1use std::future::Future;
2use std::pin::Pin;
3
4use crate::task::{Context, Poll};
5
6/// Cooperatively gives up a timeslice to the task scheduler.
7///
8/// Calling this function will move the currently executing future to the back
9/// of the execution queue, making room for other futures to execute. This is
10/// especially useful after running CPU-intensive operations inside a future.
11///
12/// See also [`task::spawn_blocking`].
13///
14/// [`task::spawn_blocking`]: fn.spawn_blocking.html
15///
16/// # Examples
17///
18/// Basic usage:
19///
20/// ```
21/// # async_std::task::block_on(async {
22/// #
23/// use async_std::task;
24///
25/// task::yield_now().await;
26/// #
27/// # })
28/// ```
29#[inline]
30pub async fn yield_now() {
31 YieldNow(false).await
32}
33
34struct YieldNow(bool);
35
36impl Future for YieldNow {
37 type Output = ();
38
39 // The futures executor is implemented as a FIFO queue, so all this future
40 // does is re-schedule the future back to the end of the queue, giving room
41 // for other futures to progress.
42 fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
43 if !self.0 {
44 self.0 = true;
45 cx.waker().wake_by_ref();
46 Poll::Pending
47 } else {
48 Poll::Ready(())
49 }
50 }
51}
52