fix(nix fmt): remove the default "." argument

[zimbatm]

Motivation

Allow treefmt to function as expected.

Context

When nix fmt is called without an argument, Nix appends the "." argument before calling the formatter. The comment in the code is:

Format the current flake out of the box

This also happens when formatting sub-folders.

This means that the formatter is now unable to distinguish, as an interface, whether the "." argument is coming from the flake or the user's intent to format the current folder. This decision should be up to the formatter.

Treefmt, for example, will automatically look up the project's root and format all the files. This is the desired behaviour. But because the "." argument is passed, it cannot function as expected.

Priorities and Process

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[fricklerhandwerk]

I think this is a good idea.

[roberth]

Could you add a brief release note, documentation and a test case?

[zimbatm]

thanks, updated the tests and added a release note

[tomberek]

Was also discussed in Formatter team meeting.

[zimbatm]

Thanks, I didn't expect this to be merged so quickly!

For visitors:

nixfmt, nixpkgs-fmt and alejandra are currently formatting stdin when no arguments are passed, so this change will cause some back-compat issues.

I would recommend those projects change their default to format the current folder instead (and format stdin on --stdin instead).

For visitors hitting this, here is a small wrapper example that will restore the previous behaviour:

{
  outputs = { self, nixpkgs, systems }:
    let
      eachSystem = nixpkgs.lib.genAttrs (import systems) (system: nixpkgs.legacyPackages.${system});
    in
    {
      formatter = eachSystem (pkgs: 
        pkgs.writeShellScriptBin "formatter" ''
          if [[ $# = 0 ]]; set -- .; fi
          exec "${pkgs.nixfmt-rfc-style}/bin/nixfmt "$@"
        '');
    };
}

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-2-25-released/55994/1