commit 943b42f7433a15abfbe6ec718281012d690e5fd8
author Nathaniel McCallum <nathaniel@mccallum.life> Sun Aug 10 01:38:17 2025 -0400
committer Nathaniel McCallum <nathaniel@mccallum.life> Sun Aug 10 02:21:11 2025 -0400
tree d1b45bc9f7ec5db2c1c5f813c8de265a3009df96
parent 31db5b5be9f078de7cae45bf3486876358702344

parser: fix parsing of trait bound polarity and for-binders

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.