rust-analyzer: fix parsing of trait bound polarity and for-binders

[npmccallum]

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.

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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[rustbot]

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

[jieyouxu]

This is a r-a-only change, so please instead submit this PR against the r-a repository.

[npmccallum]

This is a r-a-only change, so please instead submit this PR against the r-a repository.

I don't think this is true. I think my commit message just triggered the bot. This is a change to the parser crate which is hosted in this repo. I'm not sure the best way to improve my commit message.

[jieyouxu]

I don't think this is true. I think my commit message just triggered the bot. This is a change to the parser crate which is hosted in this repo. I'm not sure the best way to improve my commit message.

To clarify, rust-analyzer is a subtree of this repository, but it is not a git submodule (because that has annoying UX issue). It is in fact sync'd between this repo and rust-analyzer with https://github.com/rust-lang/josh-sync. You can see an example sync such as #144887.

Please refer to https://rustc-dev-guide.rust-lang.org/external-repos.html?highlight=josh#external-dependencies-subtrees.

I.e. the bot message is correct.

[jieyouxu]

In any case, I'll roll a rust-analyzer reviewer, if the r-a reviewer is fine reviewing/accepting it on the r-l/r repo side then disregard the above.

r? rust-analyzer

[ChayimFriedman2]

Please send rust-analyzer-only PRs to https://github.com/rust-lang/rust-analyzer/.

[npmccallum]

rust-lang/rust-analyzer#20417