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.
The Ruff autoformatter is going to be based on an intermediate representation (IR) formatted via [Wadler's algorithm](https://homepages.inf.ed.ac.uk/wadler/papers/prettier/prettier.pdf). This is architecturally similar to [Rome](https://github.com/rome/tools), Prettier, [Skip](https://github.com/skiplang/skip/blob/master/src/tools/printer/printer.sk), and others.
This PR adds a fork of the `rome_formatter` crate from [Rome](https://github.com/rome/tools), renamed here to `ruff_formatter`, which provides generic definitions for a formatter IR as well as a generic IR printer. (We've also pulled in `rome_rowan`, `rome_text_size`, and `rome_text_edit`, though some of these will be removed in future PRs.)
Why fork? `rome_formatter` contains code that's specific to Rome's AST representation (e.g., it relies on a fork of rust-analyzer's `rowan`), and we'll likely want to support different abstractions and formatting capabilities (there are already a few changes coming in future PRs). Once we've dropped `ruff_rowan` and trimmed down `ruff_formatter` to the code we currently need, it's also not a huge surface area to maintain and update.
