Rust: Model union, never, and pointer types

[hvitved]

Extracted from #20282.

DCA looks good: increase in resolvable calls by 0.353 % points.

[Copilot]

Pull Request Overview

This PR adds support for modeling union, never (!), and pointer types in Rust's type system. The changes enhance the type inference system to better handle these Rust type constructs, resulting in improved call resolution capabilities.

Key changes:

• Added new type representations for union, never, and pointer types to the type system
• Extended type mention resolution to handle these new type constructs
• Updated type inference logic to support pointer type parameters

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rust/ql/lib/codeql/rust/internal/Type.qll	Adds UnionType, NeverType, and PtrType classes with their type parameters
rust/ql/lib/codeql/rust/internal/TypeMention.qll	Adds type mention classes for never and pointer type representations
rust/ql/lib/codeql/rust/internal/TypeInference.qll	Extends type inference to handle pointer type parameters
rust/ql/lib/codeql/rust/elements/internal/UnionImpl.qll	Adds getStructField method to Union class for field access
rust/ql/test/library-tests/type-inference/type-inference.expected	Test expectations updated to reflect improved never type inference
rust/ql/test/query-tests/security/CWE-770/CONSISTENCY/PathResolutionConsistency.expected	Test expectations for improved call resolution with pointer casts
rust/ql/test/query-tests/security/CWE-696/CONSISTENCY/PathResolutionConsistency.expected	Test expectations for improved call resolution with pointer operations
rust/ql/test/extractor-tests/macro-expansion/CONSISTENCY/TypeInferenceConsistency.expected	Removes previous type inference inconsistency for union types

[paldepind]

One tiny comment, otherwise looks really good!

[paldepind]

Nice! With this I think we have support for all forms of types in Rust (maybe with the exception of function pointer types) which is really great!