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Brief: #1025, a parameter a kept lambda captures is kept (owned), not left lent for rustc to refuse

Score: compiler-bug lane (rustc leaks through a checked program); found by #1024’s build (ac8a38a6). No new NK code.

Problem

A lambda handed to a position the callee keeps, capturing a lent String parameter, with no use of the parameter afterwards, passes the checker and reaches rustc:

fn keep(f: fn(i64) -> i64) -> Vec[fn(i64) -> i64] {
    let mut v: Vec[fn(i64) -> i64] = []
    v.push(f)
    v
}
fn hand(s: String) -> i64 {
    let v = keep(fn(x) { x + s.len() })
    v.len()
}
fn main() {
    println(hand("abc"))
}

gives error: Lifetime may not live long enough. at line 7 (probed at a3ec9600). The same holds for a trait method that keeps (h.take(fn(x) { x + s.len() }), #1024’s probe without the println(s)).

Controls (probed): with let s = f"a{t}" a local in place of the parameter it runs and prints 1 (the lambda takes the local); with println(s) after the call it is refused with NK2101 (#1024).

Cause

The keeps walk (crates/nikaia-std/src/tools/keeps.nika) reads a lambda argument’s body as running during the call (keeps_blocks_in, line 467: Expr::Closure { body, .. } => keeps_block(..)), so a parameter the lambda names is not recorded as kept even when the callee keeps that position. spawn is the model: keep_what_a_task_names (line 890) records everything a task names as kept (ADR-312 D1).

Locations

To build

  1. A lambda argument at position at of a call: the parameters its body names are recorded as handed to (callee, at) (Passed, so the fixpoint asks the callee’s column; an unknown callee keeps, D2), for free calls and method calls alike; a lambda at a position the callee does not keep stays as it is (its body runs during the call). Then hand keeps s, s lowers owned and the probe prints 1. Tests in tests/compiler/tests/: the probe (.stdout 1), the trait-method form (tests/compiler/tests/a_lent_parameter_kept_by_a_trait_method_is_taken.nika without its println(s), plus println("ok") in main; .stdout ok), and a lambda handed to a non-keeping callee that stays lent and is used afterwards (fn twice(f: fn(i64) -> i64) -> i64 { f(1) + f(2) }, let n = twice(fn(x) { x + s.len() }), println(s), n; prints abc then 9; all three rows probed at a3ec9600: the first two give the rustc error, the third runs). lower-std fixpoint clean. Commit keeps: a parameter a kept lambda names is kept (#1025), Fixes #1025.

Done when

Tests pass through nikaia test; a_lent_parameter_kept_by_a_trait_method_is_taken.nika, a_trait_method_callback_is_handed_on.nika and the guard set of .claude/agents/builder.md are green; pushed. Changelog: one line.

Stop and report if

An edit outside crates/nikaia-std/src/tools/keeps.nika and its lowering (lowered/keeps.rs), the Rust caller of the keeps walk (crates/nikaia/src/contracts/keeps.rs) if a new field must reach it, crates/nikaia/tests/it/keeps.rs only to keep it passing, and the new tests; an error in another agent’s file. Read docs/builder-facts.md first.

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)

Report (at most 15 lines, to exactly /home/user/Nikaia/docs/briefs/b1025-kept-lambda-names.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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