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Brief: #1031, a kept fn copied from a name, a field or a tuple part is moved, not cloned (rustc)

Score: compiler bug (false acceptance: the checker passes, rustc refuses; every second use of a copied function value), bugs-first default of docs/priorities.md. Found by b1018; the same family as #1018 and #1008.

Issue #1031 (copied)

Found by b1018 (report of d5995dbd), widened by the architect’s probes at d7e56e49.

Problem

A function value is a copy that shares its closure (Part I, A function type stands wherever a type does). Read from a list by index it is cloned since #1018; read from a name, a field or a tuple part in a value position it lowers to a move of the Kept, so a second read passes the checker and rustc refuses.

fn main() {
    let f: fn(i64) -> i64 = fn(x) { x + 5 }
    let g = f
    println(f(1))
    println(g(2))
}

Today: rustc Borrow of moved value: f. Expected 6, 7.

Probe matrix (all at d7e56e49, nikaia run): | Row | Today | | :— | :— | | let g = f then f(1), g(2) | rustc Borrow of moved value: f | | struct H { f: fn(i64) -> i64 }: let a = h.f; let b = h.f | rustc Use of moved value: h.f | | H { f: f }, then H { f: h.f }, then let a = h.f | rustc Use of moved value: h.f | | fs.push(f) twice, then f(2) | rustc Use of moved value: f | | let t = (f, 1); let u = (f, 2) | rustc Use of moved value: f | | keep(f) to an owning fn parameter, then f(1) | 6, 7 (fine: hands_on_kept, emit_core_args.nika:369, #1008) | | let a = fs[0] | fine since #1018 |

Locations

To build

  1. The checker records every name or field read whose type is Ty::Fn and that names a place (a local, a parameter, a field; not a named function, named_fn_values), keyed by the read’s node_key, in a fact beside kept_fn_reads; the emitter writes .clone() after such a read where it stands as a value (a let initialiser, an argument, a struct literal field, a tuple part, a list element, a return), and not where it is the callee of a call (f(1), h.f(2)) or already duplicated by hands_on_kept. Test tests/compiler/tests/a_kept_fn_copied_from_a_name_or_field.nika + .stdout: rows 1-5 in one program (text in the brief), printing 6, 7, 21, 13, 21; it fails today at its line 6 (rustc Borrow of moved value: f). Commit emit: a kept fn copied from a name or a field shares the closure (#1031), Fixes #1031.

The test program (run by the architect at d7e56e49: rustc Borrow of moved value: f at line 6)

struct H { f: fn(i64) -> i64 }

fn main() {
    let f: fn(i64) -> i64 = fn(x) { x + 5 }
    let g = f
    println(f(1))
    println(g(2))
    let h = H { f: f }
    let k = H { f: h.f }
    let a = h.f
    let b = h.f
    let c = k.f
    println(a(1) + b(2) + c(3))
    let mut fs: Vec[fn(i64) -> i64] = []
    fs.push(f)
    fs.push(f)
    let d = fs[1]
    println(d(1) + f(2))
    let t = (f, 1)
    let u = (f, 2)
    let (p, _) = t
    let (q, _) = u
    println(f(1) + p(2) + q(3))
}

Expected .stdout: 6, 7, 21, 13, 21, one a line. Rows 1, 2, 4 and 5 were each traced to the move separately (one program per row, the rustc message names the copied place); a tuple part read back by a let (p, _) = t pattern is not traced separately: if it needs more than the .clone() at (f, 1), say so in Body:.

Not this brief: println(f(1) + h.f(2) + k.f(3)) in one expression gave NK2605 (can fail, because H.f can fail), unprobed further; h.f(2) alone runs. Keep field calls out of the test; if you meet it, it is a Found:.

Done when

The test passes through nikaia test and fails on the unfixed compiler; lower-std fixpoint clean; a_kept_fn_read_by_index still passes; cargo build --profile fast -p nikaia; the guard set of .claude/agents/builder.md green; pushed. Changelog: one line (a function value copied from a name, a field or a tuple part shares its closure, as Part I says, instead of a lowering rustc refused).

Stop and report if

An edit is needed outside crates/nikaia-std/src/tools/emit_*.nika, check_*.nika, check_facts.nika, their lowered files (and lowered.rs) 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)

Report (at most 15 lines, to exactly /home/user/Nikaia/docs/briefs/b1031-kept-fn-name-copy.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 a lowering other than the brief’s where rustc cannot take the brief’s, b1029 2026-10-31: say why 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:, nor an existing test’s expected .stdout/.stderr updated because your change rightly changes the message it prints, b1012b 2026-10-31: name the test 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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