[ci proposal] add cargo fmt -- --check

Description

As a developer I value the consistency cargo fmt provides to Rust codebases.
I propose we add cargo fmt -- --check to the test.yml workflow.

Considered alternatives

• do not do this
  • not enforcing consistency goes against the grain of Rust's ethos
• automated cargo fmt applying
  • I find this overly complex.

Additional context

• where should this go?

  • rnix-parser/flake.nix

    Line 35 in 65fbe7a

    cargo test ${flags}

[Ma27]

Makes sense, but I'd prefer to get most of the other changes merged before to avoid forcing people to resolve conflicts.

I'd prefer to get most of the other changes merged before to avoid forcing people to resolve conflicts

Great plan!

I'm in no rush too. I can take a crack at this in the coming weeks / months.

[darichey]

I think this can be closed now (github didn't link it to the PR)