agent: builder model: opus max_turns: 100 budget_usd: 20 —

Brief: #880, a String moved into another String variable is lowered as a view

Score: bugs lane (docs/priorities.md, bugs before moving): a checked program that rustc refuses; it blocks rs2nika R92 (#877, commit 7881ad4 on origin/wip/877-entry).

Issue text (#880, copied)

The checker accepts this program. The lowering writes the move dflt = arg as a view (__keep_frame.put(..)), and rustc refuses the generated code with “Mismatched types”.

fn pick(rest: String, first: bool) -> String {
    let mut dflt: String = ""
    let arg: String = rest.trim().clone()
    if first {
        dflt = arg
    } else {
        dflt = (arg.strip_prefix("|") ?? "").clone()
    }
    return dflt
}

fn main() {
    print(f"{pick(" ab ", true)}|{pick("|x", false)}\n")
}

Expected output: ab||x.

Locations

__keep_frame is written by crates/nikaia-std/src/tools/emit_keeps.nika (:28-67, the frame keep); an argument keep by emit_core_args.nika (:425, ({keep}).put(). Which plan marks dflt = arg as kept: start from the generated Rust of the probe and follow the keep’s key back (the keep plan, ADR-283 D24). Likely cause (issue): the later arg.strip_prefix(..) in the other branch makes arg count as lent for the whole function.
Resolve further names with nikaia run scripts/where.nika -- NAME..., not grep -r.

To build

Find why the move is lowered as a view, and lower an owned String moved into a String variable as a move. Fix it in the compiler (Rust or tools/*.nika; a .nika change needs lower-std and its lowered file committed, docs/builder-facts.md). Probe also the variant without the else branch and with dflt = arg in both branches: all three must print what Nikaia means.
Test tests/compiler/tests/string_move_not_view.nika with its .stdout (the probe above).
Finish #877 step 2: git cherry-pick 7881ad4 (R92 from origin/wip/877-entry) onto develop, build the converter (crates/rs2nika/), run its tests, commit Part of #877. Do not delete the remote branch (the owner deletes branches).

Commit of steps 1-2: ... (Fixes #880).

Done when

All three steps built; nikaia test tests/compiler --both-settings; it filters keep, view, move, and the guard set of .claude/agents/builder.md; rs2nika’s tests. Changelog: one line (a program rustc refused now builds).

Stop and report if

The fix needs a language decision (what a move of a lent binding means), or an error in another agent’s file.

Read docs/builder-facts.md.

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/b880-string-move-view.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, 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 (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); 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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