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agent: builder model: opus max_turns: 90 budget_usd: 18 effort: medium —

Brief: #1107 rest, a view of a type that copies is copied where the value is wanted (ADR-387)

Blocks: none (b26 left #1107’s rows 4-5 open for structs that copy; no move waits on it now, C1c’s FnContract does not copy)

Score: language decision by the architect (ADR-387, 2026-11-01), closes #1107. One mechanism exists at calls and is widened to four more places (effort medium).

Facts: Where things live, Traps already met, Commands

Problem

ADR-233 D4 lets a view of a value that copies go where the value is wanted only at a call, by a name (check_arguments.nika ~:420: a_view_that_copies + unviewed_type(found) == want + check_arguments_a_variable, recorded in state.facts.copied_args so the emitter writes the *). ADR-387 D1 widens it to every place that wants the value. Probe (target/fast/nikaia at e1a88e49, the architect ran it):

use std::collections

struct C { n: i64 }
struct Holder { c: C }

fn first(xs: ref Vec[C]) -> C {
    let e: ref C = xs[0]
    return e
}

fn pick(m: ref collections::BTreeMap[String, C], k: String) -> C? {
    let c = m.get(k) ?? return null
    return c
}

fn main() {
    let xs = [C { n: 1 }, C { n: 2 }]
    let e: ref C = xs[1]
    let g: C = e
    let h = Holder { c: e }
    let mut kept: Vec[C] = []
    kept.push(e)
    let ys = [3, 4]
    let f: ref i64 = ys[0]
    let i: i64 = f
    let mut m: collections::BTreeMap[String, C] = collections::BTreeMap()
    m.insert("a", C { n: 5 })
    let p = pick(m, "a") ?? return
    print(f"{first(xs).n} {g.n} {h.c.n} {kept.len()} {i} {p.n}\n")
}

Today: NK1104 at :8 (return e), NK1103 at :19 (let g: C = e), NK1106 at :20 (Holder { c: e }), NK1103 at :25 (let i: i64 = f). push and pick’s return c were not reported, perhaps only because the first errors stop a later phase: check each row alone. Expected after the fix: 1 2 2 1 3 5. Also the #1107 issue’s own rows 4-5 with C { n: i64 } (the unannotated let c = m[k] ?? return null handed back as ref C, and as C).

Locations (suspected, fairly sure)

To build

  1. The widening, the emitter’s * at each new place, and tests/compiler/tests/a_copy_view_is_copied_where_the_value_is_wanted.nika (+ .stdout) from the probe above, plus one .stderr test row that a non-copy struct is still refused (let g: D = e with D { s: String }, NK1103). Refused by the unfixed binary, passes --both-settings. Then an_unwrapped_map_view_stays_a_view (b26’s test) still passes, and cargo test -p nikaia --test it -- coalesce view map_read element_read copy. lower-std and its fixpoint. Commit check: a view of a type that copies is copied where the value is wanted (ADR-387), Fixes: #1107.

    Rules

    • Read docs/builder-facts.md first. develop in the main checkout; other agents’ uncommitted files are not yours: never touch or stage them.
    • Commit locally after each step with target/fast/nikaia run scripts/commit.nika -- REPORT (it adds exactly the report’s Files:); the driver pushes.
    • Before committing, cheap checks only: cargo build --profile fast -p nikaia, lower-std and its fixpoint when a tools/*.nika changed, cargo fmt, cargo clippy --all-targets -- -D warnings, scripts/pre-push-check.sh, the tests the brief names and the ones your change touches.
    • Version lines, CHANGELOG.md, badges and NK codes are the supervisor’s. New tests in Nikaia only.
    • A form Nikaia, std or the toolchain lacks stops the work, unless an ADR or Part I-III refuses it (name it): Result: blocked, Found: the gap with a probe, Needs: architect decision.
    • Disk: NIKAIA_CACHE_LIMIT=2G; at the end delete your own clone’s target/ and target/release/nikaia-scratch.
    • No cause after about 40 turns: stop, Result: partial with what you ruled out, the probe, and the suspected file and function.

Report (at most 15 lines, to exactly /home/user/Nikaia/docs/briefs/bugs/b33-1107-copy-view-copied.md.report.md)

Fields of docs/architect.md, Briefs and reports: Result, Files, Commit, Body (optional), Fixes, Pushed, Tests, Gate, Deviation, Found, Needs, Changelog, Owner (a question for the owner), Disk freed, for a move Rs2nika: (the kept N of M lines of rs2nika --score and the three costliest hand edits, else none), and last Attribution: with your commit trailer lines below it.

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