Module#attr

VALUE
rb_mod_attr(int argc, VALUE *argv, VALUE klass)
{
    if (argc == 2 && (argv[1] == Qtrue || argv[1] == Qfalse)) {
        ID id = id_for_attr(klass, argv[0]);
        VALUE names = rb_ary_new();

        rb_category_warning(RB_WARN_CATEGORY_DEPRECATED, "optional boolean argument is obsoleted");
        rb_attr(klass, id, 1, RTEST(argv[1]), TRUE);
        rb_ary_push(names, ID2SYM(id));
        if (argv[1] == Qtrue) rb_ary_push(names, ID2SYM(rb_id_attrset(id)));
        return names;
    }
    return rb_mod_attr_reader(argc, argv, klass);
}