Rust: Unify type inference for tuple indexing expressions

[hvitved]

This PR adresses the comment https://github.com/github/codeql/pull/20041/files#r2207752592, i.e. type inference for all field expressions, x.i, are treated uniformly, regardless of whether the type of x is a tuple or a tuple struct/enum.

[Copilot]

Pull Request Overview

This PR unifies type inference for tuple indexing expressions by treating all field expressions (x.i) uniformly, regardless of whether the type of x is a tuple or a tuple struct/enum. This aligns with the comment in the GitHub issue to standardize the handling of tuple field access.

• Introduces a new TupleTypeParameterDecl declaration type within the field expression matching framework
• Removes separate handling functions for tuple indexing expressions in favor of the unified field expression approach
• Adds test case to validate tuple field access through references

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
rust/ql/lib/codeql/rust/internal/TypeInference.qll	Main implementation: adds TupleTypeParameterDecl to the field expression matching system and removes old tuple-specific inference functions
rust/ql/lib/codeql/rust/internal/Type.qll	Minor fix: removes redundant TTuple constraint from UnitType class
rust/ql/test/library-tests/type-inference/main.rs	Adds test case for tuple field access through references
rust/ql/test/library-tests/type-inference/type-inference.expected	Updates expected test output to reflect the unified type inference approach

[Copilot]

The abstract Declaration class has a concrete implementation of getTypeParameter that returns none() for all subclasses. This suggests that either this method should be abstract (forcing subclasses to implement it) or it should have a more meaningful default implementation. Consider making this method abstract if different declaration types should handle type parameters differently.

[geoffw0]

Changes and DCA LGTM.

@aibaars FYI.