My editor runs `rustfmt` on save to format Rust code, not `cargo fmt`.
With our recent bump to the Rust 2024 edition, the formatting that
`rustfmt`/`cargo fmt` applies changed. Unfortunately, `rustfmt` and
`cargo fmt` have different behaviors for determining which edition to
use when formatting: `cargo fmt` looks for the Rust edition in
`Cargo.toml`, whereas `rustfmt` looks for it in `rustfmt.toml`. As a
result, whenever I save, I have to remember to manually run `cargo fmt`
before committing/pushing.
There is an open issue asking for `rustfmt` to also look at `Cargo.toml`
when it's present (https://github.com/rust-lang/rust.vim/issues/368),
but it seems like they "closed" that issue just by bumping the default
edition (six years ago, from 2015 to 2018).
In the meantime, this PR adds a `rustfmt.toml` file with our current
Rust edition so that both invocation have the same behavior. I don't
love that this duplicates information in `Cargo.toml`, but I've added a
reminder comment there to hopefully ensure that we bump the edition in
both places three years from now.
Douglas Creager
Charlie Marsh