll::Ready(Some(v))
+            }
         }
     }
 }