Strange order dependency for macro that matches _ and ident

[larsluthman]

A macro_rules! macro with two cases that match _ and ident might not match _ depending on the order of the cases. This works:

macro_rules! foo {
    (_) => {};
    ($a:ident) => {};
}

fn main() {
    foo!(_);
}

But this doesn't:

macro_rules! foo {
    ($a:ident) => {};
    (_) => {};
}

fn main() {
    foo!(_);
}

...giving the error message:

rustc 1.17.0-nightly (306035c21 2017-02-18)
error: expected ident, found _
 --> <anon>:8:10
  |
8 |     foo!(_);
  |          ^

Why would it expect an ident there? If _ isn't an ident, shouldn't it just move on to the second macro case and match that?

[durka]

Annoyingly, the macro expander doesn't attempt to handle this kind of ambiguity at all. It won't back out of $a:ident and retry the next rule, it just bails. It can only use "constants" (like _ or some other explicit token that isn't a macro variable) to disambiguate.

[Mark-Simulacrum]

I don't think this is really a direct issue since this basically resolves to the macro expander being unable to handle ambiguity between macro arms. As such, I'm going to close.