Equality Operator Architecture

Public syntax and semantics are specified in Values and Types and Generics, Interfaces, and Methods. This document records the compiler boundaries for v0.12.0 Phase 1.

Authored and Resolved Models

EqualityOperatorDecl is an instance member with an ordinary visibility boundary, fixed receiver shape, named right binding, bool result, and body. OperatorRequirementPredicate is a distinct where-clause node. Neither representation is encoded as an interface, a method spelling visible to users, or free-form operator-overload metadata.

Resolution places the callable form of an equality declaration in the owner's ordinary static method set under a compiler-private identity. That identity lets method visibility, declaration patterns, qualification, cross-source bodies, specialization, call targets, occurrences, and static lowering reuse existing services. Presentation always maps the identity back to authored operator syntax and never exposes the private name.

Selection and Plans

Type checking first tries an equality declaration on the exact left owner. If none applies, it asks the common receiver-coercion candidate service for one-step readonly targets. The selected right parameter is then checked exactly or through one readonly coercion. Owned operands receive an implicit readonly borrow adjustment; already borrowed operands retain their capability. More than one viable left-coercion target is a focused ambiguity, not a missing-operation error.

One TypecheckEqualityPlan records:

• source spans and concrete operand types;
• selected callable declaration identity and concrete Self type;
• left and right conversion plans;
• the right implicit-borrow adjustment.

Specialization substitutes unresolved generic operand types and rebuilds this plan once concrete types are known. Ownership, provenance, buildability, IR, and editor consumers use the recorded plan. They must not look up ==, inspect a standard type name, or select a coercion again.

Lowering and Ownership

Primitive booleans, integers, str data, and payloadless enum tags retain leaf equality lowering. Source-defined equality lowers through the ordinary static-call boundary. Receiver preparation and the implicit right borrow evaluate each operand once, apply their recorded conversion at most once, and preserve the owners after the call. != wraps the same result in logical negation.

Generic operator requirements are compile-time evidence. They add no witness, metadata, or ABI field. A specialized call must construct a concrete equality plan before IR lowering.

Standard-Library Boundary

std/str owns byte equality. String reaches it through its existing readonly str coercion. std/slice owns element equality and readonly search under where (&T == &T): bool; Vec<T> reaches that surface through its slice coercion. Iterator default methods use the same requirement and borrow each yielded owner only for the comparison so ordinary cleanup remains authoritative.

No compiler table names these types or APIs. The built-in-instance authority model is the only exception that permits the selected standard package to attach the source declaration to str or [T].

Analysis Boundary

Hover and completion format the authored declaration with a concrete owner. Definition, references, and rename use the declaration identity at the == token. The right binding retains its independent parameter identity. Semantic tokens classify operator as a keyword, == as a method declaration, and both operands as readonly parameters.

Analysis consumes resolver identities and typecheck plans. It must not scan text for operator shapes or expose the compiler-private callable identity.