stylix: switch to using treefmt

[awwpotato]

allows for using nix fmt on the repo

Things done

• Tested locally
• Tested in testbed
• Commit message follows commit convention
• Fits style guide
• Respects license of any existing code used

Notify maintainers

[trueNAHO]

Nitpick: Consider sorting the entries by moving formatter between checks and packages at line 192.

[trueNAHO]

Invalidating the formatting in /flake.nix and running nix fmt results in the following error, causing the file not to be formatted back:

$ nix fmt
warning: Git tree '<STYLIX>' is dirty
2025/03/31 18:59:07 INFO using config file: /nix/store/8p4mscr4wwmcgfdk9h5ghd7r9rlb4psx-treefmt.toml
Error: path . not inside the tree root /nix/store

On a side note, should we rename the PR to something more explicit, like:

stylix: support 'nix fmt'

[awwpotato]

Invalidating the formatting in /flake.nix and running nix fmt results in the following error, causing the file not to be formatted back:

Seems like nixfmt-tree is currently broken when overriden, I made a bug report upstream NixOS/nixpkgs#395045

[MattSturgeon]

Invalidating the formatting in /flake.nix and running nix fmt results in the following error, causing the file not to be formatted back:

Seems like nixfmt-tree is currently broken when overriden

https://nixpk.gs/pr-tracker.html?pr=391319

[MattSturgeon]

This should either be pkgs.nixfmt-tree.override or pkgs.treefmt.withConfig.

The first one uses the "simple" nixfmt+treefmt wrapper and overrides a few minor settings.

The second one allows you to configure treefmt with a blank slate; this is what nixfmt-tree uses under the hood.

[awwpotato]

Oh oops misread the pr description. Not sure if we want to introduce treefmt into this project because it's functionality overlaps with git-hooks.nix

[MattSturgeon]

For some context, treefmt is needed because nixfmt doesn't support being run against a directory; it can only be run against a file (or a list of files). That's why the nixfmt-tree wrapper exists; it provides a treefmt pre-configured to run nixfmt.

Not sure if we want to introduce treefmt into this project because its functionality overlaps with git-hooks.nix

Git hooks can just run treefmt; in nixvim we use git-hook.nix's pre-commit.hooks.treefmt options.

its functionality overlaps with git-hooks.nix

Maybe you're thinking of treefmt-nix, which is a set of modules for configuring treefmt. Essentially a much fancier version of pkgs.treefmt.withConfig. That does have some overlap regarding checks, but can be configured as needed.