object.c:1004
static VALUE
rb_obj_ivar_set(obj, iv, val)
    VALUE obj, iv, val;
{
    ID id = rb_to_id(iv);

    if (!rb_is_instance_id(id)) {
	rb_name_error(id, "`%s' is not allowed as an instance variable name", rb_id2name(id));
    }
    return rb_ivar_set(obj, id, val);
}
