generateNixPathFromInputs should check for default.nix file instead of for package set

[lilyball]

nix.generateNixPathFromInputs only adds inputs to NIX_PATH if they have a packages or legacyPackages output. This is a rather curious restriction, as the existence of such a package set does not prove that the flake source can be imported directly. It also means that flakes that can be imported, such as nix-darwin, don't end up in NIX_PATH as they don't have a package set.

Instead of testing for the package set, it should check if the flake has a default.nix file. Unfortunately I don't think Nix lets you test if a file exists, so this might require using an activation script instead of config.environment.etc. Though perhaps setting environment.etc."nix/input".source to a derivation whose outPath is a directory of symlinks might work? That way the derivation build script can test the inputs for a default.nix.

[lilyball]

I hacked this together myself, and I can confirm that environment.etc."nix/input".source = pkgs.runCommandNoCC works to build a directory. There may be an issue activating the result for anyone who already has the existing /etc/nix/inputs as a directory though (this is true for nix-darwin, I don't know if nixos's environment.etc activation is any smarter, though my belief is nix-darwin just copied it from nixos).

[gytis-ivaskevicius]

Yeah, good point. Will implement it this weekend

[gytis-ivaskevicius]

I think it's fixed by #105?