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Dead-code scan (Periphery)

A weekly, advisory scan that reports unused Swift code with Periphery. It builds + indexes the project, so it runs on macOS in hygiene.yml; it never gates (dead code is not a bug, and the detection is inherently fuzzy). Config lives in .periphery.yml.

Why advisory, never a gate

Unused-code detection cannot be fully precise: code reached only through the Objective-C runtime, reflection (NSClassFromString), selectors, or KVC looks unused to static analysis. A gate that fails on those would block real work, so the scan only produces a report a human reviews — the same "fuzzy ⇒ weekly advisory" reasoning as the unused-image scan (see CI.md).

False-positive rules (and why)

.periphery.yml is tuned against the patterns that commonly trip Periphery up in a SwiftUI/iOS app. Each rule trades a little recall for far fewer false alarms — the right bias for an advisory check.

Pattern that looks "unused" Rule Why it's a false positive
DTO properties decoded from JSON retain_codable_properties The synthesized Codable init assigns them; they're read from JSON, not from code.
#Preview / PreviewProvider retain_swift_ui_previews Tooling entry points, not dead code.
@objc members retain_objc_accessible Often called by the ObjC runtime — KVO, target/action, NotificationCenter selectors — invisibly to static analysis.
Delegate params unused in every conformance retain_unused_protocol_func_params Required by the protocol signature.
AnyCancellable / observers held for lifetime retain_assign_only_property_types Assigned for their side effect, never read.
Code used only by tests report_exclude: **/*Tests/** Tests are indexed (so they count as usage) but not reported on.

These were verified empirically: the same patterns are flagged with no config and silenced once each rule is on.

What you must still handle by hand

Static analysis genuinely cannot resolve names built at runtime. For those, annotate the declaration in code:

// periphery:ignore — instantiated via NSClassFromString("…")
final class FeatureFlagPlugin {}

func handle(event: Event, context: Context) { … } // periphery:ignore:parameters context

Unused parameters of free functions are also reported (there's no blanket suppression) — usually a real finding; annotate if intentional.

Tuning for your project

  • After make new-app, update project / schemes in .periphery.yml.
  • Add your own held types to retain_assign_only_property_types, and your own dynamic patterns as // periphery:ignore annotations.
  • retain_objc_accessible: true errs toward silence; drop it if you want to surface genuinely-dead @objc members and accept some runtime-call noise.
  • The scan reports everything each run (no baseline) — review the artifact and clean up in batches.