WIP: feat(fmt): use nixfmt-rfc-style

[tomberek]

Motivation

Make nix fmt usable by default.

• Evaluating for .#formatters.SYSTEM or inputs.nixpkgs is too slow for the moment, so disabling that override for now.
• Using the same mechanism as nix develop to obtain an ambient nixpkgs
• defaulting to nixfmt-rfc-style
• could read+evaluate ci/pinned-nixpkgs.json, this can cost another download+copy to store, but will be more stable over time in case someone has flake:nixpkgs set to a moving branch

Context

#6087

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[nixos-discourse]

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

https://discourse.nixos.org/t/formatting-team-meeting-2024-08-06/50222/2

[roberth]

This elevates a very recent Nixpkgs implementation detail to a new Nix interface that is not consistent with anything that came before in Nix. It would be more appropriate to do this in a Nixpkgs CLI.

What if we could just load the shell instead, or just stick to the plan and finish lazy trees?

[tomberek]

Agreed, this is quite fragile. What about having a version pinned in Nix itself? A reference to nixpkgs or nixfmt repos. Benefit is thst it is simple and predictable. Downside is that you don't get an automatic upgrade.

Nixpkgs-specific tooling can do additional work to match the Nixpkgs to the fmt.

[tomberek]

User research notes (done on Discord):

• nix fmt FILE1 FILE2 would only format those files.
• nix fmt: would expect this to recurse through directory and format all ".nix"
  • would want a dry-run feature
  • would expect the version of nixfmt to be tied to Nix CLI
• to change to a different formatter:
  • would expect this to be done by... a flag like "--version", but that is occupied, "--use-version"?
• how to persist the changed version?
  • .cache

[roberth]

That all seems rather ad hoc. What if we just execed nixfmt from PATH? That way you can have a default one in your NixOS config, matching your NixOS pins, or you'd get the shared correct version from the nix shell you've loaded.
Otherwise we might end up recreating a bad implementation of treefmt and a new locking mechanism that we don't really need.

[tomberek]

I'm thinking of the following order.

1. nixfmt in PATH (allows people who care to keep moving)
2. pinned version associated with each CLI release, stable and easy to understand
3. when feasible, pinned via repo (not today)

[roberth]

Isn't 3 mostly covered by the formatter flake attribute?

[infinisil]

(ping @NixOS/nix-formatting)