Reduce mutability in std-use of with_metadata_of

Author: Andreas Molzer

library/alloc/src/rc.rs

@@ -1386,7 +1386,7 @@ impl<T: ?Sized> Rc<T> {
             Self::allocate_for_layout(
                 Layout::for_value(&*ptr),
                 |layout| Global.allocate(layout),
-                |mem| mem.with_metadata_of(ptr as *mut RcBox<T>),
+                |mem| mem.with_metadata_of(ptr as *const RcBox<T>),
             )
         }
     }

library/alloc/src/sync.rs

@@ -1204,7 +1204,7 @@ impl<T: ?Sized> Arc<T> {
             Self::allocate_for_layout(
                 Layout::for_value(&*ptr),
                 |layout| Global.allocate(layout),
-                |mem| mem.with_metadata_of(ptr as *mut ArcInner<T>),
+                |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
             )
         }
     }