Trace Swift code: @Traced/@TraceAll macros (primary) + SwiftTrace bridge (secondary)

[everettjf]

Why

AppleTrace's automatic mode rebinds objc_msgSend, but Swift's static / vtable / witness dispatch never goes through objc_msgSend, so most Swift code is invisible to the auto-hook. This adds Swift tracing via a new SwiftPM package, following the two-route plan in docs/swift-tracing.md.

What

A SwiftPM package at the repo root (reuses the existing appletrace.mm core — single source of truth, included from the Xcode tree, not duplicated).

Primary route — Swift macros (AppleTrace). Source-level instrumentation that sidesteps dispatch entirely (begin/end inserted into the body at compile time), so it covers final classes, structs, and protocol methods alike:

• withSpan(_:_:) + beginSection/endSection/traceInstant/traceCounter/asyncBegin/asyncEnd/flush wrappers.
• @Traced (body macro) wraps a function body in a #function-named section.
• @TraceAll (member-attribute macro) stamps @Traced on every method with a body.

import AppleTrace
withSpan("loadFeed") { try? loadFeed() }

@TraceAll final class FeedViewModel { func reload() {}; func render() {} }

Secondary route — SwiftTrace bridge (AppleTraceAuto, optional). Zero-annotation auto-tracing by bridging johnno1962/SwiftTrace: a SwiftTrace.Decorated subclass forwards each traced method's entry/exit to APTBeginSection/APTEndSection.

import AppleTraceAuto
AppleTraceAuto.trace(aClass: FeedViewModel.self)

The two routes complement each other: SwiftTrace can't see final/statically-dispatched methods (its documented blind spot) — the macros cover those.

Verification

• swift test — 6 tests green on Swift 6.2.4 with no experimental feature flags: @Traced/@TraceAll expansion, a usage test that applies the macros and asserts the function names land in the trace, and a runtime test driving the C core through withSpan.
• swift run AppleTraceAutoExample — verifies the SwiftTrace bridge end-to-end (traces a non-final class through a protocol; exits non-zero if not captured). The bridge can't be exercised from an XCTest bundle because SwiftTrace's metadata scanning needs a normal executable / app image.
• Both AppleTrace and AppleTraceAuto build for the iOS Simulator (xcodebuild).
• New Swift Tests CI workflow runs all of the above on macos-latest.

Demo app

sample/AppleTraceSwiftDemo (SwiftUI) consumes the local package and demonstrates both routes in one guided app: tap Generate Trace to run a multi-threaded workload (@Traced/@TraceAll/withSpan spans, an AppleTraceAuto-hooked ImageLoader, counters, async arcs, a 60-frame render loop). The screen shows the trace directory and the merge/Perfetto steps. Verified on the iPhone 17 Simulator (~490 events, 5 named tracks, both routes, no hang) and built+signed for device. CI also builds it.

Build note: use -destination only (no -sdk iphonesimulator, which forces the macro plugin onto the wrong SDK).

Notes

• The existing Xcode framework / Objective-C objc_msgSend hook are unchanged.
• docs/swift-tracing.md has the full landscape survey, the rationale, and per-route limitations.

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