Switch to `cargo fmt` instead of `rustfmt` directly

[FanShupei]

Is your feature request related to a problem? Please describe.

Rust edition may introduce style change (e.g. Edition 2024 introduces). cargo fmt will use edition from Cargo.toml, but currently treefmt invokes rustfmt directly using a global option. There are changes for edition mismatch. nix fmt and cargo fmt giving different results is highly undesirable.

If the project mix up edition 2024/2021 code (allowed by Rust, appears in gradually updating edition), the situation is even worse, nix fmt can not work perfectly since there is no correct edition to specify.

Describe the solution you'd like

Switch to cargo fmt. cargo fmt will automatically detect the edition from Cargo.toml.

[acid-bong]

Additionally, a parameter for the crate root, in case it's a lone Rust package in a non-Rust monorepo, would also be useful

[jfly]

I agree cargo fmt is the right tool to use. However, we cannot use it as it does not support passing individual files to format as cli parameters (a violation of our formatter spec).

See this discussion: numtide/treefmt#30, and this longstanding PR to add support for this to cargo fmt.