nixpkgs/nixos/modules/misc/assertions.nix

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

with lib;

{

  options = {

    assertions = mkOption {
      type = types.listOf types.unspecified;
      internal = true;
      default = [];
      example = [ { assertion = false; message = "you can't enable this for that reason"; } ];
      description = ''
        This option allows modules to express conditions that must
        hold for the evaluation of the system configuration to
        succeed, along with associated error messages for the user.
      '';
    };

    warnings = mkOption {
      internal = true;
      default = [];
      type = types.listOf types.string;
      example = [ "The `foo' service is deprecated and will go away soon!" ];
      description = ''
        This option allows modules to show warnings to users during
        the evaluation of the system configuration.
      '';
    };

  };
  # impl of assertions is in <nixos/modules/system/activation/top-level.nix>
}