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    use crate::stream::{Fuse, StreamExt};
    use core::fmt;
    use core::pin::Pin;
    use futures_core::future::{FusedFuture, 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;

    pin_project! {
        /// A `Stream` that implements a `peek` method.
        ///
        /// The `peek` method can be used to retrieve a reference
        /// to the next `Stream::Item` if available. A subsequent
        /// call to `poll` will return the owned item.
        #[derive(Debug)]
        #[must_use = "streams do nothing unless polled"]
        pub struct Peekable<St: Stream> {
            #[pin]
            stream: Fuse<St>,
            peeked: Option<St::Item>,
        }
    }

    impl<St: Stream> Peekable<St> {
        pub(super) fn new(stream: St) -> Self {
            Self {
                stream: stream.fuse(),
                peeked: None,
            }
        }

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

        /// Produces a `Peek` future which retrieves a reference to the next item
        /// in the stream, or `None` if the underlying stream terminates.
        pub fn peek(self: Pin<&mut Self>) -> Peek<'_, St> {
            Peek { inner: Some(self) }
        }

        /// Peek retrieves a reference to the next item in the stream.
        ///
        /// This method polls the underlying stream and return either a reference
        /// to the next item if the stream is ready or passes through any errors.
        pub fn poll_peek(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<&St::Item>> {
            let mut this = self.project();

            Poll::Ready(loop {
                if this.peeked.is_some() {
                    break this.peeked.as_ref();
                } else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
                    *this.peeked = Some(item);
                } else {
                    break None;
                }
            })
        }
    }

    impl<St: Stream> FusedStream for Peekable<St> {
        fn is_terminated(&self) -> bool {
            self.peeked.is_none() && self.stream.is_terminated()
        }
    }

    impl<S: Stream> Stream for Peekable<S> {
        type Item = S::Item;

        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            let this = self.project();
            if let Some(item) = this.peeked.take() {
                return Poll::Ready(Some(item));
            }
            this.stream.poll_next(cx)
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            let peek_len = if self.peeked.is_some() { 1 } else { 0 };
            let (lower, upper) = self.stream.size_hint();
            let lower = lower.saturating_add(peek_len);
            let upper = match upper {
                Some(x) => x.checked_add(peek_len),
                None => None,
            };
            (lower, upper)
        }
    }

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

        delegate_sink!(stream, Item);
    }

    pin_project! {
        /// Future for the [`Peekable::peek()`](self::Peekable::peek) function from [`Peekable`]
        #[must_use = "futures do nothing unless polled"]
        pub struct Peek<'a, St: Stream> {
            inner: Option<Pin<&'a mut Peekable<St>>>,
        }
    }

    impl<St> fmt::Debug for Peek<'_, St>
    where
        St: Stream + fmt::Debug,
        St::Item: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Peek")
                .field("inner", &self.inner)
                .finish()
        }
    }

    impl<St: Stream> FusedFuture for Peek<'_, St> {
        fn is_terminated(&self) -> bool {
            self.inner.is_none()
        }
    }

    impl<'a, St> Future for Peek<'a, St>
    where
        St: Stream,
    {
        type Output = Option<&'a St::Item>;
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            let inner = self.project().inner;
            if let Some(peekable) = inner {
                ready!(peekable.as_mut().poll_peek(cx));

                inner.take().unwrap().poll_peek(cx)
            } else {
                panic!("Peek polled after completion")
            }
        }
    }