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Brief: #1016, a ref Vec[T] result lowers as &Vec<T> while its parameter lowers as &[T] (rustc E0308)

Score: S=3 (a false acceptance: the checker passes, rustc refuses, on the shortest view-returning function there is; found by the yaml lane, y6), O=0.5 M, C=3 M, area bugs.

Issue #1016 (copied; reduced by the architect at c30b7bc0)

Reduced probe:

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

Expected 2. Today rustc: Mismatched types, expected &Vec<i64>, found &[i64]. Lowered: fn f(a: &[i64]) -> &Vec<i64> { a }.

The issue’s probe (also fails, same cause):

enum P { Seq(Vec[P]), Leaf(i64) }
fn inner(a: ref Vec[P]) -> ref Vec[P] {
    if a.len() == 1 { match a[0] { P::Seq(parts) => return parts, else => {} } }
    return a
}
fn main() {
    let v = [P::Seq([P::Leaf(1), P::Leaf(2)])]
    println(f"{inner(v).len()}")
}

lowers to fn inner(a: &[P]) -> &Vec<P>. Expected 2. Parameter and result of the same view type lower to the same Rust type, &[T]; a field’s &Vec<P> (parts) coerces to &[P] at the return by itself in rustc. A .clone() of a slice view is already written .to_owned() (probed: let mut b = a.clone() over a ref Vec[i64] parameter runs), so callers that copy the result keep working; check that with a caller that does f(v).clone() and pushes.

Locations

To build

  1. A ref Vec[T] result (not mut, not nullable) lowers as &[T] with the lifetime it has today, as a ref Vec[T] parameter does; a ref Vec[T]? result stays as it is unless the parameter rule covers it too (follow what the parameter does, and say so in Body:).
  2. Test tests/compiler/tests/a_list_view_result_is_a_slice.nika with .stdout: both probes above, and a caller that copies the result (let mut c = f(v).clone(), c.push(3), prints 3). Run against the unfixed compiler first; then the existing tests whose names hold slice, ref_vec or view_result (nikaia test filters).

Only emit_function.nika, emit_item.nika, emit_ops.nika, their lowered files (and lowered.rs) and the new test are yours. Another file needed: stop, report partial with what and why.

The commit that finishes it says Fixes #1016.

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/b3-1016-ref-list-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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