agent: builder model: sonnet max_turns: 90 budget_usd: 15 —

Brief: #655 step 3, `?? return` on a field of a `mut` parameter is lent

Score: bugs lane (docs/priorities.md): the checker passes a program rustc refuses (E0507), a wrong-acceptance bug; one step, about 10 lines in one function.

Issue text (#655, item 2 and step 3)

state.flow.unrolling ?? return, where state is a mut parameter whose type does not copy, lowers by value, and rustc refuses it (E0507). A ref parameter gets .as_ref() and runs. The place to fix is lent_coalesces in check_core_ops.nika. To build 3: a field of a mut parameter read through ?? return is lent. Test.

Probe (reproduced by the architect on fbc9bfd, `target/fast/nikaia run n.nika`)

struct Unroll { names: Vec[String] }
struct Flow { unrolling: Unroll? }
struct State { flow: Flow }
fn first(mut state: State) -> i64 {
    let u = state.flow.unrolling ?? return 0
    return u.names.len()
}
fn main() {
    let mut s = State { flow: Flow { unrolling: Unroll { names: ["a"] } } }
    println(first(s).to_string())
}

Today: rustc Cannot move out of state.flow.unrolling … behind a mutable reference. Wanted: prints 1. With unrolling: String? it already runs (prints the length). The decision is settled: a mut parameter is a loan, so ADR-279 D10 (a loan on the left and a jump on the right is a view) applies; no design choice is open.

Locations

crates/nikaia-std/src/tools/check_core_ops.nika: check_core_ops_coalesce (724), the D10 arm (~774: place && state.jumps(..) && ... && (check_core_ops_lent_root(..) || check_core_ops_behind_a_view(..))); check_core_ops_lent_root (563) answers state.lent_here(root) || got_past_a_jump; find why a mut parameter’s root is not lent_here (grep fn lent_here in tools/check_*.nika) and fix it there or in lent_root, whichever is narrower.
Lowering reads lent_coalesces in tools/emit_core_flow.nika:588; no change expected there.

To build

The fix above, then lower-std with the rebuilt binary; commit the changed lowered/*.rs with it.
Tests in tests/compiler/tests/: a_mut_parameters_field_past_a_jump_is_lent.nika + .stdout (the probe), and a second where the view is then used after a write to another field of state (state.flow untouched, e.g. a new count: i64 field incremented) - if rustc refuses that, the checker must refuse it with the existing borrow code, not let it through; report which. Run: cargo test --profile fast -p nikaia --test it -- coalesce lent view, target/fast/nikaia test --both-settings --project tests/compiler. Commit Part of #655 (steps 1, 2, 4, 5 stay open).

Rules (CLAUDE.md; this block is pasted into every brief, also a fix-forward)

Read docs/builder-facts.md first. develop in the main checkout; git status at start: other agents’ uncommitted files are not yours, never touch or stage them.
git add <files> only; commit after each step; git push origin develop fast-forward, no rebase (rejected: report Pushed: rejected, go on committing).
Before pushing: cargo fmt, cargo clippy --all-targets -- -D warnings, scripts/pre-push-check.sh, and the gate of .claude/agents/builder.md (#852).
No version lines, CHANGELOG.md, badges; no new NK code. New tests in Nikaia only; never the full suite. No foreground sleep, no polling; scripts in files.
Disk: NIKAIA_CACHE_LIMIT=2G; at most two clones with target/ in the machine; delete your own clone’s target/ and target/release/nikaia-scratch at the end; report GB freed.

Report (at most 15 lines, to exactly `/home/user/Nikaia/docs/briefs/b655-mut-param-coalesce.md.report.md`)

Fields of docs/architect.md, Briefs and reports: Result: done|noop|partial|blocked (the bare word first; Owner: is a question for the owner or none, never a role), Files, Commit, Fixes, Pushed, Tests, Gate (green|red), Deviation, Found, Needs, Changelog, Owner, Disk freed. none where nothing, and then the bare word: Deviation: none (a note) cuts the batch (batch.nika compares exactly); a note goes into the commit message. Found: is only for a defect outside your change (it cuts the batch); how you solved your own step goes into the commit message. Deviation: is only a step of the brief built otherwise than written or not built; how you verified (e.g. a test not run against the unfixed compiler) goes under Tests:. A noop says why under Tests: (what was probed), not under Deviation:. Final answer, one line: done <sha>, report <path> (or BLOCKED: ... / PUSH REJECTED ...).

Attribution: Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_01GFeMqxRbtUMiGVyDMxsvEq

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Brief: #655 step 3, ?? return on a field of a mut parameter is lent