Pre-RFC: at most one repetition macro patterns

[kennytm]

@mark-i-m

Under this RFC, one would simply write:

macro_rules! foo {
  (do $b1:block $(and $b2:block)?) => {
    $b1
    $($b2)*
  }
}

Why is $b2 expanded using *?

@mark-i-m

I don’t think it has grammar ambiguities

There is ambiguity: $($x:ident)?+ today matches a?b?c and not a+. Fortunately this is easy to resolve: you just look one more token ahead and always treat ?* and ?+ to mean separate by the question mark token.

@mark-i-m

Finally, we could consider adding something more general, such as “at most N repetitions”. It’s not clear what syntax we could use for this. Perhaps something like $(pat)N?, where N is replaced with an integer. I’m not sure if this retains syntactic disambiguity. Also, it’s not clear how useful such a pattern would be.

$($x:tt){2,7}