It holds a function pointer to the HeapSizeOf::heap_size_of_children() implementation corresponding to that IDL interface. This removes the need for a clumsly TypeId-based match expression in the former heap_size_of_eventtarget() function. Source-Repo: https://github.com/servo/servo Source-Revision: 5a0a91eba794af027723051c7f039cb22088aa65