Issue #13933: Remove transmute_mut from Arc

directly use the internal pointer instead.

src/liballoc/arc.rs

@@ -160,7 +160,8 @@ impl<T: Send + Share + Clone> Arc<T> {
         // reference count is guaranteed to be 1 at this point, and we required
         // the Arc itself to be `mut`, so we're returning the only possible
         // reference to the inner data.
-        unsafe { mem::transmute::<&_, &mut _>(self.deref()) }
+        let inner = unsafe { &mut *self._ptr };
+        &mut inner.data
     }
 }