Option to generate flake attributes with cross-compilation enabled nixpkgs about flake-parts

[roberth]

Implementation note:

This could be generalized to arbitrary variants of the flake configuration.

1. move this to flake-parts https://github.com/hercules-ci/hercules-ci-agent/blob/master/variants.nix
2. add cross compilation option for setting Nixpkgs buildPlatform (setting crossSystem is wrong imo)
3. add renaming functions and options to control the copying of attributes from the otherwise invisible variants.<x>.packages.<y> to packages."${x}/${y}"

---

An alternative solution for adding cross support could be NixOS/nixpkgs#160061.

I am not aware of any documentation regarding cross compilation with flakes. Are you?
I would expect packages in packages.<system> to be runnable on system, as suggested by the nix run behavior. Ultimately Nix is all about outputs, as those are the usable end product. Where the derivation runs is secondary and is abstracted away by the remote builder concept.

That is the nature of cross compilation, but the Nix CLI behavior seems to steer towards a solution that repeats the mistakes of nixos/modules/misc/nixpkgs.nix.

[ldesgoui]

Hey,

Thanks a lot for the pointers, it did allow me to figure it out, here's how that looks like for now

{ config, inputs, ... }:
let
  rootConfig = config;
in
{
  perSystem = { config, lib, system, moduleType, ... }: {
    options =
      let
        inherit (lib) mkOption types;
      in
      {
        crossSystems = mkOption {
          type = types.listOf types.str;

          default = builtins.filter (c: system != c && config.selectCrossSystem c) rootConfig.systems;
        };

        selectCrossSystem = mkOption {
          type = types.functionTo types.bool;

          default = crossSystem:
            let
              local = lib.systems.elaborate system;
              cross = lib.systems.elaborate crossSystem;
            in
            # Can only build MacOS stuff from MacOS
            (cross.isMacOS -> local.isMacOS)
            # https://github.com/NixOS/nixpkgs/issues/137877
            && (local.isMacOS && local.isAarch64 -> !cross.isLinux)
          ;
        };

        x = mkOption {
          type = types.lazyAttrsOf moduleType;
          default = { };
        };
      };

    config = {
      _module.args.pkgs = lib.mkIf (config ? _module.args.crossSystem)
        (import inputs.nixpkgs {
          inherit system;
          inherit (config._module.args) crossSystem;
        });

      x = lib.mkIf (!(config ? _module.args.crossSystem))
        (lib.genAttrs config.crossSystems (c: { _module.args.crossSystem = c; }));
    };
  };
}

add cross compilation option for setting Nixpkgs buildPlatform (setting crossSystem is wrong imo)

I looked far and wide, that seems to only be a NixOS thing

I am not aware of any documentation regarding cross compilation with flakes [..]

Yeah, this is a philosophical question to which I chose not to find an answer :P
Feels like packages.<system> should be buildable on <system> and apps.<system> should be runnable on <system>, and all package shouldn't implicitly be apps?

As the solution is opinionated and somewhat niche, I'll just close. Feel free to re-open if you'd like