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    //! Definition of the `PollFn` combinator

    use {Stream, Poll};

    /// A stream which adapts a function returning `Poll`.
    ///
    /// Created by the `poll_fn` function.
    #[derive(Debug)]
    #[must_use = "streams do nothing unless polled"]
    pub struct PollFn<F> {
        inner: F,
    }

    /// Creates a new stream wrapping around a function returning `Poll`.
    ///
    /// Polling the returned stream delegates to the wrapped function.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::stream::poll_fn;
    /// use futures::{Async, Poll};
    ///
    /// let mut counter = 1usize;
    ///
    /// let read_stream = poll_fn(move || -> Poll<Option<String>, std::io::Error> {
    ///     if counter == 0 { return Ok(Async::Ready(None)); }
    ///     counter -= 1;
    ///     Ok(Async::Ready(Some("Hello, World!".to_owned())))
    /// });
    /// ```
    pub fn poll_fn<T, E, F>(f: F) -> PollFn<F>
    where
        F: FnMut() -> Poll<Option<T>, E>,
    {
        PollFn { inner: f }
    }

    impl<T, E, F> Stream for PollFn<F>
    where
        F: FnMut() -> Poll<Option<T>, E>,
    {
        type Item = T;
        type Error = E;

        fn poll(&mut self) -> Poll<Option<T>, E> {
            (self.inner)()
        }
    }

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