Rust: Type inference for non-overloadable operators

[paldepind]

This PR adds type inference for all non-overloadable operators in Rust. It turns out that there are only three such operators. && and || (which are not overloadable for short-circuiting reasons) and =.

The assignment operator evaluates to unit, and as we didn't have a unit type I added one.

[Copilot]

Pull Request Overview

Adds type inference support for Rust’s built-in logical (&&, ||) and assignment (=) operators by introducing a unit type.

• Implement inference rules for && and || returning bool
• Infer assignment expressions as () and track LHS types
• Define TUnit() in the type system and a UnitType class

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
rust/ql/test/library-tests/type-inference/type-inference.expected	Updated expected results to cover &&, `
rust/ql/test/library-tests/type-inference/main.rs	Added operators module with tests for logical and assignment operators
rust/ql/lib/codeql/rust/internal/TypeInference.qll	Imported builtins and added inferLogicalOperationType and inferAssignmentOperationType rules
rust/ql/lib/codeql/rust/internal/Type.qll	Extended TType with TUnit() and implemented a UnitType class

Comments suppressed due to low confidence (3)

rust/ql/lib/codeql/rust/internal/TypeInference.qll:10

• Typo in module path: Bultins should be Builtins to correctly import the stdlib Builtins module.

private import codeql.rust.frameworks.stdlib.Bultins as Builtins

rust/ql/lib/codeql/rust/internal/TypeInference.qll:257

• In typeEquality, the QL class AssignmentExpr is used but elsewhere AssignmentOperation is referenced; use a consistent AST class (likely AssignmentOperation) to ensure coverage.

exists(AssignmentExpr be |

rust/ql/lib/codeql/rust/internal/Type.qll:12

• [nitpick] Consider adding a brief comment next to TUnit() in the newtype TType declaration to explain its role representing the unit () type.

TUnit() or

[hvitved]

LGTM. I have started a DCA run.

[paldepind]

DCA is looking good. Thanks for starting it.