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

Closes #24091

src/libsyntax/ptr.rs

@@ -111,6 +111,13 @@ impl<T: Display> Display for P<T> {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> fmt::Pointer for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Pointer::fmt(&self.ptr, f)
+    }
+}
+
 impl<T: Hash> Hash for P<T> {
     fn hash<H: Hasher>(&self, state: &mut H) {
         (**self).hash(state);