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agent: builder model: opus max_turns: 110 budget_usd: 22 —

Brief: a struct whose every field has a default lowers with impl Default (E12-4b prerequisite, #125)

Score: self-hosting lane (docs/priorities.md; #125, docs/selfhosting-plan.md E12-4b): emit/mod.rs Build has 343 Build::default() callers on the Rust side; it can move to emit_build.nika only when the lowering writes the Default the Rust callers use. One compiler step, no move.

Facts

Locations

To build

  1. A struct (not extern, no generics, not seeded with hidden type parameters) whose every field has a default gets impl Default for S { fn default() -> Self { S { f: <default>, ... } } } after its declaration, each default lowered as the literal fill-in lowers it. A struct with one field without a default gets none (unchanged output). Test tests/compiler/tests/a_struct_of_defaults_has_default.nika + .stdout: the probe’s B plus a text field with a default and a nested struct field, checks that the lowered program still runs (B {}) and, if the language can call it, that it equals the literal of all defaults; if Nikaia cannot call Default::default (no such name in Part I), the test is nikaia lower output checked by a .stdout of the run only, and say under Tests: how the impl was seen.
  2. lower-std: commit any tools file whose lowering gains an impl Default (a tools struct of all-default fields), and check no Rust host code now has two Default impls for one type (rustc E0119 in the build); a host impl Default made redundant is deleted in the same commit.

Done when

The test passes; nikaia test tests/compiler --both-settings; it filtered on struct default typecheck; cargo build of the workspace (step 2); the guard set of .claude/agents/builder.md; clippy clean. Commits Part: #125; Changelog: one sentence.

Stop and report if

An open design choice (a generic struct, a default that is not a build-time value); an error in another agent’s file.

Read docs/builder-facts.md.

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/e12-4b0-struct-default-impl.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 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); 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:); 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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