Local ambiguity in macro *definition*

[jonas-schievink]

macro_rules! m {
    () => ();;
}

<anon>:2:14: 2:15 error: local ambiguity: multiple parsing options: built-in NTs tt ('lhs') or 1 other options.
<anon>:2     () => ();;
                      ^

(the equivalent of $( $lhs:tt => $rhs:tt );+ $(;)* is used to parse macros)

This leaks "lhs", which is an implementation detail, and is a bad error message.

[brson]

Still repros.

[brson]

Bad error message.

[nikomatsakis]

Seems to me to be primarily a bad error message -- it's not clear we should accept trailing ; (we don't accept trailing separators in any other context?)

[estebank]

Triage: still happens:

error: local ambiguity: multiple parsing options: built-in NTs tt ('lhs') or 1 other option.
 --> src/lib.rs:2:14
  |
2 |     () => ();;
  |              ^

error: aborting due to previous error

[Centril]

Triage: same behavior now.