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.