core/future/
poll_fn.rs

1use crate::fmt;
2use crate::future::Future;
3use crate::pin::Pin;
4use crate::task::{Context, Poll};
5
6/// Creates a future that wraps a function returning [`Poll`].
7///
8/// Polling the future delegates to the wrapped function. If the returned future is pinned, then the
9/// captured environment of the wrapped function is also pinned in-place, so as long as the closure
10/// does not move out of its captures it can soundly create pinned references to them.
11///
12/// # Examples
13///
14/// ```
15/// # async fn run() {
16/// use core::future::poll_fn;
17/// use std::task::{Context, Poll};
18///
19/// fn read_line(_cx: &mut Context<'_>) -> Poll<String> {
20///     Poll::Ready("Hello, World!".into())
21/// }
22///
23/// let read_future = poll_fn(read_line);
24/// assert_eq!(read_future.await, "Hello, World!".to_owned());
25/// # }
26/// ```
27///
28/// ## Capturing a pinned state
29///
30/// Example of a closure wrapping inner futures:
31///
32/// ```
33/// # async fn run() {
34/// use core::future::{self, Future};
35/// use core::task::Poll;
36///
37/// /// Resolves to the first future that completes. In the event of a tie, `a` wins.
38/// fn naive_select<T>(
39///     a: impl Future<Output = T>,
40///     b: impl Future<Output = T>,
41/// ) -> impl Future<Output = T>
42/// {
43///     let (mut a, mut b) = (Box::pin(a), Box::pin(b));
44///     future::poll_fn(move |cx| {
45///         if let Poll::Ready(r) = a.as_mut().poll(cx) {
46///             Poll::Ready(r)
47///         } else if let Poll::Ready(r) = b.as_mut().poll(cx) {
48///             Poll::Ready(r)
49///         } else {
50///             Poll::Pending
51///         }
52///     })
53/// }
54///
55/// let a = async { 42 };
56/// let b = future::pending();
57/// let v = naive_select(a, b).await;
58/// assert_eq!(v, 42);
59///
60/// let a = future::pending();
61/// let b = async { 27 };
62/// let v = naive_select(a, b).await;
63/// assert_eq!(v, 27);
64///
65/// let a = async { 42 };
66/// let b = async { 27 };
67/// let v = naive_select(a, b).await;
68/// assert_eq!(v, 42); // biased towards `a` in case of tie!
69/// # }
70/// ```
71///
72/// This time without [`Box::pin`]ning:
73///
74/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin
75///
76/// ```
77/// # async fn run() {
78/// use core::future::{self, Future};
79/// use core::pin::pin;
80/// use core::task::Poll;
81///
82/// /// Resolves to the first future that completes. In the event of a tie, `a` wins.
83/// fn naive_select<T>(
84///     a: impl Future<Output = T>,
85///     b: impl Future<Output = T>,
86/// ) -> impl Future<Output = T>
87/// {
88///     async {
89///         let (mut a, mut b) = (pin!(a), pin!(b));
90///         future::poll_fn(move |cx| {
91///             if let Poll::Ready(r) = a.as_mut().poll(cx) {
92///                 Poll::Ready(r)
93///             } else if let Poll::Ready(r) = b.as_mut().poll(cx) {
94///                 Poll::Ready(r)
95///             } else {
96///                 Poll::Pending
97///             }
98///         }).await
99///     }
100/// }
101///
102/// let a = async { 42 };
103/// let b = future::pending();
104/// let v = naive_select(a, b).await;
105/// assert_eq!(v, 42);
106/// # }
107/// ```
108///
109///   - Notice how, by virtue of being in an `async` context, we have been able to make the [`pin!`]
110///     macro work, thereby avoiding any need for the `unsafe`
111///     <code>[Pin::new_unchecked](&mut fut)</code> constructor.
112///
113/// [`pin!`]: crate::pin::pin!
114#[stable(feature = "future_poll_fn", since = "1.64.0")]
115pub fn poll_fn<T, F>(f: F) -> PollFn<F>
116where
117    F: FnMut(&mut Context<'_>) -> Poll<T>,
118{
119    PollFn { f }
120}
121
122/// A Future that wraps a function returning [`Poll`].
123///
124/// This `struct` is created by [`poll_fn()`]. See its
125/// documentation for more.
126#[must_use = "futures do nothing unless you `.await` or poll them"]
127#[stable(feature = "future_poll_fn", since = "1.64.0")]
128pub struct PollFn<F> {
129    f: F,
130}
131
132#[stable(feature = "future_poll_fn", since = "1.64.0")]
133impl<F: Unpin> Unpin for PollFn<F> {}
134
135#[stable(feature = "future_poll_fn", since = "1.64.0")]
136impl<F> fmt::Debug for PollFn<F> {
137    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
138        f.debug_struct("PollFn").finish()
139    }
140}
141
142#[stable(feature = "future_poll_fn", since = "1.64.0")]
143impl<T, F> Future for PollFn<F>
144where
145    F: FnMut(&mut Context<'_>) -> Poll<T>,
146{
147    type Output = T;
148
149    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
150        // SAFETY: We are not moving out of the pinned field.
151        (unsafe { &mut self.get_unchecked_mut().f })(cx)
152    }
153}