This re-use trick seems clever, but definitely needed more consideration before merging IMO. Re-use where we can determine the size & alignment are a perfect ma

This re-use trick seems clever, but definitely needed more consideration before merging IMO. Re-use where we can determine the size & alignment are a perfect match ought to be a win, but the other cases seem very situational. And The8472's attitude here seems unlikely to lead to a better Rust.