CARD: scaffold-b-typed-builders — Typed Builders / Typestate

Band 1 — Identity & Recognition

02Classification: layer=E (verification); mechanism=scaffold B.

03Intent: Make the statistically-likely wrong call un-representable, so a hallucinated edit fails cargo check before runtime — encoding protocol correctness in types rather than docstrings.

04Also Known As: typestate; phantom types; type-state builder; sealed trait; newtype wrapper; make-illegal-states-unrepresentable.

05Applicability / Recognition: Apply when — a seam has a usage protocol (order of calls, required fields, valid states); a primitive (u64, String, bool) crosses a boundary where its meaning matters; an API takes multiple same-typed args or a bool flag. Detector seed: a pub seam fn taking &str/bool/multiple u* of the same type, OR a runtime check that a struct is "ready" → recognition fires (94% of compile errors are type-level; move the check there).

Band 2 — Justification & Tradeoffs

06Motivation: A weak agent calls connect(host, port, true, false) and swaps the bools. With ConnectionBuilder requiring .tls(Tls::Enabled) and .host(Host::new(...)), the swap does not type-check; the error surfaces in the loop, not in production.

07Structure & Participants: Newtype (primitive + meaning) · Typestate marker (phantom state) · Builder (type-mandatory required fields) · Sealed trait (closed extension).

08Collaborations: Shrinks the input space Class D oracles must cover; the compiler is the Class E loop's primary checker; pairs with Class C for runtime invariants types can't express.

09Goals / Non-Goals: Goals: convert probable hallucinations to compile errors at seams. Non-Goals: NOT typestate everywhere (ergonomic cost) — scope to seam surfaces; NOT a replacement for contracts on value-range invariants.

10Consequences: (+) a whole class of misuse becomes uncompilable; (+) the type IS the protocol doc. (−) typestate ergonomics cost for human contributors; (−) over-typing fights idiom — scope tightly.

11Alternatives: runtime validation (errors surface late — in production, not the loop); a contract (Class C) when the invariant is a value property, not a protocol.

12Risks & Assumptions: assumes the protocol is type-expressible; some invariants need Class C. Sunset: none material. Strong models may be mildly distorted by over-constraint — keep newtype/typestate proportional.

13Evidence & Transfer-strength: R3-008 (misuse-resistance, theory), DR2-012/R2C-005 (94% type-level errors; type-awareness cuts compile errors, benchmark). Class: benchmark + theory. Tag: [E-mid].

Band 3 — Operation

trigger: WHEN a pub seam fn takes &str/bool/duplicate-same-type args, OR a runtime "is-ready" check exists THEN apply
mode: gate            # introduced at seam design; checked at merge
routine:
  1. Wrap each meaning-bearing primitive at the seam in a newtype.
  2. Encode call-order/required-field protocol as typestate or a type-mandatory builder.
  3. Seal extension traits; add #[must_use] where ignoring the result is a defect.
  4. Delete the now-impossible runtime validity checks.
  5. Confirm the previously-wrong call no longer compiles (add a trybuild ui test).
checker: conform T-sem `seam-protocol-typed` + trybuild compile-fail test
raid_role: layer=seams; order=after:none; batch=seam
budget: active_rules=1; first_signal=cargo check (<60s)