Module inference 

Type Inference Engine

Implements Hindley-Milner style type inference with extensions for Shape’s domain-specific features.

Bidirectional type checking

The engine supports three checking modes (see bidirectional.rs):

• Infer – purely synthesise a type from the expression structure.
• Check(T) – verify the expression against an expected type (hard constraint, emitted for annotated bindings and return positions).
• Synth(T) – synthesise with a hint (soft constraint, used for closure parameter inference from generic method signatures).

When a method call like arr.map(|x| ...) is encountered, the engine looks up the GenericMethodSignature for the receiver type, extracts the expected closure parameter types, and passes them as Synth hints so that x receives the array element type without annotation.

Sub-modules

• access – property access, index access, field resolution
• bidirectional – CheckMode and the check_expr entry point
• expressions – expression-level inference (literals, calls, closures, match, if/else, binary/unary ops)
• hoisting – optimistic pre-pass that collects property assignments to widen object types before the main inference walk
• items – top-level item inference (functions, types, impls, extends)
• operators – binary and unary operator type rules
• statements – statement-level inference (let, assignment, return, for, while, blocks)

Structs

PropertyAssignment

Collected property assignment information

PropertyAssignmentCollector

Visitor that collects all property assignments in a program

TypeInferenceEngine

Enums

CheckMode

Mode for bidirectional type checking