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    use crate::stream::{StreamExt, Fuse};
    use core::cmp;
    use core::pin::Pin;
    use futures_core::stream::{FusedStream, Stream};
    use futures_core::task::{Context, Poll};
    use pin_project_lite::pin_project;

    pin_project! {
        /// Stream for the [`zip`](super::StreamExt::zip) method.
        #[derive(Debug)]
        #[must_use = "streams do nothing unless polled"]
        pub struct Zip<St1: Stream, St2: Stream> {
            #[pin]
            stream1: Fuse<St1>,
            #[pin]
            stream2: Fuse<St2>,
            queued1: Option<St1::Item>,
            queued2: Option<St2::Item>,
        }
    }

    impl<St1: Stream, St2: Stream> Zip<St1, St2> {
        pub(super) fn new(stream1: St1, stream2: St2) -> Self {
            Self {
                stream1: stream1.fuse(),
                stream2: stream2.fuse(),
                queued1: None,
                queued2: None,
            }
        }

        /// Acquires a reference to the underlying streams that this combinator is
        /// pulling from.
        pub fn get_ref(&self) -> (&St1, &St2) {
            (self.stream1.get_ref(), self.stream2.get_ref())
        }

        /// Acquires a mutable reference to the underlying streams that this
        /// combinator is pulling from.
        ///
        /// Note that care must be taken to avoid tampering with the state of the
        /// stream which may otherwise confuse this combinator.
        pub fn get_mut(&mut self) -> (&mut St1, &mut St2) {
            (self.stream1.get_mut(), self.stream2.get_mut())
        }

        /// Acquires a pinned mutable reference to the underlying streams that this
        /// combinator is pulling from.
        ///
        /// Note that care must be taken to avoid tampering with the state of the
        /// stream which may otherwise confuse this combinator.
        pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut St1>, Pin<&mut St2>) {
            let this = self.project();
            (this.stream1.get_pin_mut(), this.stream2.get_pin_mut())
        }

        /// Consumes this combinator, returning the underlying streams.
        ///
        /// Note that this may discard intermediate state of this combinator, so
        /// care should be taken to avoid losing resources when this is called.
        pub fn into_inner(self) -> (St1, St2) {
            (self.stream1.into_inner(), self.stream2.into_inner())
        }
    }

    impl<St1, St2> FusedStream for Zip<St1, St2>
        where St1: Stream, St2: Stream,
    {
        fn is_terminated(&self) -> bool {
            self.stream1.is_terminated() && self.stream2.is_terminated()
        }
    }

    impl<St1, St2> Stream for Zip<St1, St2>
        where St1: Stream, St2: Stream
    {
        type Item = (St1::Item, St2::Item);

        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>> {
            let mut this = self.project();

            if this.queued1.is_none() {
                match this.stream1.as_mut().poll_next(cx) {
                    Poll::Ready(Some(item1)) => *this.queued1 = Some(item1),
                    Poll::Ready(None) | Poll::Pending => {}
                }
            }
            if this.queued2.is_none() {
                match this.stream2.as_mut().poll_next(cx) {
                    Poll::Ready(Some(item2)) => *this.queued2 = Some(item2),
                    Poll::Ready(None) | Poll::Pending => {}
                }
            }

            if this.queued1.is_some() && this.queued2.is_some() {
                let pair = (this.queued1.take().unwrap(), this.queued2.take().unwrap());
                Poll::Ready(Some(pair))
            } else if this.stream1.is_done() || this.stream2.is_done() {
                Poll::Ready(None)
            } else {
                Poll::Pending
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let queued1_len = if self.queued1.is_some() { 1 } else { 0 };
            let queued2_len = if self.queued2.is_some() { 1 } else { 0 };
            let (stream1_lower, stream1_upper) = self.stream1.size_hint();
            let (stream2_lower, stream2_upper) = self.stream2.size_hint();

            let stream1_lower = stream1_lower.saturating_add(queued1_len);
            let stream2_lower = stream2_lower.saturating_add(queued2_len);

            let lower = cmp::min(stream1_lower, stream2_lower);

            let upper = match (stream1_upper, stream2_upper) {
                (Some(x), Some(y)) => {
                    let x = x.saturating_add(queued1_len);
                    let y = y.saturating_add(queued2_len);
                    Some(cmp::min(x, y))
                }
                (Some(x), None) => x.checked_add(queued1_len),
                (None, Some(y)) => y.checked_add(queued2_len),
                (None, None) => None
            };

            (lower, upper)
        }
    }