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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;

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

    impl<St, Fut, F> fmt::Debug for Then<St, Fut, F>
    where
        St: fmt::Debug,
        Fut: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Then")
                .field("stream", &self.stream)
                .field("future", &self.future)
                .finish()
        }
    }

    impl<St, Fut, F> Then<St, Fut, F>
        where St: Stream,
              F: FnMut(St::Item) -> Fut,
    {
        pub(super) fn new(stream: St, f: F) -> Self {
            Self {
                stream,
                future: None,
                f,
            }
        }

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

    impl<St, Fut, F> FusedStream for Then<St, Fut, F>
        where St: FusedStream,
              F: FnMut(St::Item) -> Fut,
              Fut: Future,
    {
        fn is_terminated(&self) -> bool {
            self.future.is_none() && self.stream.is_terminated()
        }
    }

    impl<St, Fut, F> Stream for Then<St, Fut, F>
        where St: Stream,
              F: FnMut(St::Item) -> Fut,
              Fut: Future,
    {
        type Item = Fut::Output;

        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<Self::Item>> {
            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);
                    break Some(item);
                } else if let Some(item) = ready!(this.stream.as_mut().poll_next(cx)) {
                    this.future.set(Some((this.f)(item)));
                } else {
                    break None;
                }
            })
        }

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

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

        delegate_sink!(stream, Item);
    }