macro_at_most_once_rep -

# `macro_at_most_once_rep`

NOTE: This feature is only available in the 2018 Edition.

The tracking issue for this feature is: #48075

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:

    #![feature(macro_at_most_once_rep)]

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