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use futures_core::future::{Future, FusedFuture};
use futures_core::task::{Context, Poll};
use std::pin::Pin;
use pin_utils::{unsafe_pinned, unsafe_unpinned};

/// Combinator that guarantees one [`Poll::Pending`] before polling its inner
/// future.
///
/// This is created by the
/// [`FutureTestExt::pending_once`](super::FutureTestExt::pending_once)
/// method.
#[derive(Debug, Clone)]
#[must_use = "futures do nothing unless you `.await` or poll them"]
pub struct PendingOnce<Fut> {
    future: Fut,
    polled_before: bool,
}

impl<Fut: Future> PendingOnce<Fut> {
    unsafe_pinned!(future: Fut);
    unsafe_unpinned!(polled_before: bool);

    pub(super) fn new(future: Fut) -> Self {
        Self {
            future,
            polled_before: false,
        }
    }
}

impl<Fut: Future> Future for PendingOnce<Fut> {
    type Output = Fut::Output;

    fn poll(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Self::Output> {
        if self.polled_before {
            self.as_mut().future().poll(cx)
        } else {
            *self.as_mut().polled_before() = true;
            cx.waker().wake_by_ref();
            Poll::Pending
        }
    }
}

impl<Fut: FusedFuture> FusedFuture for PendingOnce<Fut> {
    fn is_terminated(&self) -> bool {
        self.polled_before && self.future.is_terminated()
    }
}