macro_rules treating ';' as an ident

[durka]

This playpen contains a macro with two *-repeating arguments, plus a non-repeating argument sandwiched between them. It works fine when the repeating arguments are supplied, but when I try to leave out the first one, rustc claims the syntax is ambiguous. But I can't see why.

[jFransham]

I have this problem with = too, in the pattern ( $fname:ident $( $arg:pat )* = ). I get "ambiguous syntax" for the = symbol.

[bluss]

It's a known limitation of macros, see https://doc.rust-lang.org/book/macros.html#syntactic-requirements.

The macro system does not deal with parse ambiguity at all.

[durka]

What's strange is that it's perfectly capable of using ; to terminate the repeating ident matcher, because ; clearly isn't an ident. But it won't use that fact to disambiguate the match at the first token.

[durka]

Also, the example at that point in the book doesn't make any sense. Neither $e:expr nor E can follow $t:ty.

[brson]

Current output:

rustc 1.15.1 (021bd294c 2017-02-08)
error: local ambiguity: multiple parsing options: built-in NTs ident ('beginning') or 1 other option.
 --> <anon>:9:25
  |
9 |     println!("{}", foo!(; f ;));
  |                         ^

Compilation failed.

[nikomatsakis]

The problem is not that ; is considered as an identifier (see e.g. https://is.gd/lJjUHz), but rather that once we start parsing a "fragment" (like $x:tt), parsing must succeed or macro parsing aborts. Fixing this turned out to be quite complex, and I expect we'll only address it in macros 2.0. Closing.