agent: builder model: opus max_turns: 80 budget_usd: 20 —

Brief: #1024, a fn parameter lent to a kept trait-method position is refused with `NK2101`, not by rustc

Score: compiler-bug lane; blocks #1012 step 2 (b1012b, test (h)). No new NK code.

Problem

trait H {
    fn take(mut self, f: fn(i64) -> i64)
}
struct Keep { held: Vec[fn(i64) -> i64] }
impl H for Keep {
    fn take(mut self, f: fn(i64) -> i64) { self.held.push(f) }
}
fn hand[T: H](mut h: T, s: String) {
    h.take(fn(x) { x + s.len() })
    println(s)
}

gives a raw error: Lifetime may not live long enough. (rustc leaks through). With let s = f"a{t}" local in place of the parameter the checker refuses it with NK2101. The refusal of a lambda that borrows at a kept position counts locals, not fn parameters.

Locations

The NK2101 path in the checker (grep NK2101 under crates/nikaia/src/check/ and crates/nikaia-std/src/tools/check_*.nika; the kept-arguments path, kept_args). Find why a captured fn parameter is not seen as borrowed there, and count it like a local. A parameter the lambda only copies (a Copy type) is not refused.

To build

The fix, and a test in tests/compiler/tests/: the probe above with its .stderr naming NK2101, and a second function where the lambda captures an i64 parameter (accepted, prints). lower-std fixpoint clean if a tools file moved. Commit check: a fn parameter lent to a kept position is refused with NK2101 (#1024), Fixes #1024.

Done when

The test passes through nikaia test, a_trait_method_callback_is_handed_on.nika still passes, the guard set of .claude/agents/builder.md is green; pushed. Changelog: one line.

Stop and report if

An edit outside the checker (crates/nikaia/src/check/, crates/nikaia-std/src/tools/check_*.nika and their lowering), crates/nikaia/src/contracts/, and the new test; an error in another agent’s file.

Not yours, never stage: every file git status shows modified or untracked at your start, and every docs/briefs/* file 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/b1024-kept-param-lent.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: #1024, a fn parameter lent to a kept trait-method position is refused with NK2101, not by rustc