agent: builder model: sonnet max_turns: 70 budget_usd: 10 —

Brief: a view of a `.to_string()` local of a number expression is returned as `&` of a local (#865)

Score: S=3 (a valid program fails in rustc, after the checker accepted it), O=1 file, bug lane; sibling of #864 (6ba5c74, same function).

Issue #865 (copied)

Found by the #864 builder (6ba5c74), reproduced by the architect on develop d33d355 (target/fast/nikaia run p.nika):

fn f(t: String) -> String {
    let s = t.len().to_string()
    return s.trim()
}
fn main() { println(f("abc")) }

rustc: Cannot return value referencing local variable 's'. The same shape with let s = f"{t}!" (#864’s neighbour) and let s = t.trim().to_string() runs.

Cause

made_here in crates/nikaia-std/src/tools/buffers.nika (about line 435) makes a let a buffer of the frame’s own (Buffer::Named("String"), so a view of it leaving the function lives in the caller’s keep) for x.to_string() only when formats(receiver) holds, and formats (about line 535) accepts only a bare name whose written type is not text. t.len().to_string(), 5.to_string(), (n + 1).to_string() fall through to makes_a_buffer, which answers Buffer::Unknown.

To build

Matrix first: let s = R.to_string(); return s.trim() for R = t.len(), 5, (n + 1) with n: i64, x.0 of a tuple of numbers, true, a call g() returning i64; note which fail.
Test tests/compiler/tests/a_view_of_a_formatted_local_is_returned_owned.nika + .stdout with the failing forms (it must fail before the fix).
Fix in formats: the text form of a receiver that is not text (ADR-282 D3) makes a buffer. A receiver that is or may be text (a String/ref String local, a .trim()) is #851’s open question (decision): leave it as today. lower-std, rebuild, commit Fixes #865.

Files: crates/nikaia-std/src/tools/buffers.nika (+ lowered/buffers.rs), the new test. Anything else: stop and report under Needs. Known facts: read docs/builder-facts.md. Pattern: joins_text (#864) right below made_here.

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/b865-to-string-local-view.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. 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

This site is open source. Improve this page.

Brief: a view of a .to_string() local of a number expression is returned as & of a local (#865)