New poll_immediate functions to immediately return from a poll

futures-util/src/future/mod.rs

@@ -68,6 +68,9 @@ pub use self::option::OptionFuture;
 mod poll_fn;
 pub use self::poll_fn::{poll_fn, PollFn};
 
+mod poll_immediate;
+pub use self::poll_immediate::{poll_immediate, PollImmediate};
+
 mod ready;
 pub use self::ready::{err, ok, ready, Ready};
 

futures-util/src/future/poll_immediate.rs

@@ -0,0 +1,126 @@
+use super::assert_future;
+use core::pin::Pin;
+use futures_core::task::{Context, Poll};
+use futures_core::{FusedFuture, Future, Stream};
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Future for the [`poll_immediate`](poll_immediate()) function.
+    ///
+    /// It will never return [Poll::Pending](core::task::Poll::Pending)
+    #[derive(Debug, Clone)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct PollImmediate<T> {
+        #[pin]
+        future: Option<T>
+    }
+}
+
+impl<T, F> Future for PollImmediate<F>
+where
+    F: Future<Output = T>,
+{
+    type Output = Option<T>;
+
+    #[inline]
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        let mut this = self.project();
+        let inner =
+            this.future.as_mut().as_pin_mut().expect("PollImmediate polled after completion");
+        match inner.poll(cx) {
+            Poll::Ready(t) => {
+                this.future.set(None);
+                Poll::Ready(Some(t))
+            }
+            Poll::Pending => Poll::Ready(None),
+        }
+    }
+}
+
+impl<T: Future> FusedFuture for PollImmediate<T> {
+    fn is_terminated(&self) -> bool {
+        self.future.is_none()
+    }
+}
+
+/// A [Stream](crate::stream::Stream) implementation that can be polled repeatedly until the future is done.
+/// The stream will never return [Poll::Pending](core::task::Poll::Pending)
+/// so polling it in a tight loop is worse than using a blocking synchronous function.
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::task::Poll;
+/// use futures::{StreamExt, future, pin_mut};
+/// use future::FusedFuture;
+///
+/// let f = async { 1_u32 };
+/// pin_mut!(f);
+/// let mut r = future::poll_immediate(f);
+/// assert_eq!(r.next().await, Some(Poll::Ready(1)));
+///
+/// let f = async {futures::pending!(); 42_u8};
+/// pin_mut!(f);
+/// let mut p = future::poll_immediate(f);
+/// assert_eq!(p.next().await, Some(Poll::Pending));
+/// assert!(!p.is_terminated());
+/// assert_eq!(p.next().await, Some(Poll::Ready(42)));
+/// assert!(p.is_terminated());
+/// assert_eq!(p.next().await, None);
+/// # });
+/// ```
+impl<T, F> Stream for PollImmediate<F>
+where
+    F: Future<Output = T>,
+{
+    type Item = Poll<T>;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.project();
+        match this.future.as_mut().as_pin_mut() {
+            // inner is gone, so we can signal that the stream is closed.
+            None => Poll::Ready(None),
+            Some(fut) => Poll::Ready(Some(fut.poll(cx).map(|t| {
+                this.future.set(None);
+                t
+            }))),
+        }
+    }
+}
+
+/// Creates a future that is immediately ready with an Option of a value.
+/// Specifically this means that [poll](core::future::Future::poll()) always returns [Poll::Ready](core::task::Poll::Ready).
+///
+/// # Caution
+///
+/// When consuming the future by this function, note the following:
+///
+/// - This function does not guarantee that the future will run to completion, so it is generally incompatible with passing the non-cancellation-safe future by value.
+/// - Even if the future is cancellation-safe, creating and dropping new futures frequently may lead to performance problems.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::future;
+///
+/// let r = future::poll_immediate(async { 1_u32 });
+/// assert_eq!(r.await, Some(1));
+///
+/// let p = future::poll_immediate(future::pending::<i32>());
+/// assert_eq!(p.await, None);
+/// # });
+/// ```
+///
+/// ### Reusing a future
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::{future, pin_mut};
+/// let f = async {futures::pending!(); 42_u8};
+/// pin_mut!(f);
+/// assert_eq!(None, future::poll_immediate(&mut f).await);
+/// assert_eq!(42, f.await);
+/// # });
+/// ```
+pub fn poll_immediate<F: Future>(f: F) -> PollImmediate<F> {
+    assert_future::<Option<F::Output>, PollImmediate<F>>(PollImmediate { future: Some(f) })
+}

futures-util/src/stream/mod.rs

@@ -89,6 +89,9 @@ pub use self::pending::{pending, Pending};
 mod poll_fn;
 pub use self::poll_fn::{poll_fn, PollFn};
 
+mod poll_immediate;
+pub use self::poll_immediate::{poll_immediate, PollImmediate};
+
 mod select;
 pub use self::select::{select, Select};
 

futures-util/src/stream/poll_immediate.rs

@@ -0,0 +1,80 @@
+use core::pin::Pin;
+use futures_core::task::{Context, Poll};
+use futures_core::Stream;
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// Stream for the [poll_immediate](poll_immediate()) function.
+    ///
+    /// It will never return [Poll::Pending](core::task::Poll::Pending)
+    #[derive(Debug, Clone)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct PollImmediate<S> {
+        #[pin]
+        stream: Option<S>
+    }
+}
+
+impl<T, S> Stream for PollImmediate<S>
+where
+    S: Stream<Item = T>,
+{
+    type Item = Poll<T>;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.project();
+        let stream = match this.stream.as_mut().as_pin_mut() {
+            // inner is gone, so we can continue to signal that the stream is closed.
+            None => return Poll::Ready(None),
+            Some(inner) => inner,
+        };
+
+        match stream.poll_next(cx) {
+            Poll::Ready(Some(t)) => Poll::Ready(Some(Poll::Ready(t))),
+            Poll::Ready(None) => {
+                this.stream.set(None);
+                Poll::Ready(None)
+            }
+            Poll::Pending => Poll::Ready(Some(Poll::Pending)),
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.stream.as_ref().map_or((0, Some(0)), Stream::size_hint)
+    }
+}
+
+impl<S: Stream> super::FusedStream for PollImmediate<S> {
+    fn is_terminated(&self) -> bool {
+        self.stream.is_none()
+    }
+}
+
+/// Creates a new stream that always immediately returns [Poll::Ready](core::task::Poll::Ready) when awaiting it.
+///
+/// This is useful when immediacy is more important than waiting for the next item to be ready.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::stream::{self, StreamExt};
+/// use futures::task::Poll;
+///
+/// let mut r = stream::poll_immediate(Box::pin(stream::iter(1_u32..3)));
+/// assert_eq!(r.next().await, Some(Poll::Ready(1)));
+/// assert_eq!(r.next().await, Some(Poll::Ready(2)));
+/// assert_eq!(r.next().await, None);
+///
+/// let mut p = stream::poll_immediate(Box::pin(stream::once(async {
+///     futures::pending!();
+///     42_u8
+/// })));
+/// assert_eq!(p.next().await, Some(Poll::Pending));
+/// assert_eq!(p.next().await, Some(Poll::Ready(42)));
+/// assert_eq!(p.next().await, None);
+/// # });
+/// ```
+pub fn poll_immediate<S: Stream>(s: S) -> PollImmediate<S> {
+    super::assert_stream::<Poll<S::Item>, PollImmediate<S>>(PollImmediate { stream: Some(s) })
+}