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Brief: an if/match argument mixing a lent view and a new value (Fixes #894)

Score: bugs lane (docs/priorities.md, bugs-first default): nikaia check passes, rustc refuses; found by the #892 builder.

Issue text (#894)

Probe

struct P { name: String }
fn look(p: ref P) -> i64 { return p.name.len() }
fn pick(t: ref P, c: bool) -> i64 { return look(if c { t } else { P { name: "bb" } }) }
fn main() {
    let t = P { name: "a" }
    println(pick(t, true))
    println(pick(t, false))
}

nikaia check passes; nikaia run: rustc Mismatched types at the P { .. } arm, Consider borrowing here. Lowered: look(if c { t } else { P { name: "bb" } }) - the view arm t is &P, the new arm is P. The same with fn keep(p: P) -> String { return p.name } (inferred lent, lowered &P). Expected: 1, then 2. Runs already: the same if with an owned local t (no ref parameter) passed to keep.

Locations

To build

  1. A lent argument (declared ref or inferred lent) given an if/match whose arms mix a view and an owned value is lowered so every arm is a view of something that lives through the call. Test: tests/compiler/tests/a_lent_argument_from_arms_view_and_new.nika + .stdout, forms if and match, declared ref and inferred lent, a struct and String.

Done when

The new test; nikaia test tests/compiler --both-settings; it filtered on handed_over text_tiers typecheck; the guard set of .claude/agents/builder.md; clippy clean. Commit with Fixes: #894; Changelog: one sentence.

Stop and report if

No form rustc accepts keeps every arm a view without a copy (then blocked with the forms tried); an open design choice; an error in another agent’s file.

Read docs/builder-facts.md.

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/b894-lent-argument-arms.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 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); 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: say so 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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