meta matches pseudo-identifiers

[gnzlbg]

This (playground) fails:

macro_rules! foo {
    ($($m:meta => $i:item)* _ => $j:item) => {};
}

foo! {
    _ => { fn f() -> i32 { 2 } }
}

fn main() { f(); }

error: local ambiguity: multiple parsing options: built-in NTs meta ('m') or 1 other option.
 --> src/main.rs:6:5
  |
6 |     _ => { fn f() -> i32 { 2 } }
  |     ^

---

This also fails (playground):

macro_rules! foo {
    ($m:meta) => {};
    (_ => { $i:item }) => {
        $i
    };
}

foo! {
    _ => { fn f() -> i32 { 2 } }
}

fn main() {  f(); }

error: expected identifier, found reserved identifier `_`
 --> src/main.rs:9:5
  |
9 |     _ => { fn f() -> i32 { 2 } }
  |     ^ expected identifier, found reserved identifier

---

I believe both cases should work because meta should not match pseudo-identifiers.

cc @petrochenkov @Centril @nrc @alexcrichton

[lolbinarycat]

triage: still broken on latest stable and nightly