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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-30 03:57:31 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-30 03:57:31 +0000 |
commit | dc0db358abe19481e475e10c32149b53370f1a1c (patch) | |
tree | ab8ce99c4b255ce46f99ef402c27916055b899ee /library/core/src/future | |
parent | Releasing progress-linux version 1.71.1+dfsg1-2~progress7.99u1. (diff) | |
download | rustc-dc0db358abe19481e475e10c32149b53370f1a1c.tar.xz rustc-dc0db358abe19481e475e10c32149b53370f1a1c.zip |
Merging upstream version 1.72.1+dfsg1.
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'library/core/src/future')
-rw-r--r-- | library/core/src/future/poll_fn.rs | 87 |
1 files changed, 87 insertions, 0 deletions
diff --git a/library/core/src/future/poll_fn.rs b/library/core/src/future/poll_fn.rs index 90cb79739..d27a9dfc1 100644 --- a/library/core/src/future/poll_fn.rs +++ b/library/core/src/future/poll_fn.rs @@ -24,6 +24,93 @@ use crate::task::{Context, Poll}; /// assert_eq!(read_future.await, "Hello, World!".to_owned()); /// # } /// ``` +/// +/// ## Capturing a pinned state +/// +/// Example of a closure wrapping inner futures: +/// +/// ``` +/// # async fn run() { +/// use core::future::{self, Future}; +/// use core::task::Poll; +/// +/// /// Resolves to the first future that completes. In the event of a tie, `a` wins. +/// fn naive_select<T>( +/// a: impl Future<Output = T>, +/// b: impl Future<Output = T>, +/// ) -> impl Future<Output = T> +/// { +/// let (mut a, mut b) = (Box::pin(a), Box::pin(b)); +/// future::poll_fn(move |cx| { +/// if let Poll::Ready(r) = a.as_mut().poll(cx) { +/// Poll::Ready(r) +/// } else if let Poll::Ready(r) = b.as_mut().poll(cx) { +/// Poll::Ready(r) +/// } else { +/// Poll::Pending +/// } +/// }) +/// } +/// +/// let a = async { 42 }; +/// let b = future::pending(); +/// let v = naive_select(a, b).await; +/// assert_eq!(v, 42); +/// +/// let a = future::pending(); +/// let b = async { 27 }; +/// let v = naive_select(a, b).await; +/// assert_eq!(v, 27); +/// +/// let a = async { 42 }; +/// let b = async { 27 }; +/// let v = naive_select(a, b).await; +/// assert_eq!(v, 42); // biased towards `a` in case of tie! +/// # } +/// ``` +/// +/// This time without [`Box::pin`]ning: +/// +/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin +/// +/// ``` +/// # async fn run() { +/// use core::future::{self, Future}; +/// use core::pin::pin; +/// use core::task::Poll; +/// +/// /// Resolves to the first future that completes. In the event of a tie, `a` wins. +/// fn naive_select<T>( +/// a: impl Future<Output = T>, +/// b: impl Future<Output = T>, +/// ) -> impl Future<Output = T> +/// { +/// async { +/// let (mut a, mut b) = (pin!(a), pin!(b)); +/// future::poll_fn(move |cx| { +/// if let Poll::Ready(r) = a.as_mut().poll(cx) { +/// Poll::Ready(r) +/// } else if let Poll::Ready(r) = b.as_mut().poll(cx) { +/// Poll::Ready(r) +/// } else { +/// Poll::Pending +/// } +/// }).await +/// } +/// } +/// +/// let a = async { 42 }; +/// let b = future::pending(); +/// let v = naive_select(a, b).await; +/// assert_eq!(v, 42); +/// # } +/// ``` +/// +/// - Notice how, by virtue of being in an `async` context, we have been able to make the [`pin!`] +/// macro work, thereby avoiding any need for the `unsafe` +/// <code>[Pin::new_unchecked](&mut fut)</code> constructor. +/// +/// [`pin!`]: crate::pin::pin! #[stable(feature = "future_poll_fn", since = "1.64.0")] pub fn poll_fn<T, F>(f: F) -> PollFn<F> where |