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

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.