The tracking issue for this feature is: TODO(mark-i-m)

With this feature gate enabled, one can use ? as a Kleene operator meaning "0 or 1 repetitions" in a macro definition. Previously only + and * were allowed.

For example:

#![allow(unused_variables)]
#![feature(macro_at_most_once_rep)]

fn main() {
macro_rules! foo {
    (something $(,)?) // `?` indicates `,` is "optional"...
        => {}
}
}