Swift Concurrency Migration

Migrate from evidence, one isolation boundary at a time. Preserve behavior before changing architecture, and keep every target buildable between migration waves.

Inspect before editing

Determine the compiler, language mode, deployment targets, and effective settings for every affected target. Do not infer them from the newest syntax in one file.

xcodebuild -version
xcrun swiftc --version
xcodebuild -list -project App.xcodeproj
xcodebuild -showBuildSettings -project App.xcodeproj -target App \
  | rg 'SWIFT_VERSION|SWIFT_STRICT_CONCURRENCY|SWIFT_DEFAULT_ACTOR_ISOLATION|SWIFT_APPROACHABLE_CONCURRENCY|IPHONEOS_DEPLOYMENT_TARGET|MACOSX_DEPLOYMENT_TARGET'

For a package, inspect // swift-tools-version, each target's swiftSettings, and the resolved compiler: