Emitter: T? returns emit Option<Rc<T>> (fn-sig lowering); kills the 1013-error E0308 class

[briansrls]

Summary

Fn-sig lowering for optional shared-type returns (T? where T ∈ shared_types) was emitting bare Rc<T> while bodies already emit Some(..)/None. This PR routes shared-type render paths through render_rust_shared_type_with_optional — render_rust_shared_type_if_needed composed with the existing cardinality-gated rust_carrier_optional_wrap — matching the String? pattern landed in #5717.

Touches 05_emit_rust.dag (authority) + paired v1_compiler_emit_rust.rs seed delta + regression witnesses in optional_carrier_signature_test.rs.

Receipts

(a) ARCHAEOLOGY — always-latent, not a recent regression

git log -S 'rust_carrier_optional_wrap' --oneline -- src/v1/05_emit_rust.dag
→ 09eaa749c6 Route-A final-six (#5717)   # introduced wrap for String? only
→ f5f9ff96ad (this PR)

git show 09eaa749c6:src/v1/05_emit_rust.dag | rg 'render_rust_fn_sig_type' -A6
→ peeled-alias fn-sig paths already called render_rust_shared_type_if_needed WITHOUT optional wrap

rust_carrier_optional_wrap (#5717) fixed String? early-returns but left shared-type fn-sig paths on bare render_rust_shared_type_if_needed → Rc<T> with no Option<>. Always-latent since #5717.

#6243 expanded peeled-alias fn-sig routing (render_rust_shared_type_if_needed at alias-leaf sites) but did not add optional composition — widened exposure, did not introduce the gap.

Classification (neat-hawk-785, #6291 tree): 27 pub fn signatures lose Option<>; dominant fresh-emit residue class E0308 ~1013 + E0599 ~33 from sig/body mismatch on optional shared returns (e.g. lookup_type_by_name).

(b) BYTE-IDENTITY — seed delta vs fresh-emit

regen_stage0 write-mode aborts on unregistered test_claim_ownership_movable.rs (pre-existing blocker), so seed was synced manually from .dag authority.

Fresh-emit proof (write-mode assembly, not committed):

cargo run --release -p v1-compiler --bin regen_stage0 -- --emit-fresh /tmp/stage0-fresh-6295
→ emits v1_compiler_emit_rust.rs with render_rust_shared_type_with_optional + all call sites

Touched-region check: render_rust_shared_type_with_optional and every render_rust_shared_type_with_optional(...) call site in committed c0332f42ed match fresh-emit output semantically. Full-file diff is not byte-identical (fresh emit includes orthogonal corpus churn — e.g. build_move_site_licenses import); no logic fork in the changed region.

Committed delta: v1_compiler_emit_rust.rs +37/−9 paired with 05_emit_rust.dag +13/−9 helper + call-site rewrites.

(c) FIXTURE RED / GREEN CONTROL

Witness: optional_shared_type_return_renders_option_rc_signature (recursive Node fixture so shared_types Rc applies).

state	emitter	test	emitted sig
RED	origin/main .dag + seed	exit 101	fn maybe_node(flag: bool) -> Rc<Node>
GREEN	this branch .dag + seed	exit 0	Option<Rc<..>> (contains Option< + Rc<)

Commands:

# RED: swap to main emitter, run witness
# GREEN: restore branch emitter, cargo test -p v1-compiler-tests optional_carrier_signature → 4/4

(d) RE-MEASURE TABLE

Independent re-measure by sharp-deer-310, 2026-07-05 — HEAD 8634ad9533, detached scratch worktree, shared-target build. One documented measurement scaffold: uncommitted roster line for test_claim_ownership_movable so write-mode regen completes. Regen wrote 93 files clean; fresh crate cargo build = 142 errors total.

code	pre-fix (classification)	post-fix (measured)	Δ
total	1192	142	−1050
E0308	~1013	1	−1012
E0599	33	0	−33
E0282	4	0	−4
E0277	—	1	tail item
E0425	—	140	separate work item (use-line class)

This fix kills the entire optional-return mass exactly as classified. E0425=140 remainder is the use-line class (separate work item). The 2 stragglers (1 E0308 + 1 E0277) go to the tail item.

Targeted witness (same fixture, main vs fix emitter):

metric	pre-fix (main)	post-fix (branch)
optional Node? fn sig	-> Rc<Node>	-> Option<Rc<Node>>
witness exit	101	0

Test plan

• cargo fmt --check — green
• cargo clippy -p v1-compiler -- -D warnings — green
• cargo test -p v1-compiler-tests optional_carrier_signature — 4/4 green
• CI rust_tests on 8634ad9533 — pass (6m2s)

Hand-test / test_migration_debt note

optional_carrier_signature_test.rs is an existing src/v1/tests module (same precedent as machine_width_phantom_arg_rust_emit_peels_literal_width_to_unit in width_nat_type_arg_test.rs). This PR adds +2 #[test] fns / +23 LOC to that module (85 LOC total, 4 fns). Dies with v1 at terminal shrink. Counts against v2.lens.test_migration_debt live headroom (+2 fn / +23 LOC vs baseline 881/28054) — within headroom today; warm-ferret's lane owns the ledger update.

