parser: fix parsing of trait bound polarity and for-binders #20417
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The rustc AST allows both `for<>` binders and `?` polarity
modifiers in trait bounds, but they are parsed in a specific
order and validated for correctness:
1. `for<>` binder is parsed first.
2. Polarity modifiers (`?`, `!`) are parsed second.
3. The parser validates that binders and polarity modifiers
do not conflict:
```rust
if let Some(binder_span) = binder_span {
match modifiers.polarity {
BoundPolarity::Maybe(polarity_span) => {
// Error: "for<...> binder not allowed with ? polarity"
}
}
}
```
This implies:
- `for<> ?Sized` → Valid syntax. Invalid semantics.
- `?for<> Sized` → Invalid syntax.
However, rust-analyzer incorrectly had special-case logic that
allowed `?for<>` as valid syntax. This fix removes that incorrect
special case, making rust-analyzer reject `?for<> Sized` as a
syntax error, matching rustc behavior.
This has caused confusion in other crates (such as syn) which
rely on these files to implement correct syntax evaluation.
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I have fixed the upstream bug which led to this mistake: rust-lang/rust-analyzer#20417
npmccallum
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I have fixed the upstream bug which led to this mistake: rust-lang/rust-analyzer#20417
|
Interesting, the current parsing used to work (and even added specifically in #12444), but that changed in rust-lang/rust#127054. |
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BTW, in what way does |
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The rustc AST allows both
for<>binders and?polarity modifiers in trait bounds, but they are parsed in a specific order and validated for correctness:for<>binder is parsed first.?,!) are parsed second.This implies:
for<> ?Sized→ Valid syntax. Invalid semantics.?for<> Sized→ Invalid syntax.However, rust-analyzer incorrectly had special-case logic that allowed
?for<>as valid syntax. This fix removes that incorrect special case, making rust-analyzer reject?for<> Sizedas a syntax error, matching rustc behavior.This has caused confusion in other crates (such as syn) which rely on these files to implement correct syntax evaluation.