Rust: Remove unused impl type

[paldepind]

I just realized that the type for impl blocks is in fact unused after the recent changes to method resolution.

[Copilot]

Pull Request Overview

This PR removes now-unused support for treating impl blocks as types following recent method resolution changes.

• Deleted the isImplSelfTypeParam predicate and the ImplMention class in TypeMention.qll.
• Removed the TImpl variant and the ImplType class in Type.qll.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rust/ql/lib/codeql/rust/internal/TypeMention.qll	Removed ImplMention class and related predicate
rust/ql/lib/codeql/rust/internal/Type.qll	Dropped TImpl from TType union and deleted ImplType

Comments suppressed due to low confidence (2)

rust/ql/lib/codeql/rust/internal/TypeMention.qll:191

• Removal of the ImplMention class could affect type resolution for impl blocks. Please add or update tests to ensure no regressions in resolving self-path type parameters.

class ImplMention extends TypeMention, ImplItemNode {

rust/ql/lib/codeql/rust/internal/Type.qll:15

• The TImpl variant and ImplType class have been removed. Verify with tests that all type-resolution scenarios formerly handled by this variant are still correctly covered.

TImpl(Impl i) or

[geoffw0]

I assume we don't expect to need this type again in future.

[paldepind]

I assume we don't expect to need this type again in future.

Right now it doesn't look very likely. I think maybe if in some future the shared library handles method resolution something like it could be needed. But that's highly speculative and even if it does happen it could look completely different.

First DCA filed for some reason but the looks fine (no changes).