Changed semantics of throttle to non-dropping variant with backpressure

examples/throttle.rs

@@ -0,0 +1,27 @@
+//! Spawns a timed task which gets throttled.
+
+fn main() {
+    #[cfg(feature = "unstable")]
+    {
+        use async_std::prelude::*;
+        use async_std::task;
+
+        task::block_on(async {
+            use async_std::stream;
+            use std::time::Duration;
+
+            // emit value every 1 second
+            let s = stream::interval(Duration::from_nanos(1000000)).enumerate();
+
+            // throttle for 2 seconds
+            let s = s.throttle(Duration::from_secs(2));
+
+            s.for_each(|(n, _)| {
+                dbg!(n);
+            })
+            .await;
+            // => 0 .. 1 .. 2 .. 3
+            // with a pause of 2 seconds between each print
+        })
+    }
+}

src/stream/stream/mod.rs

@@ -117,7 +117,6 @@ use std::time::Duration;
 cfg_unstable! {
     use std::future::Future;
     use std::pin::Pin;
-    use std::time::Duration;
 
     use crate::stream::into_stream::IntoStream;
     use crate::stream::{FromStream, Product, Sum};
@@ -316,7 +315,33 @@ extension_trait! {
             TakeWhile::new(self, predicate)
         }
 
-        fn throttle(self, d: Duration) -> Throttle<Self, Self::Item>
+        #[doc = r#"
+            Limit the amount of items yielded per timeslice in a stream.
+
+            # Examples
+            ```ignore
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::stream;
+            use std::time::Duration;
+
+            // emit value every 1 second
+            let s = stream::interval(Duration::from_nanos(1000000)).enumerate();
+
+            // throttle for 2 seconds
+            let s = s.throttle(Duration::from_secs(2));
+
+            s.for_each(|(n, _)| {
+                dbg!(n);
+            })
+            .await;
+            // => 0 .. 1 .. 2 .. 3
+            // with a pause of 2 seconds between each print
+            #
+            # }) }
+            ```
+        "#]
+        fn throttle(self, d: Duration) -> Throttle<Self>
         where
             Self: Sized,
         {

src/stream/stream/throttle.rs

@@ -7,60 +7,50 @@ use futures_timer::Delay;
 use crate::stream::Stream;
 use crate::task::{Context, Poll};
 
-/// A stream that only yields one element once every `duration`, and drops all others.
+/// A stream that only yields one element once every `duration`, and applies backpressure. Does not drop any elements.
 /// #[doc(hidden)]
 #[allow(missing_debug_implementations)]
-pub struct Throttle<S, T> {
+pub struct Throttle<S> {
     stream: S,
     duration: Duration,
     delay: Option<Delay>,
-    last: Option<T>,
 }
 
-impl<S: Unpin, T> Unpin for Throttle<S, T> {}
+impl<S: Unpin> Unpin for Throttle<S> {}
 
-impl<S: Stream> Throttle<S, S::Item> {
+impl<S: Stream> Throttle<S> {
     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, S::Item> {
+impl<S: Stream> Stream for Throttle<S> {
     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));
-                } else {
                 *self.as_mut().delay() = None;
-                }
+            } else {
+                return Poll::Pending;
             }
         }
 
         match self.as_mut().stream().poll_next(cx) {
-            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);
+            Poll::Pending => {
                 cx.waker().wake_by_ref(); // Continue driving even though emitting Pending
-                    return Poll::Pending;
+                Poll::Pending
             }
-
+            Poll::Ready(None) => Poll::Ready(None),
+            Poll::Ready(Some(v)) => {
                 *self.as_mut().delay() = Some(Delay::new(self.duration));
                 Poll::Ready(Some(v))
             }