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Brief: a list of views handed back may be a literal or grown with extend (#1071)

Score: blocks #928 step 4 (yaml’s events, ADR-yaml-1 D7; y21-events run 2 blocked on it, draft in local branch wip/y21-events). A compiler defect found by the yaml lane; fixed in this lane because only yaml needs it now. It changes crates/: name in the report what it decided (the supervisor raises the version), Changelog: one sentence. The commit says Fixes #1071.

Facts: Where things live, Traps already met

Problem

A list handed back whose elements hold views of a ref parameter is accepted when it is built as let mut out: Vec[E] = [], out.push(..), return out (#655, #1000), but refused NK2302 (“walk keeps n after this call, but n: ref N doesn’t say whose data it points into”) when it is (a) returned as a list literal, return [E { name: n.name }], (b) started from a non-empty literal, let mut out: Vec[E] = [E { name: n.name }], or (c) also grown with out.extend(f(x)), recursive or not (the refusal then lands on the push line). for e in walk(x) { out.push(e) } in place of (c) passes. Found by the yaml lane (#928 step 4, events, run y21-events): the event walk is recursive over a borrowed tree.

Probes (today NK2302; wanted the printed line)

(a), wanted 1 hello:

struct N {
	name: String,
}

struct E {
	name: String,
	value: String,
}

fn walk(n: ref N) -> Vec[E] sync {
	return [E { name: n.name, value: "" }]
}

fn word_of(input: String) -> N {
	return N { name: input.trim() }
}

fn main() {
	let n = word_of(" hello ")
	let es = walk(n)
	println(f"{es.len()} {es[0].name}")
}

(b)+(c), recursive, wanted 2 hello:

struct N {
	name: String,
	kids: Vec[N],
}

struct E {
	name: String,
}

fn walk(n: ref N) -> Vec[E] sync {
	let mut out: Vec[E] = [E { name: n.name }]
	for x in n.kids {
		out.extend(walk(x))
	}
	return out
}

fn word_of(input: String) -> N {
	let leaf = N { name: input.trim(), kids: [] }
	return N { name: input.trim(), kids: [leaf] }
}

fn main() {
	let n = word_of(" hello ")
	let es = walk(n)
	println(f"{es.len()} {es[1].name}")
}

Locations (suspected, high confidence)

To build (after a tools/*.nika change, lower-std regenerates its lowered/*.rs; commit both)

  1. A list literal of the result’s element struct, returned or bound as a list handed back, and extend on a list handed back, are treated as pushes into it: probes (a) and (b)+(c) print 1 hello and 2 hello. Tests tests/compiler/tests/list_literal_of_views_handed_back.nika and recursive_walk_extends_a_list_of_views.nika, each with .stdout.
  2. What a list literal or extend brings that is not a view of a declared parameter stays refused as today (e.g. a view of a by-value parameter, NK2302); the existing tests keep their output; tests/compiler --both-settings green.

Probes checked by the architect on target/fast/nikaia at 11f808ef. Files you may change: the tools/*.nika under Locations with their lowered/*.rs, the new tests named under To build, and an existing test’s expected output your change rightly changes (name it in Body:). If the fix needs a language decision, stop: Result: blocked, Needs: architect decision. Stop about 15 turns before max_turns, commit what is finished, report partial.

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/yaml/y25-1071-list-of-views-literal-extend.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_01UVxJDveRM5qKZKyCqq6Y24

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