alloc: impl fmt::Pointer for Rc, Arc and Box

Closes #24091

src/liballoc/rc.rs

@@ -634,6 +634,13 @@ impl<T: fmt::Debug> fmt::Debug for Rc<T> {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> fmt::Pointer for Rc<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Pointer::fmt(&*self._ptr, f)
+    }
+}
+
 /// A weak version of `Rc<T>`.
 ///
 /// Weak references do not count when determining if the inner value should be