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//! Stream combinator that can be used in the futures::select! macro. It is
//! similar to futures::StreamExt::select_next_some, but instead of only
//! resolving the Some variants, this one will return Option<T>. This is
//! useful when you want to do some action after one stream completes.
//! The bulk of this code was copied from futures::stream::SelectNextSome
//! (Copyright (c) 2016 Alex Crichton, Copyright (c) 2017 The Tokio Authors).

use core::pin::Pin;
use futures::future::{FusedFuture, Future};
use futures::stream::{FusedStream, StreamExt};
use futures::task::{Context, Poll};

pub trait SelectNextAnyExt {
    fn select_next_any(&mut self) -> SelectNextAny<'_, Self>
    where
        Self: Unpin + FusedStream;
}

impl<T> SelectNextAnyExt for T
where
    T: Unpin + FusedStream,
{
    fn select_next_any(&mut self) -> SelectNextAny<'_, Self> {
        SelectNextAny::new(self)
    }
}

/// Future for the [`select_next_any`](super::StreamExt::select_next_any)
/// method.
#[derive(Debug)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct SelectNextAny<'a, St: ?Sized> {
    stream: &'a mut St,
}

impl<'a, St: ?Sized> SelectNextAny<'a, St> {
    fn new(stream: &'a mut St) -> Self {
        SelectNextAny { stream }
    }
}

impl<St: ?Sized + FusedStream + Unpin> FusedFuture for SelectNextAny<'_, St> {
    fn is_terminated(&self) -> bool {
        self.stream.is_terminated()
    }
}

impl<St: ?Sized + FusedStream + Unpin> Future for SelectNextAny<'_, St> {
    type Output = Option<St::Item>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        assert!(
            !self.stream.is_terminated(),
            "SelectNextAny polled after terminated"
        );

        self.stream.poll_next_unpin(cx)
    }
}