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    //! Definition of the `PollFn` adapter combinator

    use super::assert_future;
    use core::fmt;
    use core::pin::Pin;
    use futures_core::future::Future;
    use futures_core::task::{Context, Poll};

    /// Future for the [`poll_fn`] function.
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    pub struct PollFn<F> {
        f: F,
    }

    impl<F> Unpin for PollFn<F> {}

    /// Creates a new future wrapping around a function returning [`Poll`].
    ///
    /// Polling the returned future delegates to the wrapped function.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::future::poll_fn;
    /// use futures::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());
    /// # });
    /// ```
    pub fn poll_fn<T, F>(f: F) -> PollFn<F>
    where
        F: FnMut(&mut Context<'_>) -> Poll<T>
    {
        assert_future::<T, _>(PollFn { f })
    }

    impl<F> fmt::Debug for PollFn<F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("PollFn").finish()
        }
    }

    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)
        }
    }