agent: builder model: opus max_turns: 100 budget_usd: 20 —

Brief: #997, a let-bound lambda with a `String` parameter reaches rustc

Score: bug lane; blocks K3a part 2 (k3a2-comptime-compute.md, last in this batch: comptime_run.rs:393 binds known by let). S=1, a lowering defect on the self-hosting path. Builders have no GitHub: the issue text is copied below.

Problem

A lambda bound by let with an untyped String parameter, then handed on as fn(String) -> T? sync, passes the checker and reaches rustc: it lowers as |n| known(&n), rustc infers n: str (E0277, Sized). The same lambda written straight into the call runs. With a map read as the tail, fn(n) { more.get(n) } also misses the .copied() (Option<&i64> for i64?). Found by b995 (0cf0b173); blocks K3a part 2 (#125): comptime_run.rs:393 is let known = |name| known(name).or_else(|| ahead.get(name).cloned()).

Probe (target/fast at 0cf0b173)

fn known(n: String) -> i64? {
    if n == "a" { return 1 }
    return null
}
fn ask(f: fn(String) -> i64? sync, k: String) -> i64? {
    return f(k)
}
fn main() {
    let w = fn(n) { known(n) }
    print(ask(w, "a") ?? 0)
}

rustc: The size for values of type str cannot be known at :9. Expected 1. Second row: a local BTreeMap[String, i64] more, let w = fn(n) { known(n) ?? more.get(n) } and let v = fn(n) { more.get(n) }, each through ask. Third row: the element a String / an enum with a String part (?.clone() written, per ADR-293 D15).

Locations

Lowering of a lambda: emit_core_closure, crates/nikaia-std/src/tools/emit_core_flow.nika:229 (writes |{names}| without types); kept lambdas emit_flow_kept.
The type the checker gives a let-bound lambda’s parameter: follow Expr::Lambda in check_core_leaves.nika / check_core_args.nika (nikaia run scripts/where.nika -- emit_core_closure).

To build

Probe on a fresh build; rows 1-3.
Lower a let-bound lambda so rustc accepts it (any lowering rustc accepts: e.g. the parameter’s type written, |n: &str| or |n: String|, matched to how the call site passes it), and its map-read tail copied as a return of it is.
Test tests/compiler/tests/let_lambda_string_param.nika + .stdout (rows 1-3), through nikaia test; lower-std fixpoint; the it guard set. Commit Fixes #997.

Notes

Read docs/builder-facts.md; CLAUDE.md rules for builders; report form of docs/architect.md, Briefs and reports.

Do not touch: the uncommitted files git status shows at start (crates/rs2nika/convert/nikaia.*, docs/selfhosting-plan.md, the k1/k3a1 drafts, other reports): another session’s. Changelog: one sentence.

Rules (CLAUDE.md; this block is pasted into every brief, also a fix-forward)

Read docs/builder-facts.md first. develop in the main checkout; git status at start: other agents’ uncommitted files are not yours, never touch or stage them.
git add <files> only; commit after each step; git push origin develop fast-forward, no rebase (rejected: report Pushed: rejected, go on committing).
Before pushing: cargo fmt, cargo clippy --all-targets -- -D warnings, scripts/pre-push-check.sh, and the gate of .claude/agents/builder.md (#852).
No version lines, CHANGELOG.md, badges; no new NK code. New tests in Nikaia only; never the full suite. No foreground sleep, no polling; scripts in files.
A form Nikaia, std or the toolchain lacks stops the work: no code around it unless an ADR or Part I-III refuses it (name it). Else Result: blocked, Found: gap + probe, Needs: owner decision (CLAUDE.md, A missing form stops the work).
Disk: NIKAIA_CACHE_LIMIT=2G; at most two clones with target/ in the machine; delete your own clone’s target/ and target/release/nikaia-scratch at the end; report GB freed.
Commit and push before you write the report; the report’s Pushed: line names the pushed sha. A report without a Pushed: line, or with a placeholder left in (TC, DISK), is not done and cuts the batch (k3a1 2026-10-29: Result: done, nothing committed).

Report (at most 15 lines, to exactly `/home/user/Nikaia/docs/briefs/b997-let-lambda-string-param.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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Brief: #997, a let-bound lambda with a String parameter reaches rustc