Milestone 6x — Region parameters (OD-3; plan and acceptance)

Status: complete 2026-10-23 (ADR 0006: implement OD-3 before C3). Design: od3-design.md. Normative: type-system.md §25.

Increments

#	Content	Spec	Status
X1	Syntax and types: region parameters, region arguments (Token<src>, StringView<r>, Slice<r, T>, borrow(r) T, static); erasure; scoped structs, enums, instances and collections; fields and payloads (N0270); scoped type arguments and region contents; N0273	§25.1–§25.5	done 2026-10-23
X2	Flows and borrow checking: signature flows (§25.6) replacing return sources; writes into locals and through references, collection writes, call flows into borrow mut arguments, N0272; type parameters as flow names; scoped generic instances (N0274); drop check	§25.6–§25.7	done 2026-10-23
X3	Programs and tests: zero-copy tokenizer, CSV field splitting into Vector<StringView>, arena-linked graph; compile-fail tests per rule (≥ 2 each) and near-misses; property test with scoped elements; every v0 corpus program unchanged	—	done 2026-10-23

Acceptance criteria

1. Every rule of §25.4 and §25.7 has at least two compile-fail tests and one near-miss compile-pass test (the M2 standard, ownership.md §6).
2. The v0 corpus (all of tests/corpus) passes unchanged.
3. Three programs run: a zero-copy tokenizer, a field splitter returning Vector<StringView>, and a graph of nodes linked by references inside one Arena. Clean under ASan+UBSan, GCC/clang and inside CosmoOS (c2.py, both arches).
4. Docs: type-system.md §25, ownership.md E3/E4, lifetimes.md, diagnostics.md, grammar.md, od3-design.md (status), roadmap.md, risks.md.

Acceptance results (2026-10-23)

#	Criterion	Evidence	Result
1	≥ 2 compile-fail and ≥ 1 near-miss per rule	check-fail/region-declarations.nova (N0270 ×3, N0273 ×3, unknown borrow(r), region values and pool boxes in fields and vectors, duplicate regions), parse-fail/region-*.nova (static, interfaces, order), check-fail/region-flows.nova (rule 1 ×2, rule 2 ×2 incl. a cross-arena link, rule 3 ×3, rule 4 ×2, N0272 ×3, rule 6 ×2), check-fail/region-instances.nova (N0274 ×3). Near-misses: run-pass/region-near-misses.nova and the accepted lines of those files	met
2	v0 corpus unchanged	every earlier test passes; 10 annotations changed, only by code (N0246 → N0270 for scoped fields) or because a case became legal by §25.4 (Vector<borrow i32>, SlotMap<borrow i32>, Handle<borrow i32>)	met
3	Three programs	run-pass/region-tokenizer.nova (zero-copy tokens and Vector<StringView> fields), run-pass/region-graph.nova (arena-linked nodes, re-linking), plus CollectionPropertyTests "views" (6 seeds: Vector<StringView> against a model, then mutating the string after the vector's last use). Clean under clang/GCC, ASan+UBSan and the fault sweep; c2.py on both arches	met
4	Docs	type-system.md §25, grammar.md, ownership.md E3/E4, lifetimes.md §8, diagnostics.md, od3-design.md, ADR 0006	met

Limits recorded (not hidden): one loan set per value (ADR 0006), so a struct with two regions keeps both alive together; arrays and slices of scoped elements stay rejected; a moved-out value with a scoped drop still counts its scope end as a use (conservative); methods cannot declare regions of their own; impl blocks for scoped types stay rejected.