ADR 0005 — M6: user generics and interfaces; built-in collections; floats deferred

• Status: Accepted (project owner, 2026-10-22, when starting M6: "Built-in types" for collections, "Adapt Ex13" for floats)
• Scope: what M6 implements in the language, and what stays built in.

Decision

1. User generics and interfaces are real language features in M6. This covers generic functions, structs and enums, monomorphisation, interfaces, impl blocks and bounds (language-spec.md Ex12, Ex13, OD-10: static dispatch only).
2. Generic bodies are checked once, generically (type-system.md §20):
   • a type parameter is an opaque type: a move type that may need drop, supporting only what its bounds allow;
   • move and borrow checking also run on the generic version;
   • each instantiation is then monomorphised for C. Errors are reported at the definition, not per instantiation.
3. Collections are built-in generic types, like Box (ADR 0003) and the regions (ADR 0004): Vector<T, A>, String<A>, StringView, SlotMap<T> with Handle<T>, and StableVector<T>. Their memory is managed by the C runtime. The user-visible API is the one memory-model.md §4 describes. A Nova core library implementation waits for unsafe (M8).
4. Library conversion is deferred to M8. Result, Box, the regions and the collections become Nova library types once raw pointers and unsafe exist. The surface syntax stays.
5. Floats stay out. Ex13 runs with integer geometry. Floats (f32/f64 arithmetic, formatting, the kernel-profile ban) are an open item to schedule separately (OD-23 in open-decisions.md).

Consequences

• M6 adds the generic machinery (type parameters, instantiation, inference, interfaces) that M8's library will need. The collections' semantics (invalidation through borrows, H2) are checked by the same borrow checker.
• Ex13 and Ex12 run in adapted forms. Ex12's return if (…) a else b is written as an if statement, since if expressions do not exist in v0.