# concat.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.14/src/stream/stream/concat.rs`.
Keywords: rust, rustlang, rust-lang

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

    pin_project! {
        /// Future for the [`concat`](super::StreamExt::concat) method.
        #[derive(Debug)]
        #[must_use = "futures do nothing unless you `.await` or poll them"]
        pub struct Concat<St: Stream> {
            #[pin]
            stream: St,
            accum: Option<St::Item>,
        }
    }

    impl<St> Concat<St>
    where St: Stream,
          St::Item: Extend<<St::Item as IntoIterator>::Item> +
                    IntoIterator + Default,
    {
        pub(super) fn new(stream: St) -> Self {
            Self {
                stream,
                accum: None,
            }
        }
    }

    impl<St> Future for Concat<St>
    where St: Stream,
          St::Item: Extend<<St::Item as IntoIterator>::Item> +
                    IntoIterator + Default,
    {
        type Output = St::Item;

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

            loop {
                match ready!(this.stream.as_mut().poll_next(cx)) {
                    None => {
                        return Poll::Ready(this.accum.take().unwrap_or_default())
                    }
                    Some(e) => {
                        if let Some(a) = this.accum {
                            a.extend(e)
                        } else {
                            *this.accum = Some(e)
                        }
                    }
                }
            }
        }
    }

    impl<St> FusedFuture for Concat<St>
    where St: FusedStream,
          St::Item: Extend<<St::Item as IntoIterator>::Item> +
                    IntoIterator + Default,
    {
        fn is_terminated(&self) -> bool {
            self.accum.is_none() && self.stream.is_terminated()
        }
    }