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

Brief: #996, `return m.get(k)` as `String?` from a lent map of literal text reaches rustc

Score: bug lane, a false acceptance (checker passes, rustc E0308; S=2) in a form every move over a name map writes (return m.get(k)); K3a part 2 waits on the owner (#1012). Builders have no GitHub: the issue text is copied below, narrowed by the architect’s probes.

Problem (issue #996, narrowed)

A function that hands back a lent map’s text element as String? passes the checker and reaches rustc. Probes on a5663bf6:

use std::collections

fn word(m: collections::BTreeMap[String, String], k: String) -> String? {
    return m.get(k)
}

fn main() {
    let mut w: collections::BTreeMap[String, String] = collections::BTreeMap()
    w.insert("b", "two")
    print(word(w, "b") ?? "none")
}

Lowered today: fn word(m: &collections::BTreeMap<&'static str, &'static str>, k: &str) -> Option<&'static str> { m.get(nikaia_std::index::as_text(&k)) }; rustc: expected Option<&'static _>, found Option<&&_>. The map holds literal text (the &'static str tier, which copies), so the read must be copied out: m.get(..).copied(). Expected output two.
The same with let r = m.get(k); return r: same E0308 (cover it too).
With an owned value (w.insert("b", f"two{n}")) the checker already refuses with NK1102 and the ?.clone() help: correct, keep it so (it is ADR-293 D15’s rule).
Numbers (-> i64?) work: the read is copied.

So the defect: the literal-text tier is a value that copies (!takes_away), and the returned (and let-then-returned) method-form read does not get its .copied().

Locations

crates/nikaia-std/src/tools/check_reads.nika:91 a_map_read_kept (ADR-293 D15): inserts copied_reads when the wanted T? copies or does not take away; the NK1102 refusal is below it. Check whether the return of a method-form m.get(k) reaches it, and with which found/want (read.is_a_view() returns early: a literal-text read may be typed as a view there).
crates/nikaia-std/src/tools/emit_expr.nika:199: writes .copied() for a node in copied_reads; emit_core_calls.nika:1482: .copied() for copied_walks. Check that the method-call path reaches one of them.
Text tiers (which text lowers &'static str): text_tiers.nika; use nikaia run scripts/where.nika -- NAME for further definitions.
Lowered twins: crates/nikaia-std/src/tools/lowered/*.rs (written by lower-std only).

To build

Fix it so both rows lower with the copy (.copied() or any lowering rustc accepts and ADR-293 D15 allows) and print two; the NK1102 row stays refused. New test tests/compiler/tests/a_returned_map_read_of_literal_text_is_copied.nika + .stdout (both function forms, one print each; take the .stdout from the real run), and a refusal test for the owned row only if none exists yet (nikaia run scripts/where.nika -- NK1102 lists the tests). lower-std fixpoint (lower-std, build, lower-std: the second run changes nothing). Fixes #996.

Done when

The new test passes through nikaia test (copy it to the scratchpad to nikaia run it alone first), the lower-std fixpoint is clean, the gate of .claude/agents/builder.md is green; pushed. Commit subject: check: a returned map read of literal text is copied (#996) (or emit: where the fix lies), Fixes #996. Changelog: one line.

Stop and report if

The fix needs an owned String map value returned as String? without ?.clone() (that is ADR-293 D15’s refusal, not this defect); an error in another agent’s file.

Not yours, never stage: crates/rs2nika/convert/nikaia.{contracts,derived,proofs} and every docs/briefs/* file that git status shows modified or untracked except your report.

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.
A form Nikaia, std or the toolchain lacks stops the work: no code around it unless an ADR or Part I-III refuses it (name it). Else Result: blocked, Found: gap + probe, Needs: owner decision (CLAUDE.md, A missing form stops the work).
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.
Commit and push before you write the report; the report’s Pushed: line names the pushed sha. A report without a Pushed: line, or with a placeholder left in (TC, DISK), is not done and cuts the batch (k3a1 2026-10-29: Result: done, nothing committed).

Report (at most 15 lines, to exactly `/home/user/Nikaia/docs/briefs/b996-map-read-literal-text.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 (redesign) for a brief with Shape: redesign and no Convert: line). 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); a process event such as another session resetting or rewriting your files is no Found:, it goes under Needs: (m3a 2026-10-28); a converter draft that came out wrong is no Found:, it is one of the costliest edits under Rs2nika:, which the converter lane reads (e12-4b 2026-10-27); 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, nor a step another commit on develop already built, shown by your test failing with that commit’s change reverted, b889 2026-10-27: name the commit in Body:, nor a test’s expected .stdout/.stderr taken from what the program really prints where the brief’s sketch of it was short or wrong, b934 2026-10-29: say so in Body:, nor a lowering that follows the ADR governing it where the brief’s expected lowering contradicts that ADR, b933 2026-10-29: name the ADR in Body:, nor re-keying an allow-list entry (KNOWN) whose hash changed for the same program and the same reason, b-fixforward-sweep2 2026-10-28: 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: #996, return m.get(k) as String? from a lent map of literal text reaches rustc