agent: builder model: sonnet max_turns: 70 budget_usd: 12 effort: low —

Brief: #1130, `drain()` on `BTreeMap`, `HashSet` and `BTreeSet` (ADR-386)

Blocks: none (repays C1b’s clone in ledger_items_stored; C1c also edits ledger_items.nika: if its commit is on develop, add beside it, do not reorder)

Score: std counterpart decided by the architect (ADR-386), self-hosting debt (a clone written round a missing form). Small: one ledger form known (HashMap::drain), effort low.

Issue #1130

drain() on BTreeMap, HashSet and BTreeSet (ADR-386, architect’s decision 2026-11-01). Only Vec::drain and collections::HashMap::drain are in std.contracts (:2971, :2982); C1b (547e6462) had to clone every set out of a borrowed BTreeMap for want of it.

Probe (target/fast/nikaia at e1a88e49):

use std::collections
fn main() {
    let mut m: collections::BTreeMap[String, Vec[i64]] = collections::BTreeMap()
    m.insert("a", [1, 2])
    for (k, v) in m.drain() {
        print(f"{k} {v.len()}\n")
    }
}

NK1171: collections::BTreeMap has no method called drain. Expected: a 2.

Facts: Where things live, Traps already met, Commands

Locations

crates/nikaia-std/std.contracts: [fn."collections::HashMap::drain"] (:2982) is the model; add BTreeMap::drain (Seq[(K, V)] sync ends sized), HashSet::drain (Seq[T] sync sized), BTreeSet::drain (Seq[T] sync ends sized), each taking self by value.
Lowering: HashSet to Rust drain(); BTreeMap/BTreeSet have no stable drain: into_iter(), or std::mem::take(&mut m).into_iter() where the container is a place still named after the loop (ADR-386 D2). Find where HashMap::drain is lowered with nikaia run scripts/where.nika -- drain (emitter, tools/emit_*.nika); crates/nikaia-std/src/map.rs:249 is std’s own HashMap::drain.

To build

The three ledger entries and their lowering; test tests/compiler/tests/drain_every_map_and_set.nika (+ .stdout): a BTreeMap drained in key order with each value handed to a Vec that keeps it, a HashSet drained into a sum, a BTreeSet drained in order, and a BTreeMap field of a let mut struct drained and then len() 0. Fails on the unfixed binary (NK1171), passes --both-settings.
Repay C1b’s clone: ledger_items_stored (crates/nikaia-std/src/tools/ledger_items.nika:488) takes each unit’s map over with for (key, code) in unit.drain() and extend(code.resolved) / extend(code.free) with no .iter().cloned(); contracts/mod.rs:444’s caller keeps passing owned maps (adapt to what the lowering asks). lower-std, its fixpoint, cargo test -p nikaia --lib contracts::, --test it -- throws ledger contract.

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/b32-1130-drain-every-map.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.

Result:, Gate: (green or red <check>), Deviation:, Found: and the other fields with nothing to say hold the bare word: done, none. Anything after it reads as content.
Deviation: only for a step of the brief built otherwise than written, or not built. Found: only for a defect outside your change. Everything else (your choices, how you solved the step, why a test’s expected output changed) goes into Body:; how you verified goes into Tests:.
Final answer, one line: done <sha>, report <path> (or BLOCKED: ... / PUSH REJECTED ...).

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Brief: #1130, drain() on BTreeMap, HashSet and BTreeSet (ADR-386)