Three edition 2021 compatible sets of changes in preparation for the
edition 2025 split out from #11724.
In edition 2025, `gen` is a keyword, so we escape it as `r#gen`. `ref`
and `ref mut` are not allowed anymore for `&T` and `&mut T`, so we
remove them. `cargo fmt` now formats inside of macros, which the 2021
formatter doesn't undo.
## Summary
This is the pattern I see in a variety of crates, and I believe this is
preferred if you don't _need_ an owned `String`, since you can avoid the
allocation. This could be pretty impactful for us?
konsti
Charlie Marsh