In reply to @ity:itycodes.org
Why is foo for bob true while bar for alice is false

But I've managed to figure it out:

{ config, lib, ... }:
with lib;

{
  options = {
    users = mkOption {
      type = types.attrsOf (types.submodule (args: let user = args.config; in {
        options = {
          foo = mkEnableOption "foo";
          bar = mkEnableOption "bar";
        };
        config = {
          foo = mkIf user.bar true;
        };
      }));
      default = {};
    };
  };
}

I have no idea why I don't know this, but how to achieve

a = [x y] ++ (mkIf condition [z]);

?
This snippet is wrong