Traits Cookbook

This subsection of the guide is a focused, example-driven reference for Nitpick's trait / impl / dyn surface as it stands at the close of the v0.31.1.x phase (Phase 2 of the v0.31.x cycle).

Traits in Nitpick are a static abstraction over behaviour: a trait declares a set of method signatures, an impl T:for:U block supplies those signatures for a concrete type U, and call sites dispatch in two modes — static (the type is known at compile time and the method is resolved directly) and dynamic (the value is held behind a dyn T fat pointer and the method is resolved through a vtable at run time).

Chapters

1. Basics — declaring a trait, writing an impl T:for:U, and calling methods via UFCS (x.foo()).
2. Dyn surface — dyn T as a local variable, struct field, and function parameter; how concrete → dyn coercion works.
3. Borrow rules — $$i dyn T and $$m dyn T borrows, the source-side conflict rules, and the call-site interaction.
4. Return-borrow methods — $$i / $$m return types, the from <ident> source-binding clause, the multi-borrow defaults (TRAIT-RET-DEC-002/005), and why dyn dispatch is rejected for return-borrow methods (ARIA-053). New in v0.31.8.x.

Each chapter is short and self-contained.

Conventions used in these chapters

• Examples are minimal func:main programs that compile and run with npkc.
• "concrete type" means a named struct or primitive that satisfies a trait.
• "dyn T" means the trait-object form: a {data, vtable} fat pointer.
• Diagnostics are quoted as the compiler emits them, with the ARIA-NNN code.
• Where bug regressions are referenced, the fixture number is given in parentheses, e.g. (bug374).

Validation status (v0.31.1.10)

• Compiler CTest: 107/107 (modulo 2 known pre-existing flakes).
• K core tests: 151/151.
• K proofs: 11/11.
• Trait/dyn-related bug regressions: bug363 ... bug378.

Scope and non-scope

In scope for Phase 2 / v0.31.1.x:

• trait declaration with method signatures.
• impl T:for:U with one method per signature.
• UFCS dispatch on concrete values (c.bump()).
• dyn T as a local variable, struct field, and function parameter.
• Concrete → dyn T coercion at assignment and call sites.
• $$i dyn T and $$m dyn T borrows with source-side conflict checking (ARIA-019 / ARIA-023 / ARIA-026).

Not yet in scope (deferred to later v0.31.x phases or v0.32.x+):

• Trait inheritance / super-traits.
• Associated types and associated constants.
• Generic trait bounds (func:f = T($$i T:x) where T: Trait).
• derive for arbitrary traits (only derive(Display) exists today — see comptime cookbook).
• obj (the "owned trait object" form discussed in PLAN.md D-9 follow-on).