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    use {Async, Future, Poll};
    use core::fmt;
    use stream::Stream;

    /// Future for the `flatten_stream` combinator, flattening a
    /// future-of-a-stream to get just the result of the final stream as a stream.
    ///
    /// This is created by the `Future::flatten_stream` method.
    #[must_use = "streams do nothing unless polled"]
    pub struct FlattenStream<F>
        where F: Future,
              <F as Future>::Item: Stream<Error=F::Error>,
    {
        state: State<F>
    }

    impl<F> fmt::Debug for FlattenStream<F>
        where F: Future + fmt::Debug,
              <F as Future>::Item: Stream<Error=F::Error> + fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("FlattenStream")
                .field("state", &self.state)
                .finish()
        }
    }

    pub fn new<F>(f: F) -> FlattenStream<F>
        where F: Future,
              <F as Future>::Item: Stream<Error=F::Error>,
    {
        FlattenStream {
            state: State::Future(f)
        }
    }

    #[derive(Debug)]
    enum State<F>
        where F: Future,
              <F as Future>::Item: Stream<Error=F::Error>,
    {
        // future is not yet called or called and not ready
        Future(F),
        // future resolved to Stream
        Stream(F::Item),
        // EOF after future resolved to error
        Eof,
        // after EOF after future resolved to error
        Done,
    }

    impl<F> Stream for FlattenStream<F>
        where F: Future,
              <F as Future>::Item: Stream<Error=F::Error>,
    {
        type Item = <F::Item as Stream>::Item;
        type Error = <F::Item as Stream>::Error;

        fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
            loop {
                let (next_state, ret_opt) = match self.state {
                    State::Future(ref mut f) => {
                        match f.poll() {
                            Ok(Async::NotReady) => {
                                // State is not changed, early return.
                                return Ok(Async::NotReady)
                            },
                            Ok(Async::Ready(stream)) => {
                                // Future resolved to stream.
                                // We do not return, but poll that
                                // stream in the next loop iteration.
                                (State::Stream(stream), None)
                            }
                            Err(e) => {
                                (State::Eof, Some(Err(e)))
                            }
                        }
                    }
                    State::Stream(ref mut s) => {
                        // Just forward call to the stream,
                        // do not track its state.
                        return s.poll();
                    }
                    State::Eof => {
                        (State::Done, Some(Ok(Async::Ready(None))))
                    }
                    State::Done => {
                        panic!("poll called after eof");
                    }
                };

                self.state = next_state;
                if let Some(ret) = ret_opt {
                    return ret;
                }
            }
        }
    }

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