nixpkgs/nixos/lib/eval-config.nix

# From an end-user configuration file (`configuration.nix'), build a NixOS
# configuration object (`config') from which we can retrieve option
# values.

# !!! Please think twice before adding to this argument list!
# Ideally eval-config.nix would be an extremely thin wrapper
# around lib.evalModules, so that modular systems that have nixos configs
# as subcomponents (e.g. the container feature, or nixops if network
# expressions are ever made modular at the top level) can just use
# types.submodule instead of using eval-config.nix
evalConfigArgs@{
  # !!! system can be set modularly, would be nice to remove,
  #     however, removing or changing this default is too much
  #     of a breaking change. To set it modularly, pass `null`.
  system ? builtins.currentSystem,
  # !!! is this argument needed any more? The pkgs argument can
  # be set modularly anyway.
  pkgs ? null,
  # !!! what do we gain by making this configurable?
  #     we can add modules that are included in specialisations, regardless
  #     of inheritParentConfig.
  baseModules ? import ../modules/module-list.nix,
  # !!! See comment about args in lib/modules.nix
  extraArgs ? { },
  # !!! See comment about args in lib/modules.nix
  specialArgs ? { },
  modules,
  modulesLocation ? (builtins.unsafeGetAttrPos "modules" evalConfigArgs).file or null,
  # !!! See comment about check in lib/modules.nix
  check ? true,
  prefix ? [ ],
  lib ? import ../../lib,
  extraModules ?
    let
      e = builtins.getEnv "NIXOS_EXTRA_MODULE_PATH";
    in
    lib.optional (e != "") (import e),
}:

let
  inherit (lib) optional;

  evalModulesMinimal =
    (import ./default.nix {
      inherit lib;
      # Implicit use of feature is noted in implementation.
      featureFlags.minimalModules = { };
    }).evalModules;

  pkgsModule = rec {
    _file = ./eval-config.nix;
    key = _file;
    config = lib.mkMerge (
      (optional (system != null) {
        # Explicit `nixpkgs.system` or `nixpkgs.localSystem` should override
        # this.  Since the latter defaults to the former, the former should
        # default to the argument. That way this new default could propagate all
        # they way through, but has the last priority behind everything else.
        nixpkgs.system = lib.mkDefault system;
      })
      ++ (optional (pkgs != null) {
        # This should be default priority, so it conflicts with any user-defined pkgs.
        nixpkgs.pkgs = pkgs;
      })
    );
  };

  withWarnings =
    x:
    lib.warnIf (evalConfigArgs ? extraArgs)
      "The extraArgs argument to eval-config.nix is deprecated. Please set config._module.args instead."
      lib.warnIf
      (evalConfigArgs ? check)
      "The check argument to eval-config.nix is deprecated. Please set config._module.check instead."
      x;

  legacyModules =
    lib.optional (evalConfigArgs ? extraArgs) {
      config = {
        _module.args = extraArgs;
      };
    }
    ++ lib.optional (evalConfigArgs ? check) {
      config = {
        _module.check = lib.mkDefault check;
      };
    };

  allUserModules =
    let
      # Add the invoking file (or specified modulesLocation) as error message location
      # for modules that don't have their own locations; presumably inline modules.
      locatedModules =
        if modulesLocation == null then
          modules
        else
          map (lib.setDefaultModuleLocation modulesLocation) modules;
    in
    locatedModules ++ legacyModules;

  noUserModules = evalModulesMinimal ({
    inherit prefix specialArgs;
    modules =
      baseModules
      ++ extraModules
      ++ [
        pkgsModule
        modulesModule
      ];
  });

  # Extra arguments that are useful for constructing a similar configuration.
  modulesModule = {
    config = {
      _module.args = {
        inherit
          noUserModules
          baseModules
          extraModules
          modules
          ;
      };
    };
  };

  nixosWithUserModules = noUserModules.extendModules { modules = allUserModules; };

  withExtraAttrs =
    configuration:
    configuration
    // {
      inherit extraArgs;
      inherit (configuration._module.args) pkgs;
      inherit lib;
      extendModules = args: withExtraAttrs (configuration.extendModules args);
    };
in
withWarnings (withExtraAttrs nixosWithUserModules)