Rollup merge of rust-lang#74025 - tmiasko:try-unwrap, r=Amanieu

Remove unnecessary release from Arc::try_unwrap

The thread that recovers the unique access to Arc inner value (e.g., drop
when ref-count strong reaches zero, successful try_unwrap), ensures that
other operations on Arc inner value happened before by synchronizing
with release operations performed when decrementing the reference counter.

When try_unwrap succeeds, the current thread recovers the unique access
to Arc inner value, so release is unnecessary.

r? @Amanieu

Author: Manishearth

src/liballoc/sync.rs

@@ -419,8 +419,7 @@ impl<T> Arc<T> {
     #[inline]
     #[stable(feature = "arc_unique", since = "1.4.0")]
     pub fn try_unwrap(this: Self) -> Result<T, Self> {
-        // See `drop` for why all these atomics are like this
-        if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
+        if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
             return Err(this);
         }