mirror of https://github.com/rust-lang/rust
154 lines
4.3 KiB
Rust
154 lines
4.3 KiB
Rust
use crate::fmt;
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use crate::future::Future;
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use crate::pin::Pin;
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use crate::task::{Context, Poll};
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/// Creates a future that wraps a function returning [`Poll`].
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///
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/// Polling the future delegates to the wrapped function. If the returned future is pinned, then the
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/// captured environment of the wrapped function is also pinned in-place, so as long as the closure
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/// does not move out of its captures it can soundly create pinned references to them.
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///
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/// # Examples
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///
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/// ```
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/// # async fn run() {
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/// use core::future::poll_fn;
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/// use std::task::{Context, Poll};
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///
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/// fn read_line(_cx: &mut Context<'_>) -> Poll<String> {
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/// Poll::Ready("Hello, World!".into())
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/// }
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///
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/// let read_future = poll_fn(read_line);
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/// assert_eq!(read_future.await, "Hello, World!".to_owned());
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/// # }
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/// ```
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///
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/// ## Capturing a pinned state
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///
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/// Example of a closure wrapping inner futures:
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///
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/// ```
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/// # async fn run() {
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/// use core::future::{self, Future};
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/// use core::task::Poll;
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///
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/// /// Resolves to the first future that completes. In the event of a tie, `a` wins.
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/// fn naive_select<T>(
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/// a: impl Future<Output = T>,
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/// b: impl Future<Output = T>,
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/// ) -> impl Future<Output = T>
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/// {
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/// let (mut a, mut b) = (Box::pin(a), Box::pin(b));
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/// future::poll_fn(move |cx| {
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/// if let Poll::Ready(r) = a.as_mut().poll(cx) {
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/// Poll::Ready(r)
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/// } else if let Poll::Ready(r) = b.as_mut().poll(cx) {
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/// Poll::Ready(r)
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/// } else {
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/// Poll::Pending
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/// }
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/// })
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/// }
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///
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/// let a = async { 42 };
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/// let b = future::pending();
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/// let v = naive_select(a, b).await;
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/// assert_eq!(v, 42);
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///
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/// let a = future::pending();
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/// let b = async { 27 };
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/// let v = naive_select(a, b).await;
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/// assert_eq!(v, 27);
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///
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/// let a = async { 42 };
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/// let b = async { 27 };
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/// let v = naive_select(a, b).await;
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/// assert_eq!(v, 42); // biased towards `a` in case of tie!
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/// # }
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/// ```
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///
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/// This time without [`Box::pin`]ning:
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///
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/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin
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///
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/// ```
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/// # async fn run() {
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/// use core::future::{self, Future};
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/// use core::pin::pin;
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/// use core::task::Poll;
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///
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/// /// Resolves to the first future that completes. In the event of a tie, `a` wins.
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/// fn naive_select<T>(
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/// a: impl Future<Output = T>,
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/// b: impl Future<Output = T>,
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/// ) -> impl Future<Output = T>
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/// {
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/// async {
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/// let (mut a, mut b) = (pin!(a), pin!(b));
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/// future::poll_fn(move |cx| {
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/// if let Poll::Ready(r) = a.as_mut().poll(cx) {
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/// Poll::Ready(r)
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/// } else if let Poll::Ready(r) = b.as_mut().poll(cx) {
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/// Poll::Ready(r)
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/// } else {
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/// Poll::Pending
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/// }
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/// }).await
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/// }
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/// }
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///
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/// let a = async { 42 };
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/// let b = future::pending();
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/// let v = naive_select(a, b).await;
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/// assert_eq!(v, 42);
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/// # }
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/// ```
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///
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/// - Notice how, by virtue of being in an `async` context, we have been able to make the [`pin!`]
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/// macro work, thereby avoiding any need for the `unsafe`
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/// <code>[Pin::new_unchecked](&mut fut)</code> constructor.
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///
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/// [`pin!`]: crate::pin::pin!
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#[stable(feature = "future_poll_fn", since = "1.64.0")]
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pub fn poll_fn<T, F>(f: F) -> PollFn<F>
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where
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F: FnMut(&mut Context<'_>) -> Poll<T>,
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{
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PollFn { f }
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}
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/// A Future that wraps a function returning [`Poll`].
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///
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/// This `struct` is created by [`poll_fn()`]. See its
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/// documentation for more.
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#[must_use = "futures do nothing unless you `.await` or poll them"]
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#[stable(feature = "future_poll_fn", since = "1.64.0")]
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pub struct PollFn<F> {
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f: F,
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}
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#[stable(feature = "future_poll_fn", since = "1.64.0")]
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impl<F: Unpin> Unpin for PollFn<F> {}
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#[stable(feature = "future_poll_fn", since = "1.64.0")]
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impl<F> fmt::Debug for PollFn<F> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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f.debug_struct("PollFn").finish()
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}
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}
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#[stable(feature = "future_poll_fn", since = "1.64.0")]
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impl<T, F> Future for PollFn<F>
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where
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F: FnMut(&mut Context<'_>) -> Poll<T>,
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{
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type Output = T;
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fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
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// SAFETY: We are not moving out of the pinned field.
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(unsafe { &mut self.get_unchecked_mut().f })(cx)
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}
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}
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