1 error[E0658]: Using the `?` macro Kleene operator for "at most one" repetition is unstable (see issue #48075)
2 --> $DIR/feature-gate-macro_at_most_once_rep.rs:14:20
4 14 | macro_rules! m { ($(a)?) => {} }
7 = help: add #![feature(macro_at_most_once_rep)] to the crate attributes to enable
9 error: aborting due to previous error