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agent: builder model: opus max_turns: 110 budget_usd: 30 —

Brief: meta::Field, a function over a reflected field (Part II 10.3, ADR-grammar-2 D4, #1019 step 3 prerequisite)

Facts: Traps already met

Score: #1019 step 3 (D4’s name: function) cannot be written without it; S=A, O=~1.5M tokens, C=~1M tokens, checker only. The decision is the architect’s (2026-10-31, Language decisions: obvious), written into Part II 10.3 and ADR-grammar-2 D4.

Problem (probe it with target/fast/nikaia run at your start, in your scratchpad)

@meta::Attribute(field)
pub struct Name { value: String }

struct Contact {
	@Name("full_name")
	name: String,
	age: i64 = 7,
	email: String?,
}

fn wire_name(f: meta::Field) -> String {
	return f.attribute(Name)?.value ?? f.name
}

fn show[T: Struct]() {
	for f in T::fields {
		let w = wire_name(f)
		println(f"{f.name} -> {w}")
	}
}

fn main() {
	show[Contact]()
}

Today (architect’s probe, 2026-10-31): NK1102 “wire_name expects f to be meta::Field, but you’re passing $Field”, then NK2105 “f was passed to wire_name, which keeps it”. Wanted: name -> full_name, age -> age, email -> email. The same body written inline in the loop (let w = f.attribute(Name)?.value ?? f.name) already prints that.

Decision (Part II 10.3, A reflected field is a meta::Field; ADR-grammar-2 D4 amendment)

Locations

To build

  1. meta::Field as a parameter’s type, one copy per field, no move; NK1320 elsewhere. Tests in tests/compiler/tests/: a_function_takes_a_reflected_field.nika + .stdout (the probe above, plus a second function that reads f.ty::kind and is called from the same loop, so two copies differ by type), a_reflected_field_kept_past_the_build_is_refused.nika + .stderr (struct Holder { f: meta::Field } gives NK1320; one program, one finding). Commit check: a function takes a reflected field, meta::Field (Part II 10.3), Part: #1019.

Done when

Both tests pass through nikaia test (one program alone: docs/builder-facts.md, Traps already met), each failed once against the compiler without your change (say so under Tests:); reflected_field_ty, match_on_a_kind, type_as_argument and the grammar_* tests still pass; lower-std fixpoint clean; the cheap checks of Rules. Changelog: one line.

Stop and report if

the copy per field needs the build-time runner (comptime_run.rs) rather than the checker’s copies; an edit outside check_*.nika, emit_*.nika, ty.nika, their lowered files, the move analysis and the new tests; a form beyond Part II 10.3’s paragraph; an error in another agent’s file. Stop about 15 turns before max_turns, commit what is finished and report partial.

Not yours, never stage: every file git status shows modified or untracked at your start, and every docs/briefs/* file except your report.

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/tools/j3-1019-meta-field.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 a lowering other than the brief’s where rustc cannot take the brief’s, b1029 2026-10-31: say why 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:, nor an existing test’s expected .stdout/.stderr updated because your change rightly changes the message it prints, b1012b 2026-10-31: name the test 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_01UVxJDvLe1uS47UcqA9y1U2

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