agent: builder model: opus max_turns: 110 budget_usd: 22 —

Brief: a text `+` of a lent field, an owned local in an arm of a lent argument (Fixes #897)

Score: bugs lane (docs/priorities.md, bugs-first default): nikaia check passes, rustc refuses; found by the #894 builder.

Issue text (#897)

Found by the #894 builder (b82ab26). nikaia check passes both probes; rustc refuses the lowered code.

Probe 1: a text `+` of a field read through a lent parameter

struct P { name: String }
fn look(p: ref P) -> i64 { return p.name.len() }
fn f(t: ref P) -> i64 { return look(P { name: t.name + "!" }) }
fn main() { println(f(P { name: "ab" })) }

rustc: cannot move out of t.name which is behind a shared reference. Lowered nikaia_std::concat::plus(t.name, "!"). Expected 3.

Probe 2: an owned local named in an arm of a lent argument

struct P { name: String }
fn look(p: ref P) -> i64 { return p.name.len() }
fn main() {
    let c = true
    let t = P { name: "a" }
    let a = look(if c { t } else { P { name: "bb" } })
    println(a + t.name.len())
}

rustc: borrow of moved value t: the arm moves t into the if. Passing t to a lent parameter lends it (look(t) does); an arm of a lent argument must lend it too. Expected 2.

Locations

Probe 1: crates/nikaia-std/src/tools/emit_core_leaves.nika:213 emit_core_concatenated (lends only an Expr::Index side as &str); the + call at emit_core_flow.nika:399.
Probe 2: crates/nikaia-std/src/tools/check_arm_leaves.nika:153 lent_arms_view_and_new (#894: an arm counts as viewed only if it is a view or a lent name; an owned local goes to made, names == 0 returns false); emit through nikaia_std::arm::Arm (crates/nikaia-std/src/arm.rs), decided in check_arguments.nika:334.

To build

A side of a text + that is a place the function does not own (a field through a lent parameter or view, at any depth) is lent as &str, as an indexed side is. Test tests/compiler/tests/a_text_plus_of_a_lent_field.nika + .stdout (probe 1, also as a let and a return).
An owned local named in an arm of an argument to a lent parameter is lent through the call (Arm::View(&t) beside Arm::Own(..)), not moved; the local stays usable after the call. Both arms owned locals: both lent. Test tests/compiler/tests/an_owned_local_lent_through_arms.nika + .stdout (probe 2, if and match, struct and String).

Done when

Both tests (each fails on the unfixed compiler); nikaia test tests/compiler --both-settings; it filtered on handed_over text_tiers typecheck; the guard set of .claude/agents/builder.md; clippy clean. One commit per step; the last says Fixes: #897; Changelog: one sentence.

Stop and report if

Step 2 needs a language decision (an arm of a lent argument that moves is meant to move): then blocked with the forms tried, step 1 committed; an error in another agent’s file.

Read docs/builder-facts.md.

Not yours (another lane’s uncommitted files in the checkout, 2026-10-27): crates/rs2nika/convert/**. Never stage them.

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/b897-lent-field-plus-and-arm-local.md.report.md`)

Fields of docs/architect.md, Briefs and reports: Result: done|noop|partial|blocked (the bare word alone on that line: Result: done, never a summary after it, which cuts the batch, b869a 2026-10-27; Owner: is a question for the owner or none, never a role), Files, Commit, Body (optional, one paragraph for the commit message), Fixes, Pushed, Tests, Gate (green|red), Deviation, Found, Needs, Changelog, Owner, Disk freed, and for a move Rs2nika: (the kept N of M lines of rs2nika --score, then the three costliest hand edits; none for a task that moves no Rust). 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 (a choice the brief left to you, such as keep what X needs and say so, is no deviation: it goes into the commit message; nor is building two steps in one commit when every step is built, e12-3a 2026-10-27: say so in Body:); 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: a text + of a lent field, an owned local in an arm of a lent argument (Fixes #897)