Rust: Infer more () types

[hvitved]

Although unit types are not interesting in themselves, it makes a difference now that we are keeping track of expressions without a known type.

Thanks to @paldepind for originally suggesting to have ShorthandReturnTypeMention (like we have ShorthandSelfParameterMention).

DCA confirms that we now have fewer expression without types.

[Copilot]

Pull Request Overview

This PR enhances Rust type inference to handle implicit unit return types (()) for functions without explicit return types. The main implementation adds a ShorthandReturnTypeMention class that represents the implicit () return type using the function's name as a placeholder element.

Key changes:

• Implemented ShorthandReturnTypeMention to handle implicit unit return types for functions
• Enhanced block expression type inference to detect unit-type blocks (no tail expression and no return statements)
• Updated documentation examples to use valid Rust syntax with function bodies

Reviewed Changes

Copilot reviewed 10 out of 20 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rust/ql/lib/codeql/rust/internal/TypeMention.qll	Added ShorthandReturnTypeMention class and getReturnTypeMention() function to handle implicit unit return types
rust/ql/lib/codeql/rust/internal/TypeInference.qll	Updated type inference to use new return type mention API, enhanced block expression handling with isUnitBlockExpr(), and inferBlockExprType()
rust/ql/test/library-tests/type-inference/main.rs	Added test cases for async blocks returning unit type and local functions
rust/schema/annotations.py	Fixed documentation examples to use valid Rust syntax
rust/ql/test/extractor-tests/generated/*.rs	Updated generated test files with corrected documentation examples
rust/ql/lib/codeql/rust/elements/*.qll	Updated documentation in generated QL files
rust/ql/test/library-tests/type-inference/CONSISTENCY/PathResolutionConsistency.expected	Updated test expectations with corrected line numbers
rust/ql/.generated.list	Updated checksums for regenerated files
rust/ql/test/extractor-tests/generated/.generated_tests.list	Updated checksums for regenerated test files

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[paldepind]

Nice! I think it's great to get these covered.

Three thoughts (in addition to my comments):

• Superficially it looks like it would make sense to use getReturnTypeMention here in getTypeMention for FunctionPosition. Is there a reason not to do that?

• I would've guessed that knowing more types certainly would reduce "Nodes With Type At Length Limit", but it increases per the DCA report. I think it would be worth it to take a look at one of the affected projects and see what's up.

• Just for the future, it would have been really nice if the changes to annotations.py and the generated files has been a separate commit.

[hvitved]

• Superficially it looks like it would make sense to use getReturnTypeMention here in getTypeMention for FunctionPosition. Is there a reason not to do that?

Done, thanks.

• I would've guessed that knowing more types certainly would reduce "Nodes With Type At Length Limit", but it increases per the DCA report. I think it would be worth it to take a look at one of the affected projects and see what's up.

I think I was able to figure it out: panic!(...) was incorrectly inferred to have type (), and this meant that we could end up with a receiver having both type () and some other type, which meant that we would allow for a method call both with and without an implicit borrow, which can then explode. The fix is to first make sure that panic!(...) gets the type !, and then subsequently to filter away ! types when matching receiver types against types of self parameters.

• Just for the future, it would have been really nice if the changes to annotations.py and the generated files has been a separate commit.

Yes, sorry, I should have done that in a separate commit.

[paldepind]

Thanks for the PR and for addressing my comments.