`_` gets greedily consumed in macro pattern matching repetitions

[DropDemBits]

In a repetition of a rule (e.g. expr, ident) that can cover _, and matching a _ directly, the _ is matched by the repetition causing the _ rule to fail and produces an expected leaf: `_` error.

rustc on the latest stable and nightly matches these macro invocations without any errors.

Minimal reproducible examples:

Example 1

macro_rules! seq1 {
    ($($a:ident)* _) => {};
}

// `_` overlaps with `$a:ident` but rustc matches it under the `_` token
seq1![a b c d _]; // expected leaf: `_`

Example 2

macro_rules! seq2 {
    ($($a:expr => $b:ident)* _ => $c:expr) => {};
}

// `_ => ou` overlaps with `$a:expr => $b:ident` but rustc matches it under `_ => $c:expr`
seq2![a => b c => d _ => ou]; // expected leaf: `_`

[edwin0cheng]

Um... TIL

However, The following code is not working in rustc :

playground

macro_rules! seq1 {
    ($($a:ident)* e) => {};
}

seq1![a b c d e];  <<- `error: local ambiguity: multiple parsing options: built-in NTs ident ('a') or 1 other option.`

So underscore _ is special cased ? sadface