poll_fn.rs.html -- source
Description: Source to the Rust file `src/libcore/future/poll_fn.rs`.
Keywords: rust, rustlang, rust-lang

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    use crate::fmt;
    use crate::future::Future;
    use crate::pin::Pin;
    use crate::task::{Context, Poll};

    /// Creates a future that wraps a function returning `Poll`.
    ///
    /// Polling the future delegates to the wrapped function.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(future_poll_fn)]
    /// # async fn run() {
    /// use core::future::poll_fn;
    /// use core::task::{Context, Poll};
    ///
    /// fn read_line(_cx: &mut Context<'_>) -> Poll<String> {
    ///     Poll::Ready("Hello, World!".into())
    /// }
    ///
    /// let read_future = poll_fn(read_line);
    /// assert_eq!(read_future.await, "Hello, World!".to_owned());
    /// # };
    /// ```
    #[unstable(feature = "future_poll_fn", issue = "72302")]
    pub fn poll_fn<T, F>(f: F) -> PollFn<F>
    where
        F: FnMut(&mut Context<'_>) -> Poll<T>,
    {
        PollFn { f }
    }

    /// A Future that wraps a function returning `Poll`.
    ///
    /// This `struct` is created by the [`poll_fn`] function. See its
    /// documentation for more.
    ///
    /// [`poll_fn`]: fn.poll_fn.html
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    #[unstable(feature = "future_poll_fn", issue = "72302")]
    pub struct PollFn<F> {
        f: F,
    }

    #[unstable(feature = "future_poll_fn", issue = "72302")]
    impl<F> Unpin for PollFn<F> {}

    #[unstable(feature = "future_poll_fn", issue = "72302")]
    impl<F> fmt::Debug for PollFn<F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("PollFn").finish()
        }
    }

    #[unstable(feature = "future_poll_fn", issue = "72302")]
    impl<T, F> Future for PollFn<F>
    where
        F: FnMut(&mut Context<'_>) -> Poll<T>,
    {
        type Output = T;

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
            (&mut self.f)(cx)
        }
    }