Rust: Implement support for inference of type aliases

[paldepind]

This PR implements support for type aliases. I can think of two ways to implement type aliases:

1. Use subtyping and getABaseTypeMention.
2. Effectively "inline" the type alias by having type mentions that resolve to a type alias give the alias definition as their type in resolveTypeAt.

This PR does the latter, for a few reasons:

• The approach handles nested type aliases with no extra work. Right now our support for subtyping doesn't consider subtypes within subtypes.
• For something like if b { let x: Alias1 = ...; x } else { let y: Alias2 = ...; y } where Alias1 and Alias2 refer to the same type, immediately unfolding the alias means that the entire if expression only gets one inferred type, and not both (identical) type aliases.
• When we get to handle constrained trait implementations we need to check if type are equal, and that might be easier when there are no aliases in the way and types are already "normalized".
• We only apply subsumption at specific program points (arguments to functions, etc.) whereas this approach means that aliases are handled everywhere.

[Copilot]

Pull Request Overview

This PR implements support for type alias inference in Rust by inlining alias definitions to normalize types. Key changes include the addition of a new helper method (unwrapSnd) for the PairOption type, updates to type alias definitions (including nested aliases), and adjustments to test cases to reflect these changes.

Files not reviewed (1)

• rust/ql/lib/codeql/rust/internal/TypeMention.qll: Language not supported

Comments suppressed due to low confidence (1)

rust/ql/test/library-tests/type-inference/main.rs:541

• [nitpick] The generic parameter 'A3' is not very descriptive. Consider using a more meaningful name to improve code clarity.

type AnotherPair<A3> = PairOption<S2, A3>;

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[Copilot]

Typo in comment: 'withing' should be corrected to 'within'.

[hvitved]

Very nice, one suggestion.

[paldepind]

Thanks for reviewing :)