Yeah, the opt-in specific changes shouldn't be big. Adding a `type_fulfills_X` and trait related tests would be enough. What I can't estimate, though, is the fallout for each case. The biggest one seems to be `Send`.

Fallout changes are pretty isolated in per-module commits (as much as possible :D). However, it's probably easier from a reviewing/rebasing perspective to check/work on trait implementations if they are in separate reviews. TBH, I'm happy either way. I like the idea of this RFC landing all together, although that probably means it'll take longer and it'll be more painful rebase-wise. :smile: