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Brief: #1016 continued, a ref Vec[T] result (plain and nullable) lowers as &[T] like its parameter (rustc E0308)

Score: S=3 (a builder was stopped on it: b3-1016 ended partial with a working patch, only for want of one file; a false acceptance: the checker passes, rustc refuses), O=0.5 M, C=2 M (the patch exists), area bugs. First in this batch.

What the last run found (b3-1016 report)

The patch docs/briefs/bugs/b4-1016.patch (copied from the last run’s scratchpad) and the test draft docs/briefs/bugs/b4-1016.test.nika / b4-1016.test.stdout (covering a function value too) make both probes print 2, f(v).clone() then push give 3, and trait method, method field result, typed let and for over the result run. It broke passing a function as a value: fn(ref Vec[i64]) -> ref Vec[i64] keeps its result &Vec<i64> in emit_core_types.nika:139-145 (a code type’s result, code.result -> inner), so it must get a_list_read_as_a_slice as its parameters do; also the Call-lifetime result near emit_core_types.nika:172. If the patch is gone, rebuild it from the brief’s locations below.

Architect’s decision (2026-10-31): emit_core_types.nika is yours in this brief. And a ref Vec[T]? result follows the same rule (Option<&[T]>): probed today, it fails the same way:

fn f(a: ref Vec[i64]) -> ref Vec[i64]? {
    if a.len() == 0 {
        return null
    }
    return a
}
fn main() {
    let v = [1, 2]
    let n = f(v)?.len() ?? 0
    println(f"{n}")
}

rustc: expected &Vec<i64>, found &[i64] at return a. Expected 2. If a nullable ref Vec[T] parameter still lowers as Option<&Vec<T>>, make it Option<&[T]> too, so parameter and result agree; if that grows beyond this run, build the plain result (step 1) alone, and report partial naming what is left.

The issue’s probes (copied)

fn f(a: ref Vec[i64]) -> ref Vec[i64] {
    return a
}
fn main() {
    let v = [1, 2]
    println(f"{f(v).len()}")
}

Expected 2; lowered today fn f(a: &[i64]) -> &Vec<i64> { a }. And the enum probe of #1016 (inner(a: ref Vec[P]) -> ref Vec[P] returning a field parts of a variant), expected 2.

Locations

To build

  1. A ref Vec[T] result (not mut) lowers as &[T] with today’s lifetime, in a function, a method, a trait method and a function type; one commit.
  2. The nullable result ref Vec[T]? the same (Option<&[T]>), with the parameter if needed (above).
  3. Test tests/compiler/tests/a_list_view_result_is_a_slice.nika with .stdout: both issue probes, the nullable probe, a caller that copies (let mut c = f(v).clone(), c.push(3), prints 3), a function passed as a value. Run against the unfixed compiler first; then the tests whose names hold slice, ref_vec, view_result, function_value.

Yours: emit_function.nika, emit_item.nika, emit_ops.nika, emit_core_types.nika, their lowered files and lowered.rs, the new test. Another file needed: stop, report partial with what and why.

The commit that finishes it says Fixes #1016 and removes the three attachments (git rm docs/briefs/bugs/b4-1016.patch docs/briefs/bugs/b4-1016.test.nika docs/briefs/bugs/b4-1016.test.stdout).

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/bugs/b4-1016-list-view-result-slice.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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