Don't try to recover keyword as non-keyword identifier
Fixesrust-lang/rust#149692.
On beta after rust-lang/rust#146978, we ICE on
```rs
macro_rules! m {
($id:item()) => {}
}
m!(Self());
```
where `Self` in the macro invocation is a keyword not a "normal" identifier, while attempting to recover an missing keyword before an identifier. Except, `Self` *is* a keyword, so trying to parse that as a non-reserved identifier expectedly fails.
I suspect rust-lang/rust#146978 merely unmasked a possible code path to hit this case; this logic has been so for a good while. Previously, on stable, the error message looks something like
```rs
error: expected identifier, found keyword `Self`
--> src/lib.rs:5:4
|
5 | m!(Self());
| ^^^^ expected identifier, found keyword
error: missing `fn` or `struct` for function or struct definition
--> src/lib.rs:5:4
|
2 | ($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).
9ef76797ea