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943b42f743
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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