Drop nixfmt-classic from nixpkgs

[jfly]

When doing the stable release, we announced that "The classic nixfmt will stay available for some more time as pkgs.nixfmt-classic" (https://github.com/NixOS/nixpkgs/blob/b3bc1e31d0fc920cc11a79ed4f4bd202961addfb/doc/release-notes/rl-2511.section.md#highlights-sec-nixpkgs-release-2511-highlights).

We should decide exactly how long that is. It seems reasonable to me to wait until the next release (26.05). Concretely, I propose that:

1. We add a deprecation warning to pkgs.nixfmt-classic now, with the expectation that 25.11 is released with the deprecation warning.
2. Once 25.11 is cut off from master, we remove pkgs.nixfmt-classic, with the expectation that 26.05 is released with no pkgs.nixfmt-classic.

The plan

• Do immediately: Move nixfmt-classic to aliases.nix. (It's currently in all-packages).

• Do immediately: Add a deprecation warning like this:

  nixfmt-classic = (
    if lib.oldestSupportedReleaseIsAtLeast 2605 then
      throw "nixfmt-classic has been removed as it is deprecated and unmaintained."
    else if lib.oldestSupportedReleaseIsAtLeast 2511 then
      lib.warnOnInstantiate
        "nixfmt-classic is deprecated and unmaintained. We recommend switching to nixfmt."
     else
       lib.id
  ) haskellPackages.nixfmt.bin;

• Once 25.11 is the oldest supported version in master: list the deprecation in nixpkgs' release notes.

• Once 26.05 is the oldest supported version in master: list the removal in nixpkgs' release notes.

• Once 26.05 is the oldest supported version in master: simplify the above to just the throw.

  • TBD what (if anything) we do with haskellPackages.nixfmt.bin

[sternenseemann]

Please make sure to keep nixfmt building until then. Currently nixfmt-0.6.0 has too strict bounds to work with Stackage LTS 24 without further workarounds:

Encountered missing or private dependencies:
    base >=4.12.0 && <4.19,
    filepath >=1.4.2 && <1.5,
    megaparsec >=9.0.1 && <9.6

This could be fixed by a simple Hackage revision (in this case) to

base < 4.21
megaparsec < 9.8
filepath < 1.6

[jfly]

@sternenseemann, thanks for the heads up.

We have had some turnover in the team (I don't even know how to do a release to Hackage, because we thought we'd never have to). I know very little about the nixpkgs Haskell package set. Can we relax those constraints with a patch over in nixpkgs (knowing this will go away soon)? Or would it be simplest to get the change on Hackage?

[sternenseemann]

We are currently working around this in Nixpkgs, but that is some extra code we need to keep track of. Also it does not help for anyone using cabal-install or whatever to install directly from Hackage.

Making a new release is not necessary, a Hackage revision can simply be made in the Hackage web interface. @infinisil of the current team (?) has access.

[infinisil]

Then I think that's the signal that it should be deprecated it Nixpkgs, instead of spending any more effort on it. It's already deprecated on Hackage as of recently: #161.

[sternenseemann]

Nevermind, I'll just note you won't fix this.

[nixos-discourse]

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

https://discourse.nixos.org/t/formatting-team-meeting-2025-09-30/70099/1