Post #2029201

@jamey A follow-up: I tried this example in the Rust Playground and apparently it is not allowed! rustc does not allow ambiguity around meta-variables; it will only parse $e:expr if no other meta-variables or tokens can be parsed at that point. There go my hopes of getting quartic macro expansion D: I'll continue exploring the possibilities here. I think this limitation can be handled by the packrat style I proposed, perhaps with some adjustment of cache keys.