codegen: surface Ty::Borrow into OwnershipKind::SharedBorrow / ExclBorrow at emission #221

Context

Surfaced in the paired producer review of typed-wasm proposal 0001 — ephapax#165 comment §2-Q2 (Ptr-kind mapping), 2026-05-30.

OwnershipKind (src/ephapax-wasm/src/ownership.rs:44-48) defines four variants:

pub enum OwnershipKind {
    Unrestricted = 0,
    Linear = 1,
    SharedBorrow = 2,
    ExclBorrow = 3,
}

SharedBorrow and ExclBorrow are defined but never emitted. emit_ownership_section (src/ephapax-wasm/src/lib.rs:1293-1298) maps every function parameter to either Linear (if Ty::is_linear() returns true) or Unrestricted (otherwise). Ty::Borrow distinctions surfaced from the source language collapse into Unrestricted at codegen.

Result: typed-wasm's verifier (crates/typed-wasm-verify/) sees only 2 of 4 possible OwnershipKind values from ephapax-emitted modules. The L7 (aliasing) enforcement that distinguishes SharedBorrow-vs-ExclBorrow is silently inapplicable.

Impact

1. L7 enforcement is partial for ephapax modules. typed-wasm's verify_function (per the verifier's L7 pass) reports ExclBorrowAliased errors when an ExclBorrow parameter is used more than once — but ephapax never emits ExclBorrow, so this check fires zero times against ephapax modules even when the source would warrant it.

2. Cross-language L13 verification (multi-module) loses signal. When typed-wasm gets cross-module verification (gated on a future typedwasm.region-imports carrier — see typed-wasm#94 / #region-imports), a module that imports an ephapax-emitted region cannot tell whether the export was Shared or Excl because both came over the wire as Unrestricted.

3. Convergent producer-review finding. Both 2026-05-30 reviewers of proposal 0001 independently noticed the same gap on the ephapax side (parallel session's review at Review request: typed-wasm carrier-section proposal for L2–L6 / L15 enforcement (typed-wasm#76) #165 (comment) captured "borrows erased pre-codegen" in its summary table).

What needs to change

Wire Ty::Borrow distinctions through emit_ownership_section. Specifically:

• Inspect each function parameter's Ty (not just Ty::is_linear()).
• For Ty::Borrow { linearity: Linearity::Linear, mutable: true, .. } → emit OwnershipKind::ExclBorrow.
• For Ty::Borrow { linearity: Linearity::Linear, mutable: false, .. } → emit OwnershipKind::SharedBorrow.
• For everything else, current mapping (Linear / Unrestricted) is unchanged.

The exact Ty::Borrow discriminator may need to consult both ephapax-linear and ephapax-affine sublanguages — per [[feedback_affinescript_ephapax_siblings_not_impl_proof]] the dyadic property is per-binding, not module-level. Both sublanguages produce Ty::Borrow at the AST level; the lowering may differ on how &mut vs & is surfaced. Inspect Ty::is_linear() (ephapax-syntax/src/lib.rs:153-164) for the current single-bit projection and extend.

Acceptance

• emit_ownership_section (lib.rs:1284-1320) inspects Ty::Borrow and emits SharedBorrow / ExclBorrow where appropriate.
• Round-trip test in src/ephapax-wasm/src/ownership.rs covers a function with one Ty::Borrow parameter of each polarity (& and &mut).
• At least one example in examples/ demonstrates a function signature that emits SharedBorrow.
• typed-wasm verifier round-trip continues to pass on existing ephapax fixtures.

Out of scope

• The dyadic-sublanguage IR doesn't need to change. The fix is at the codegen lowering only.
• This issue does NOT touch region/capability tracking (gated on typed-wasm proposal 0001+0002 acceptance and ephapax-side IR work — see ephapax#165 §3 for the broader scope).

Refs

• ephapax#165 §2-Q2 — paired producer review surfacing this gap
• ephapax#165 §6 open question Claude/ephapax linear types e00 zs #4 — recommended this issue be filed
• typed-wasm#65 — typedwasm.ownership section rename (this section's owner-neutralisation)
• typed-wasm proposal 0001 §"PtrKind encoding" — (Scalar, PtrOwning, PtrBorrow, PtrExclusive) maps to ephapax's (Unrestricted, Linear, SharedBorrow, ExclBorrow)

🤖 Filed 2026-05-30 from paired-review meander.