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    use core::fmt;
    use core::pin::Pin;
    use futures_core::future::Future;
    use futures_core::ready;
    use futures_core::stream::{FusedStream, Stream};
    use futures_core::task::{Context, Poll};
    #[cfg(feature = "sink")]
    use futures_sink::Sink;
    use pin_project_lite::pin_project;

    struct StateFn<S, F> {
        state: S,
        f: F,
    }

    pin_project! {
        /// Stream for the [`scan`](super::StreamExt::scan) method.
        #[must_use = "streams do nothing unless polled"]
        pub struct Scan<St: Stream, S, Fut, F> {
            #[pin]
            stream: St,
            state_f: Option<StateFn<S, F>>,
            #[pin]
            future: Option<Fut>,
        }
    }

    impl<St, S, Fut, F> fmt::Debug for Scan<St, S, Fut, F>
    where
        St: Stream + fmt::Debug,
        St::Item: fmt::Debug,
        S: fmt::Debug,
        Fut: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Scan")
                .field("stream", &self.stream)
                .field("state", &self.state_f.as_ref().map(|s| &s.state))
                .field("future", &self.future)
                .field("done_taking", &self.is_done_taking())
                .finish()
        }
    }

    impl<St: Stream, S, Fut, F> Scan<St, S, Fut, F> {
        /// Checks if internal state is `None`.
        fn is_done_taking(&self) -> bool {
            self.state_f.is_none()
        }
    }

    impl<B, St, S, Fut, F> Scan<St, S, Fut, F>
    where
        St: Stream,
        F: FnMut(&mut S, St::Item) -> Fut,
        Fut: Future<Output = Option<B>>,
    {
        pub(super) fn new(stream: St, initial_state: S, f: F) -> Self {
            Self {
                stream,
                state_f: Some(StateFn {
                    state: initial_state,
                    f,
                }),
                future: None,
            }
        }

        delegate_access_inner!(stream, St, ());
    }

    impl<B, St, S, Fut, F> Stream for Scan<St, S, Fut, F>
    where
        St: Stream,
        F: FnMut(&mut S, St::Item) -> Fut,
        Fut: Future<Output = Option<B>>,
    {
        type Item = B;

        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<B>> {
            if self.is_done_taking() {
                return Poll::Ready(None);
            }

            let mut this = self.project();

            Poll::Ready(loop {
                if let Some(fut) = this.future.as_mut().as_pin_mut() {
                    let item = ready!(fut.poll(cx));
                    this.future.set(None);

                    if item.is_none() {
                        *this.state_f = None;
                    }

                    break item;
                } else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
                    let state_f = this.state_f.as_mut().unwrap();
                    this.future.set(Some((state_f.f)(&mut state_f.state, item)))
                } else {
                    break None;
                }
            })
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.is_done_taking() {
                (0, Some(0))
            } else {
                self.stream.size_hint() // can't know a lower bound, due to the predicate
            }
        }
    }

    impl<B, St, S, Fut, F> FusedStream for Scan<St, S, Fut, F>
    where
        St: FusedStream,
        F: FnMut(&mut S, St::Item) -> Fut,
        Fut: Future<Output = Option<B>>,
    {
        fn is_terminated(&self) -> bool {
            self.is_done_taking() || self.future.is_none() && self.stream.is_terminated()
        }
    }

    // Forwarding impl of Sink from the underlying stream
    #[cfg(feature = "sink")]
    impl<S, Fut, F, Item> Sink<Item> for Scan<S, S, Fut, F>
    where
        S: Stream + Sink<Item>,
    {
        type Error = S::Error;

        delegate_sink!(stream, Item);
    }