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

    /// Creates a stream of a single element.
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::stream::{self, StreamExt};
    ///
    /// let stream = stream::once(async { 17 });
    /// let collected = stream.collect::<Vec<i32>>().await;
    /// assert_eq!(collected, vec![17]);
    /// # });
    /// ```
    pub fn once<Fut: Future>(future: Fut) -> Once<Fut> {
        assert_stream::<Fut::Output, _>(Once::new(future))
    }

    pin_project! {
        /// A stream which emits single element and then EOF.
        #[derive(Debug)]
        #[must_use = "streams do nothing unless polled"]
        pub struct Once<Fut> {
            #[pin]
            future: Option<Fut>
        }
    }

    impl<Fut> Once<Fut> {
        pub(crate) fn new(future: Fut) -> Self {
            Self { future: Some(future) }
        }
    }

    impl<Fut: Future> Stream for Once<Fut> {
        type Item = Fut::Output;

        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            let mut this = self.project();
            let v = match this.future.as_mut().as_pin_mut() {
                Some(fut) => ready!(fut.poll(cx)),
                None => return Poll::Ready(None),
            };

            this.future.set(None);
            Poll::Ready(Some(v))
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.future.is_some() {
                (1, Some(1))
            } else {
                (0, Some(0))
            }
        }
    }

    impl<Fut: Future> FusedStream for Once<Fut> {
        fn is_terminated(&self) -> bool {
            self.future.is_none()
        }
    }