chore: remove unused flatten_layers_to_masks pass

[detail-app]

Remove the flatten_layers_to_masks function from pcb-ir's IPC process
pipeline, along with its four dedicated tests in process.rs and the
flatten-specific block in gerber/export.rs's
real_board_export_parseback_and_svg_paths_smoke test (whose
compose_for_rendering half is retained).

This pass had no production callers. Its only production caller — the
--flat render option in ipc2581 render — was deliberately removed in
#1026, replaced by instance-aware artwork lowering. The remaining
call-sites were all tests, which don't count as production consumers.
process is also the only sibling module in ipc/mod.rs without a
pub use re-export, so flatten_layers_to_masks was never part of the
advertised external API. The FeatureKind::FlattenedBucket enum variant
(now never constructed) is left in place in feature.rs to avoid a
breaking change to the public enum.

History

• Last materially changed August 2026 (Compact repeated panel artifacts #1036 @akhilles)

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Dead Code PRs can be configured here.

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Note

Low Risk
Dead-code deletion with no production callers; behavior of Gerber export and live IPC pipelines is unchanged aside from removed test coverage for the flatten pass.

Overview
Removes the dead flatten_layers_to_masks IPC process pass, which lowered each layer to artwork, composed a single unioned fill mask per layer, rewrote features as FeatureKind::FlattenedBucket, and cleared placement groups. Nothing in the repo calls it anymore after the old --flat render path was dropped.

process.rs: deletes the public function and four unit tests (mask union, stroke expansion, cutouts, placement groups). Test imports are trimmed so FeatureKind is only pulled in where still needed.

gerber/export.rs: shortens real_board_export_parseback_and_svg_paths_smoke by dropping the flatten-and-SVG block; the compose_for_rendering → artwork → mask → SVG path stays.

FeatureKind::FlattenedBucket remains on the public enum but is never constructed, avoiding a breaking API change.

^{Reviewed by Cursor Bugbot for commit 77e0b2f. Bugbot is set up for automated code reviews on this repo. Configure here.}

[devin-ai-integration]

Devin Review found 1 potential issue.

[devin-ai-integration]

🟡 Public flattening API disappears

External users calling flatten_layers_to_masks lose the public function without a compatibility path. process remains public, so direct callers stop compiling after upgrading.

Prompt for agents

The published pcb-ir crate exposes dialects::ipc::process publicly, making flatten_layers_to_masks callable by downstream crates despite its lack of a root re-export. Preserve compatibility, preferably by retaining and deprecating the function before a planned breaking release. If immediate removal is required, treat it as a public API break and coordinate the crate version and migration documentation accordingly.

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[github-actions]

Build Performance

Board	Base (median)	Head (median)	Change
demo/b/Demeter	67ms ±7	75ms ±15	+12.8%
demo/b/DM0001/Governor	88ms ±5	122ms ±7	1.39× ±0.11 slower
demo/b/DM0002/Marlow	69ms ±7	73ms ±13	+5.8%
demo/b/DM0003/Bramble	85ms ±5	83ms ±7	-1.6%
demo/b/Feign	60ms ±9	60ms ±8	-1.5%
demo/b/Renfield	64ms ±6	65ms ±7	+0.8%
demo/b/Seward	67ms ±5	67ms ±5	-0.7%
arduino/b/Nano	62ms ±3	61ms ±2	-0.8%
arduino/b/unoQ	146ms ±6	142ms ±3	-2.4%

_{Measured with hyperfine. Times show median ±stddev.}

[cursor]

Approved. Cursor Bugbot and Cursor Security Agent completed with no findings that need human review, and this is a small unused-code cleanup rather than a user-facing functional change. No reviewers were assigned.

  

_{Sent by Cursor Approval Agent: Pull Request Router and Approver}