Don't try to recover keyword as non-keyword identifier

[jieyouxu]

Fixes #149692.

On beta after #146978, we ICE on

macro_rules! m {
    ($id:item()) => {}
}

m!(Self());

where Self in the macro invocation is a keyword not a "normal" identifier, while attempting to recover an missing keyword before an identifier. Except, Self is a keyword, so trying to parse that as a non-reserved identifier expectedly fails.

I suspect #146978 merely unmasked a possible code path to hit this case; this logic has been so for a good while. Previously, on stable, the error message looks something like

error: expected identifier, found keyword `Self`
 --> src/lib.rs:5:4
  |
5 | m!(Self());
  |    ^^^^ expected identifier, found keyword

error: missing `fn` or `struct` for function or struct definition
 --> src/lib.rs:5:4
  |
2 |     ($id:item()) => {}
  |      -------- while parsing argument for this `item` macro fragment
...
5 | m!(Self());
  |    ^^^^
  |
help: if you meant to call a macro, try
  |
5 | m!(Self!());
  |        +

I considered restoring this diagnostic, but I'm not super convinced it's worth the complexity (and to me, it's not super clear what the user actually intended here).

[rustbot]

r? @petrochenkov

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[jieyouxu]

Hm actually wait the previous error message does make sense, it's just the suggestion, let me see.

EDIT: hm, not very convinced it's worth it.

[petrochenkov]

@bors r+

[rust-bors]

📌 Commit 79c4727 has been approved by petrochenkov

It is now in the queue for this repository.

[tgross35]

@rustbot label +stable-nominated

This fixed a beta regression at the time and probably should have been backported, unfortunately it is now a stable regression #151658.

[apiraino]

stable backport accepted during t-compiler triage, though we don't feel it deserve in isolation a dot release.

(Backport in #151714)

@rustbot label +stable-accepted