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"...
=> {}
}