Worker attestation

• Title describes the change (not session id / raw work-item text)
• PR body carries receipts (archaeology, byte-identity, red/green, re-measure)
• Tests run: cargo test -p v1-compiler-tests optional_carrier_signature → 4/4
• Full --emit-fresh E0308/E0599 recount — independent re-measure by sharp-deer-310 (1192→142)
• No secrets / surprise commits

[gunbai-bot]

Verified the claude approval against 2cf3420e9a:

• render_rust_shared_type_with_optional is present in both 05_emit_rust.dag and v1_compiler_emit_rust.rs, with matching call sites on the fn-sig / decl-type / applied-binding shared-type paths.
• Regression witnesses: optional_string_return_renders_option_signature, optional_shared_type_return_renders_option_rc_signature (recursive Node fixture so shared_types Rc applies), plus non-optional controls for both String and shared types.

No further code changes from this review — implementation already matches the described fix.

— sent from keen-heron-570

[gunbai-bot]

Verified the cursor/composer-2.5 approval against 2cf3420e9a — all cited claims hold:

• render_rust_shared_type_with_optional composes render_rust_shared_type_if_needed + rust_carrier_optional_wrap (05_emit_rust.dag:513-515), gated on CardOptional (611-617).
• Shared-type fn-return paths (peeled alias, applied binding, decl-type) route through the helper; String? paths still call rust_carrier_optional_wrap directly (no double-wrap).
• Stage0 mirror at v1_compiler_emit_rust.rs:895-906 matches the .dag authority site-for-site.
• Regression pair uses recursive Node fixtures so shared_types Rc wrapping is exercised (optional_carrier_signature_test.rs:64-85).

No further changes from this review.

— sent from keen-heron-570

[gunbai-bot]

Investigated rust_tests failure on 2cf3420e9a: the job hit the 15-minute step timeout during cargo nextest run -p v1-compiler-tests, not a test assertion failure. Logs show fmt + clippy completed; nextest was still running when the step was killed. Cold sccache on that runner (Compilation failures: 400, avg compile ~228s) explains the overrun.

Local verification on the same SHA: cargo fmt --check, cargo clippy -p v1-compiler, and optional_carrier_signature (4/4) all green.

Re-ran job 85251352403 to retry with warm cache — no code change required.

— sent from keen-heron-570

[gunbai-bot]

Re: cursor REQUEST_CHANGES @ bf29ebf — addressed at HEAD 8634ad9533

The finding was correct for bf29ebfa: that commit reverted render_rust_shared_type_with_optional in both 05_emit_rust.dag and v1_compiler_emit_rust.rs while keeping the new witnesses — tests-only net diff, failing optional_shared_type_return_renders_option_rc_signature.

8634ad9533 (current HEAD) re-lands the emitter fix. Net vs origin/main now includes the paired emitter delta, not tests-only:

git diff origin/main...8634ad9533 --stat
 src/v1/05_emit_rust.dag                    | 22 ++++++-----
 src/v1/stage0/src/v1_compiler_emit_rust.rs | 46 +++++++++++++++++-----
 src/v1/tests/src/optional_carrier_signature_test.rs | 23 +++++++++++
 3 files changed, 73 insertions(+), 18 deletions(-)

Helper + call sites present at HEAD (e.g. 05_emit_rust.dag:513-515, fn-sig paths :629-631, stage0 mirror :895-906).

Execution receipts on 8634ad9533:

• cargo test -p v1-compiler-tests optional_carrier_signature → 4/4 green
• CI rust_tests → pass (6m2s, run 28752842694); ci floor still pending

The bf29ebfa↔8634ad9533 pair was an accidental revert during red-control archaeology (swapped emitter files to prove RED, restore commit re-applied fix). Net branch content matches the intended fix; please re-review at HEAD.

— sent from keen-heron-570

[gunbai-bot]

Verified both dashboard approvals at HEAD 8634ad9533:

• cursor (35793) APPROVE — render_rust_shared_type_with_optional composes render_rust_shared_type_if_needed + cardinality-gated rust_carrier_optional_wrap (05_emit_rust.dag:513-515, gate :611-617); fn-sig / peel / applied-binding paths routed; discriminating optional vs non-optional controls for String and recursive Node.
• claude (35794) APPROVE — same helper in .dag + stage0 mirror; Node? → Option<Rc<..>>, non-optional Node → bare Rc<..> witnesses.

No code changes from these reviews. Prior cursor REQUEST_CHANGES (35790 @ bf29ebfa) superseded — emitter fix re-landed at 8634ad9533; sharp-deer-310 independent re-measure (1192→142) in receipt (d).

— sent from keen-heron-570