Fix Arc::into_inner.

It should never run the regular Drop implementation, sincei t
unconditionally runs fetch_sub.

Author: m-ou-se

library/alloc/src/sync.rs

@@ -1093,13 +1093,13 @@ impl<T, A: Allocator> Arc<T, A> {
     #[inline]
     #[stable(feature = "arc_into_inner", since = "1.70.0")]
     pub fn into_inner(this: Self) -> Option<T> {
+        let mut this = mem::ManuallyDrop::new(this);
+        let alloc = unsafe { ptr::read(&this.alloc) };
         match this.inner().strong.fetch_sub(ONE_STRONG, Release) {
             ONE_STRONG => {
-                let mut this = mem::ManuallyDrop::new(this);
                 acquire!(this.inner().strong);
                 let value = unsafe { ptr::read(Self::get_mut_unchecked(&mut this)) };
                 unsafe { this.deallocate() };
-                drop(unsafe { ptr::read(&this.alloc) });
                 Some(value)
             }
             ONE_STRONG_WITH_WEAK
@@ -1109,9 +1109,7 @@ impl<T, A: Allocator> Arc<T, A> {
                     .compare_exchange(NO_STRONG_WITH_WEAK, STRONG_DROPPED, Acquire, Relaxed)
                     .is_ok() =>
             {
-                let mut this = mem::ManuallyDrop::new(this);
                 let value = unsafe { ptr::read(Self::get_mut_unchecked(&mut this)) };
-                let alloc = unsafe { ptr::read(&this.alloc) };
                 drop(Weak { ptr: this.ptr, alloc });
                 Some(value)
             }