Rust: Improve handling of Self type parameter

[paldepind]

This PR fixes two issues in how we handle Self type parameters. They're separate but related issues, so I've bunched them into one PR. Together these fixes should address https://github.com/github/codeql/pull/20682/files#r2464739113.

1/ Self in a default for a trait type parameter

If we have a default like this:

trait TraitWithSelfTp<A = Option<Self>> { ... }

Then, when TraitWithSelfTp is used as a trait bound, the meaning of the Self depends where that trait bound occurs. For instance, T : TraitWithSelfTp is the same as T : TraitWithSelfTp<T> and not T : TraitWithSelfTp<Self_TraitWithSelfTp>

The type mention Self in the default refers to the the Self type parameter for TraitWithSelfTp, but this needs to be translated at trait bounds.

2/ Self in supertraits

When Self occurs in the trait hierarchy

trait Sub: Super<Self> { ... }
trait SubSub : Sub { ... }

then those Self occurrences need to be instantiated such that the shared conditionSatisfiesConstraintTypeAt machinery calculates the correct trait hierarchy. Above, we should have that SubSub implements Super<Self_SubSub> and not some other Self parameter. To this end, the type mention for supertraits must instantiate the supertrait's Self type parameter.

[Copilot]

Pull Request Overview

This PR adds support for handling trait type parameters with defaults that contain the implicit Self type parameter. The main enhancement allows the CodeQL type inference system to correctly resolve type defaults like trait TraitWithSelfTp<A = Option<Self>> by substituting Self with the concrete implementing type.

Key changes:

• Added logic to detect and substitute Self type parameters in trait type parameter defaults
• Added support for resolving Self type parameters from trait bounds, impl blocks, and type parameter bounds
• Added comprehensive test cases covering various scenarios where Self appears in type parameter defaults

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
rust/ql/lib/codeql/rust/internal/TypeMention.qll	Implements core logic for detecting and resolving Self type parameters in trait defaults across different contexts
rust/ql/test/library-tests/type-inference/main.rs	Adds test module trait_default_self_type_parameter with various test cases and corresponding call in main function
rust/ql/test/library-tests/type-inference/CONSISTENCY/PathResolutionConsistency.expected	Updates expected line numbers in consistency test results (shifted by test additions)

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[Copilot]

The getSelfTypeParameter method lacks documentation explaining its purpose and the three distinct contexts where Self type parameters can be resolved. Consider adding a comment block explaining: (1) impl blocks, (2) trait supertraits, and (3) type parameter bounds.

[Copilot]

[nitpick] The comment on lines 186-187 could be more specific. It would be clearer to state that when a default type parameter is the Self type parameter (e.g., A = Option<Self>), it must be substituted with the concrete type from the trait implementation context.

[hvitved]

Very nice work; some trivial comments.

[hvitved]

Remove

[hvitved]

getSelfTraitBoundArg?

[hvitved]

I would move this up before SelfTypeParameterMention.

[paldepind]

Thanks for the review. I've addressed the comments.

The DCA report shows a small slowdown on average. When I evaluate locally I'm not seeing anything that sticks out (bad joins, etc.), so it might be noise or unavoidable.