Module#attr_writer

static VALUE
rb_mod_attr_writer(int argc, VALUE *argv, VALUE klass)
{
    int i;
    VALUE names = rb_ary_new2(argc);

    for (i=0; i<argc; i++) {
        ID id = id_for_attr(klass, argv[i]);
        rb_attr(klass, id, FALSE, TRUE, TRUE);
        rb_ary_push(names, ID2SYM(rb_id_attrset(id)));
    }
    return names;
}