Rust: Don't propagate impl in return position into function bodies

[paldepind]

With this PR: When an impl Trait type appears in a function's return type annotation, don't propagate it into the function itself.

I think this makes more sense as the annotation is for users of the function, and it's on purpose not the correct type to use inside the function.

[Copilot]

Pull request overview

This PR improves Rust type inference by preventing impl Trait return types from being propagated into function bodies. The change correctly recognizes that impl Trait in return position is an abstraction for function consumers, not the actual type used within the function implementation.

Key changes:

• Refactored function body type inference to filter out impl Trait return types
• Function bodies now correctly infer concrete types instead of opaque impl Trait types
• Updated test expectations to reflect the corrected type inference behavior

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/TypeInference.qll	Refactored type annotation logic by extracting function body type inference into a dedicated predicate with proper filtering to exclude ImplTraitReturnType and its type parameters
rust/ql/test/library-tests/type-inference/type-inference.expected	Auto-generated test expectations showing that function bodies now infer concrete types (e.g., dyn Future, S1, S2) instead of impl Trait types

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[hvitved]

Makes sense 👍