tokio/tokio-stream/src/stream_ext/chain.rs

use crate::stream_ext::Fuse;
use crate::Stream;

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

pin_project! {
    /// Stream returned by the [`chain`](super::StreamExt::chain) method.
    pub struct Chain<T, U> {
        #[pin]
        a: Fuse<T>,
        #[pin]
        b: U,
    }
}

impl<T, U> Chain<T, U> {
    pub(super) fn new(a: T, b: U) -> Chain<T, U>
    where
        T: Stream,
        U: Stream,
    {
        Chain { a: Fuse::new(a), b }
    }
}

impl<T, U> Stream for Chain<T, U>
where
    T: Stream,
    U: Stream<Item = T::Item>,
{
    type Item = T::Item;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T::Item>> {
        use Poll::Ready;

        let me = self.project();

        if let Some(v) = ready!(me.a.poll_next(cx)) {
            return Ready(Some(v));
        }

        me.b.poll_next(cx)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        super::merge_size_hints(self.a.size_hint(), self.b.size_hint())
    }
}

impl<T, U> FusedStream for Chain<T, U>
where
    T: Stream,
    U: FusedStream<Item = T::Item>,
{
    fn is_terminated(&self) -> bool {
        self.a.is_terminated() && self.b.is_terminated()
    }
}