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Brief: #1006, every set has iter() (ADR-collections-2 D3)

Score: std lane, blocks self-hosting row K3a part 2 (k3a2-comptime-compute.md, next in this batch: computed needs named.iter().any(..) over a BTreeSet[String]). S=1, a missing std entry the owner decided on 2026-10-30. Builders have no GitHub: the issue’s To build is copied below.

Decision (already written: ADR-collections-2 D3, Part III std::collections)

s.iter() on collections::BTreeSet[T] lends the elements one at a time, smallest first, as Vec::iter() does: Seq[ref(self) T] sync ends sized. On collections::HashSet[T] the same, in no order (as HashMap::keys): Seq[ref(self) T] sync sized (Rust’s HashSet iterator does not run backwards). Lowered to Rust’s .iter() (a trusted HashSet is TrustedSet). Pipeline methods on the set itself (s.any(f)) are refused by D3: do not add them. Do not edit the ADR or the specification.

Probe (target/fast at 9d401a96)

use std::collections

fn main() {
    let mut s: collections::BTreeSet[String] = collections::BTreeSet()
    s.insert("b")
    s.insert("a")
    println(s.iter().any(fn(n) { n == "a" }))
}

Today: NK1171 collections::BTreeSet has no method called iter; the same on a HashSet[i64] (h.iter().count()). for n in s runs already (smallest first).

To build

  1. crates/nikaia-std/std.contracts: [fn."collections::BTreeSet::iter"] and [fn."collections::HashSet::iter"] with the signatures above (pub = true, sync = true, touches = []), next to each kind’s other entries, each with a one-line comment (smallest first / no order, as HashMap::keys says it). Move the orphan comment A copy of the ordered set … (it sits above BTreeSet::first) onto BTreeSet::clone.
  2. Lowering: both lower to Rust’s .iter(), also inside crates/nikaia-std/src/tools/*.nika under lower-std (K3a’s code is lowered there). Add a row wherever the self-hosted checker or lowering needs one (find with nikaia run scripts/where.nika -- <name>; compare BTreeMap::iter/keys). If only the contract is needed, say so in Body:.
  3. Test tests/compiler/tests/a_set_is_walked_by_iter.nika + .stdout, through nikaia test --project tests/compiler (output to a file, grep it). Rows: on a BTreeSet[String] iter().any, iter().all, iter().filter(..).count(), iter().map(fn(s) { s.clone() }).collect() into a Vec[String] printed (sorted order), iter().rev() first element; on a HashSet[i64] iter().count() and a sum (order-free output); the K3a form named.iter().any(fn(name) { left_out.contains(name) || absent.contains(name) }) over three BTreeSet[String]. Then the K3a form once through target/fast/nikaia lower in a scratch file and compiled by rustc (as the k3a2 report probed), not committed.
  4. lower-std fixpoint (no diff, or the diff committed), the gate of .claude/agents/builder.md. Commit Fixes #1006.

Notes

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/b1006-set-iter.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 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:); 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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