Focus on stable Nix with npins instead of Flakes

[infinisil]

Focus on stable Nix and use npins for dependencies.
With this PR, the flake.nix relies on default.nix, rather than the other way around.

• Simplifies the whole thing due to not having to mess with system, see Provide builtins.currentSystem as a input to flakes or make system a parameter nix#3843
• Makes the lockfile way smaller, see Reuse input lock files nix#7730
• flake-compat is not necessary anymore

I believe the only negative impact is that downstream Flake users can't use the follows mechanism anymore to override nixpkgs. Unfortunately there's no way to give a better error in such a case.

Instead what works is

import inputs.nixfmt {
  inherit system;
  nixpkgs = inputs.nixpkgs;
}

Though I don't really expect this to be very necessary.

As a side benefit, this allows passing in a patched Nixpkgs just using e.g. pkgs.applyPatches, without needing Nix to support such functionality like in NixOS/nix#6530.

[Mic92]

Also how can I do offline evaluation with npins? With flakes, I can nix flake archive to get a gcroot over all inputs.

[Mic92]

Also is there a way to avoid the nixpkgs re-import? This can be costly in terms of memory? Is it maybe possible to just override nixfmt from nixpkgs with this version?

[Mic92]

Much better:

with import <nixpkgs> {};
nixfmt.overrideAttrs (old: {
  name = "nixfmt-2024-03-15";
  src = fetchFromGitHub {
    owner = "NixOS";
    repo = "nixfmt";
    rev = "136005fc3dd34ff0d6af9f8c57cd1d3d7d342537";
    sha256 = "sha256-g0MTMQZeRlKFQiKdEVc1IQ73nNeRwu0+GtUIvcm2NV4=";
  };
})

[Mic92]

Ah. There is actually: nixfmt-rfc-style

[infinisil]

Also how can I do offline evaluation with npins? With flakes, I can nix flake archive to get a gcroot over all inputs.

I didn't know about nix flake archive, but it's a hard sell, it's documentation is pretty much just FIXME. After looking at the source code to understand what it does, it doesn't seem to gcroot anything. Also this won't work for offline builds anyways since it doesn't include any dependencies of the derivations, but let me know if I'm mistaken.

With stable Nix there's a relatively easy way to gcroot all inputs to a derivation using .inputDerivation (introduced by me a while ago, admittedly also underdocumented):

nix-build -A inputDerivation -o inputs

Though this won't gcroot any Nix inputs, so if you do a gc you'll still need internet, so it's not a complete solution either. Would be nice to see something like npins that also makes this easy.

[Mic92]

Well. It still has more documentation than inputDerivation, which doesn't appear in any manual.