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    //! Definition of the `PollFn` combinator

    use super::assert_stream;
    use core::fmt;
    use core::pin::Pin;
    use futures_core::stream::Stream;
    use futures_core::task::{Context, Poll};

    /// Stream for the [`poll_fn`] function.
    #[must_use = "streams do nothing unless polled"]
    pub struct PollFn<F> {
        f: F,
    }

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

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

    /// Creates a new stream wrapping a function returning `Poll<Option<T>>`.
    ///
    /// Polling the returned stream calls the wrapped function.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::stream::poll_fn;
    /// use futures::task::Poll;
    ///
    /// let mut counter = 1usize;
    ///
    /// let read_stream = poll_fn(move |_| -> Poll<Option<String>> {
    ///     if counter == 0 { return Poll::Ready(None); }
    ///     counter -= 1;
    ///     Poll::Ready(Some("Hello, World!".to_owned()))
    /// });
    /// ```
    pub fn poll_fn<T, F>(f: F) -> PollFn<F>
    where
        F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
    {
        assert_stream::<T, _>(PollFn { f })
    }

    impl<T, F> Stream for PollFn<F>
    where
        F: FnMut(&mut Context<'_>) -> Poll<Option<T>>,
    {
        type Item = T;

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