Ensure unreachable branch is eliminated (#2708)

Author: EFanZh

Diff for: futures-util/src/future/select.rs

@@ -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