nixpkgs/nixos/modules/system/activation/specialisation.nix

{ config, lib, pkgs, extendModules, noUserModules, ... }:

let
  inherit (lib)
    concatStringsSep
    mapAttrs
    mapAttrsToList
    mkOption
    types
    ;

  # This attribute is responsible for creating boot entries for
  # child configuration. They are only (directly) accessible
  # when the parent configuration is boot default. For example,
  # you can provide an easy way to boot the same configuration
  # as you use, but with another kernel
  # !!! fix this
  children =
    mapAttrs
      (childName: childConfig: childConfig.configuration.system.build.toplevel)
      config.specialisation;

in
{
  options = {

    specialisation = mkOption {
      default = { };
      example = lib.literalExpression "{ fewJobsManyCores.configuration = { nix.settings = { core = 0; max-jobs = 1; }; }; }";
      description = lib.mdDoc ''
        Additional configurations to build. If
        `inheritParentConfig` is true, the system
        will be based on the overall system configuration.

        To switch to a specialised configuration
        (e.g. `fewJobsManyCores`) at runtime, run:

        ```
        sudo /run/current-system/specialisation/fewJobsManyCores/bin/switch-to-configuration test
        ```
      '';
      type = types.attrsOf (types.submodule (
        local@{ ... }:
        let
          extend =
            if local.config.inheritParentConfig
            then extendModules
            else noUserModules.extendModules;
        in
        {
          options.inheritParentConfig = mkOption {
            type = types.bool;
            default = true;
            description = lib.mdDoc "Include the entire system's configuration. Set to false to make a completely differently configured system.";
          };

          options.configuration = mkOption {
            default = { };
            description = lib.mdDoc ''
              Arbitrary NixOS configuration.

              Anything you can add to a normal NixOS configuration, you can add
              here, including imports and config values, although nested
              specialisations will be ignored.
            '';
            visible = "shallow";
            inherit (extend { modules = [ ./no-clone.nix ]; }) type;
          };
        }
      ));
    };

  };

  config = {
    system.systemBuilderCommands = ''
      mkdir $out/specialisation
      ${concatStringsSep "\n"
      (mapAttrsToList (name: path: "ln -s ${path} $out/specialisation/${name}") children)}
    '';
  };

  # uses extendModules to generate a type
  meta.buildDocsInSandbox = false;
}