1use super::assert_future;
2use core::pin::Pin;
3use futures_core::task::{Context, Poll};
4use futures_core::{FusedFuture, Future, Stream};
5use pin_project_lite::pin_project;
6
7pin_project! {
8 /// Future for the [`poll_immediate`](poll_immediate()) function.
9 ///
10 /// It will never return [Poll::Pending](core::task::Poll::Pending)
11 #[derive(Debug, Clone)]
12 #[must_use = "futures do nothing unless you `.await` or poll them"]
13 pub struct PollImmediate<T> {
14 #[pin]
15 future: Option<T>
16 }
17}
18
19impl<T, F> Future for PollImmediate<F>
20where
21 F: Future<Output = T>,
22{
23 type Output = Option<T>;
24
25 #[inline]
26 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
27 let mut this = self.project();
28 let inner =
29 this.future.as_mut().as_pin_mut().expect("PollImmediate polled after completion");
30 match inner.poll(cx) {
31 Poll::Ready(t) => {
32 this.future.set(None);
33 Poll::Ready(Some(t))
34 }
35 Poll::Pending => Poll::Ready(None),
36 }
37 }
38}
39
40impl<T: Future> FusedFuture for PollImmediate<T> {
41 fn is_terminated(&self) -> bool {
42 self.future.is_none()
43 }
44}
45
46/// A [Stream](crate::stream::Stream) implementation that can be polled repeatedly until the future is done.
47/// The stream will never return [Poll::Pending](core::task::Poll::Pending)
48/// so polling it in a tight loop is worse than using a blocking synchronous function.
49/// ```
50/// # futures::executor::block_on(async {
51/// use futures::task::Poll;
52/// use futures::{StreamExt, future, pin_mut};
53/// use future::FusedFuture;
54///
55/// let f = async { 1_u32 };
56/// pin_mut!(f);
57/// let mut r = future::poll_immediate(f);
58/// assert_eq!(r.next().await, Some(Poll::Ready(1)));
59///
60/// let f = async {futures::pending!(); 42_u8};
61/// pin_mut!(f);
62/// let mut p = future::poll_immediate(f);
63/// assert_eq!(p.next().await, Some(Poll::Pending));
64/// assert!(!p.is_terminated());
65/// assert_eq!(p.next().await, Some(Poll::Ready(42)));
66/// assert!(p.is_terminated());
67/// assert_eq!(p.next().await, None);
68/// # });
69/// ```
70impl<T, F> Stream for PollImmediate<F>
71where
72 F: Future<Output = T>,
73{
74 type Item = Poll<T>;
75
76 fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
77 let mut this = self.project();
78 match this.future.as_mut().as_pin_mut() {
79 // inner is gone, so we can signal that the stream is closed.
80 None => Poll::Ready(None),
81 Some(fut) => Poll::Ready(Some(fut.poll(cx).map(|t| {
82 this.future.set(None);
83 t
84 }))),
85 }
86 }
87}
88
89/// Creates a future that is immediately ready with an Option of a value.
90/// Specifically this means that [poll](core::future::Future::poll()) always returns [Poll::Ready](core::task::Poll::Ready).
91///
92/// # Caution
93///
94/// When consuming the future by this function, note the following:
95///
96/// - 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.
97/// - Even if the future is cancellation-safe, creating and dropping new futures frequently may lead to performance problems.
98///
99/// # Examples
100///
101/// ```
102/// # futures::executor::block_on(async {
103/// use futures::future;
104///
105/// let r = future::poll_immediate(async { 1_u32 });
106/// assert_eq!(r.await, Some(1));
107///
108/// let p = future::poll_immediate(future::pending::<i32>());
109/// assert_eq!(p.await, None);
110/// # });
111/// ```
112///
113/// ### Reusing a future
114///
115/// ```
116/// # futures::executor::block_on(async {
117/// use futures::{future, pin_mut};
118/// let f = async {futures::pending!(); 42_u8};
119/// pin_mut!(f);
120/// assert_eq!(None, future::poll_immediate(&mut f).await);
121/// assert_eq!(42, f.await);
122/// # });
123/// ```
124pub fn poll_immediate<F: Future>(f: F) -> PollImmediate<F> {
125 assert_future::<Option<F::Output>, PollImmediate<F>>(PollImmediate { future: Some(f) })
126}
127