Adjust argument type for mutable with_metadata_of (#75091)

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>),
             )
         }
     }

library/core/src/ptr/mut_ptr.rs

@@ -80,10 +80,14 @@ impl<T: ?Sized> *mut T {
     #[unstable(feature = "set_ptr_value", issue = "75091")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[inline]
-    pub fn with_metadata_of<U>(self, mut val: *mut U) -> *mut U
+    pub fn with_metadata_of<U>(self, val: *const U) -> *mut U
     where
         U: ?Sized,
     {
+        // Prepare in the type system that we will replace the pointer value with a mutable
+        // pointer, taking the mutable provenance from the `self` pointer.
+        let mut val = val as *mut U;
+        // Pointer to the pointer value within the value.
         let target = &mut val as *mut *mut U as *mut *mut u8;
         // SAFETY: In case of a thin pointer, this operations is identical
         // to a simple assignment. In case of a fat pointer, with the current