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    use core::pin::Pin;
    use futures_core::ready;
    use futures_core::stream::{FusedStream, Stream};
    use futures_core::task::{Context, Poll};
    use pin_project_lite::pin_project;

    pin_project! {
        /// Stream for the [`chain`](super::StreamExt::chain) method.
        #[derive(Debug)]
        #[must_use = "streams do nothing unless polled"]
        pub struct Chain<St1, St2> {
            #[pin]
            first: Option<St1>,
            #[pin]
            second: St2,
        }
    }

    // All interactions with `Pin<&mut Chain<..>>` happen through these methods
    impl<St1, St2> Chain<St1, St2>
    where St1: Stream,
          St2: Stream<Item = St1::Item>,
    {
        pub(super) fn new(stream1: St1, stream2: St2) -> Self {
            Self {
                first: Some(stream1),
                second: stream2,
            }
        }
    }

    impl<St1, St2> FusedStream for Chain<St1, St2>
    where St1: Stream,
          St2: FusedStream<Item=St1::Item>,
    {
        fn is_terminated(&self) -> bool {
            self.first.is_none() && self.second.is_terminated()
        }
    }

    impl<St1, St2> Stream for Chain<St1, St2>
    where St1: Stream,
          St2: Stream<Item=St1::Item>,
    {
        type Item = St1::Item;

        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>> {
            let mut this = self.project();
            if let Some(first) = this.first.as_mut().as_pin_mut() {
                if let Some(item) = ready!(first.poll_next(cx)) {
                    return Poll::Ready(Some(item))
                }
            }
            this.first.set(None);
            this.second.poll_next(cx)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            if let Some(first) = &self.first {
                let (first_lower, first_upper) = first.size_hint();
                let (second_lower, second_upper) = self.second.size_hint();

                let lower = first_lower.saturating_add(second_lower);

                let upper = match (first_upper, second_upper) {
                    (Some(x), Some(y)) => x.checked_add(y),
                    _ => None
                };

                (lower, upper)
            } else {
                self.second.size_hint()
            }
        }
    }