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Brief: #1016 step 3, a ref Vec[T] with a named lifetime, and a typed let of one, lowers as a slice (rustc E0308)

Score: S=3 (false acceptance: the checker passes, rustc refuses; found by b4-1016 under Found:), O=0.3 M, C=2 M (emit only, the rule b4 built for the plain case), area bugs. Third in this batch.

What b4-1016 built (6832794b) and left

A ref Vec[T] parameter and result lower as &[T]. Two places still write &Vec<T>, so a slice reaches them and rustc refuses. Probed by the architect on 3adb2b60:

fn longer(a: ref Vec[i64], b: ref Vec[i64]) -> ref Vec[i64] {
    if a.len() >= b.len() {
        return a
    }
    return b
}
fn twice(a: ref Vec[i64]) -> i64 {
    return longer(a, a).len()
}
fn typed(a: ref Vec[i64]) -> i64 {
    let w: ref Vec[i64] = a
    return w.len()
}
fn main() {
    let v = [1, 2]
    println(f"{twice(v)}")
    println(f"{typed([4, 5, 6])}")
    println(f"{longer(v, [7]).len()}")
}

Lowered today: fn longer<'r>(a: &'r Vec<i64>, b: &'r Vec<i64>) -> &'r Vec<i64> (a function with two view parameters and a view result takes named lifetimes, and then keeps &'r Vec<T>), so longer(a, a) in twice (a: &[i64]) is Arguments to this function are incorrect at 8:5; and let w: &Vec<i64> = a is Mismatched types at 11:5. Expected output 2, 3, 2. Both causes traced in the lowered .rs; the hand-edited Rust (&'r [i64], let w: &[i64]) is plain coercion.

Not in this brief (filed separately by the architect): a function type with two view parameters and a view result (fn(ref Vec[i64], ref Vec[i64]) -> ref Vec[i64], also with String) lowers to Fn(&[i64], &[i64]) -> &[i64] with no lifetime (rustc Missing lifetime specifier); and the nullable ref Vec[T]? (b4’s step 2, Option<&Vec<T>> today, consistent on both sides except a plain view returned as a nullable result), which is #1016 step 4.

Locations

To build

  1. A ref Vec[T] parameter and result with a named lifetime ('r, 'r2, …) lowers as &'r [T]; one commit.
  2. A typed let x: ref Vec[T] = .. lowers its type as &[T]; one commit (or with 1).
  3. Test tests/compiler/tests/a_list_view_named_lifetime_is_a_slice.nika with .stdout (2, 3, 2): the probe above. Run it against the unfixed compiler first; then a_list_view_result_is_a_slice and the tests whose names hold slice, ref_vec, lifetime.

Yours: emit_function.nika, emit_stmt.nika, emit_ops.nika, their lowered files and lowered.rs, the new test. Another file needed: stop, report partial with what and why. The commit says Part of #1016 (step 4 is left), and removes b4’s 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/b7-1016-named-lifetime-and-typed-let-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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