Rust: Type inference for impl trait types with type parameters

[paldepind]

We currently can't handle impl types with type parameters inside them, such as this example where T occurs within the impl:

fn get<T: Clone>(x: T) -> impl MyTrait<T> {
    S3(x)
}

The example is from the draft PR: #19954

This PR fixes the problem by letting type parameters of the function where the return position impl occurs also induce a type parameter of the impl type.

In the above example get has a type parameter T which occurs in the impl in return position. Hence a type parameter corresponding to T is added to the impl type. When get is called an a specific type is known for T then that type will be instantiated inside the returned impl type.

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 + Bar will have both the type Foo and Bar. That is a perfectly fine implementation, but I believe this approach has two advantages:

1. We don't loose the information that the type is an impl type.
2. We don't infer multiple types for the same term. Right now that occurs only due to inaccuracies, and I think it's nicer to keep it that way. This will make it easier to figure out where we have type blowup.

[Copilot]

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 impl trait types by creating ImplTraitTypeParameter objects
• Updates type inference logic to properly handle and rank these new type parameters
• Extends test coverage with various scenarios of parameterized impl trait 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]

Mostly questions to aid my own understanding...

[geoffw0]

All concerns addressed. 👍