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Merged
merged 1 commit into from
Feb 18, 2023
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Ensure unreachable branch is eliminated #2708

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14 changes: 12 additions & 2 deletions futures-util/src/future/select.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -99,14 +99,24 @@ where
type Output = Either<(A::Output, B), (B::Output, A)>;

fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
/// When compiled with `-C opt-level=z`, this function will help the compiler eliminate the `None` branch, where
/// `Option::unwrap` does not.
#[inline(always)]
fn unwrap_option<T>(value: Option<T>) -> T {
match value {
None => unreachable!(),
Some(value) => value,
}
}

let (a, b) = self.inner.as_mut().expect("cannot poll Select twice");

if let Poll::Ready(val) = a.poll_unpin(cx) {
return Poll::Ready(Either::Left((val, self.inner.take().unwrap().1)));
return Poll::Ready(Either::Left((val, unwrap_option(self.inner.take()).1)));
}

if let Poll::Ready(val) = b.poll_unpin(cx) {
return Poll::Ready(Either::Right((val, self.inner.take().unwrap().0)));
return Poll::Ready(Either::Right((val, unwrap_option(self.inner.take()).0)));
}

Poll::Pending
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