Rust: Model async return types as dyn Future

[hvitved]

Instead of assigning the type Future directly, we assign dyn Future, just like we assign dyn FnOnce to closures. As @paldepind notes on that PR, using impl Future would be more faithful, but in terms of our type inference implementation the effect is the same.

DCA is fine.

[Copilot]

Pull request overview

This PR updates the type inference for Rust async functions to model their return types as dyn Future instead of the raw Future trait type. This change aligns with how closures are already modeled as dyn FnOnce, providing consistency in the type system representation. The change affects both async blocks and async functions.

• Introduces a new mentions.rs file to ensure the dyn Future and dyn FnOnce types are available in the type system
• Updates type inference logic to use dyn Future with appropriate type parameters
• Adjusts test expectations to reflect the new type representation (from trait Future to dyn Future and from Output to dyn(Output))

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
rust/tools/builtins/mentions.rs	New file that declares type mentions for dyn Future and dyn FnOnce to ensure these trait object types are available for type inference
rust/ql/test/library-tests/type-inference/type-inference.expected	Updated test expectations to reflect async return types now being represented as dyn Future with dyn(Output) parameters instead of trait Future with Output parameters
rust/ql/lib/codeql/rust/internal/TypeInference.qll	Core implementation changes: updated getFutureTraitType() to return DynTraitType, added getDynFutureOutputTypeParameter() helper, and replaced references to use the new dyn-based type parameters throughout async handling code

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[paldepind]

Nice! Thanks for doing this.