We may request an RFC if:
- the lint is on (warn or higher level) by default, and is expected to affect a lot of users
- the lint is controversial
- the lint sets a (new) direction for Rust -- for example, changing an existing pattern to a different one, even if the pattern isn't widely used
- e.g., deprecating a syntax to make room for a possible new language feature
If in doubt, you can always raise the idea on Zulip first.