Ensure unreachable branch is eliminated

Author: EFanZh

futures-util/src/future/select.rs

@@ -99,14 +99,31 @@ where
     type Output = Either<(A::Output, B), (B::Output, A)>;
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        /// # Safety
+        ///
+        /// `value` must be a `Some` value.
+        #[inline(always)]
+        unsafe fn take_unchecked<T>(value: &mut Option<T>) -> T {
+            // `Option::unwrap_unchecked` is introduced in 1.58.0, so we can’t use it here.
+            match value.take() {
+                // SAFETY: the safety contract must be upheld by the caller.
+                None => core::hint::unreachable_unchecked(),
+                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)));
+            // SAFETY: `a` refers to a future lives inside `self.inner`, which means `self.inner` must be a `Some`
+            // value.
+            return Poll::Ready(Either::Left((val, unsafe { take_unchecked(&mut self.inner) }.1)));
         }
 
         if let Poll::Ready(val) = b.poll_unpin(cx) {
-            return Poll::Ready(Either::Right((val, self.inner.take().unwrap().0)));
+            // SAFETY: `b` refers to a future lives inside `self.inner`, which means `self.inner` must be a `Some`
+            // value.
+            return Poll::Ready(Either::Right((val, unsafe { take_unchecked(&mut self.inner) }.0)));
         }
 
         Poll::Pending