Rust: Type inference for impl trait types with type parameters #20119
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Pull Request Overview
This PR implements type inference for impl trait types that contain type parameters from their enclosing function. The implementation allows proper handling of return position impl types that reference function type parameters, such as fn get<T: Clone>(x: T) -> impl MyTrait<T>.
Key changes:
- Adds support for type parameters within
impltrait types by creatingImplTraitTypeParameterobjects - Updates type inference logic to properly handle and rank these new type parameters
- Extends test coverage with various scenarios of parameterized
impltrait return types
Reviewed Changes
Copilot reviewed 8 out of 9 changed files in this pull request and generated 1 comment.
Show a summary per file
| File | Description |
|---|---|
| main.rs | Adds comprehensive test cases for impl trait types with type parameters |
| PathResolutionConsistency.expected | Updates line number references due to test additions |
| TypeMention.qll | Implements type resolution for ImplTraitTypeParameter |
| TypeInference.qll | Extends type parameter ranking system to handle ImplTraitTypeParameter |
| Type.qll | Core implementation of ImplTraitTypeParameter class and related logic |
| ImplTraitTypeReprImpl.qll | Adds getFunctionReturnPos() method to locate containing function |
| .gitattributes & .generated.list | Removes generated file markers for ImplTraitTypeReprImpl.qll |
geoffw0
reviewed
Jul 25, 2025
rust/ql/lib/codeql/rust/elements/internal/ImplTraitTypeReprImpl.qll
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geoffw0
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Jul 28, 2025
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We currently can't handle
impltypes with type parameters inside them, such as this example whereToccurs within theimpl:The example is from the draft PR: #19954
This PR fixes the problem by letting type parameters of the function where the return position
imploccurs also induce a type parameter of theimpltype.In the above example
gethas a type parameterTwhich occurs in theimplin return position. Hence a type parameter corresponding toTis added to theimpltype. Whengetis called an a specific type is known forTthen that type will be instantiated inside the returnedimpltype.This implementation is a bit different from the draft one in #19954. That implementation relies on inferring trait types directly. I.e. elements of
impl Foo + Barwill have both the typeFooandBar. That is a perfectly fine implementation, but I believe this approach has two advantages:impltype.