Data-race safety in the Swift 6 language mode is designed for incremental migration. You can address data-race safety issues in your projects module-by-module, and you can enable the compiler’s actor isolation and Sendable checking as warnings in the Swift 5 language mode, allowing you to assess your progress toward eliminating data races before turning on the Swift 6 language mode.

Complete data-race safety checking can be enabled as warnings in the Swift 5 language mode using the -strict-concurrency compiler flag.

Using the Swift compiler

To enable complete concurrency checking when running swift or swiftc directly at the command line, pass -strict-concurrency=complete:

~ swift -strict-concurrency=complete main.swift

Using SwiftPM

In a SwiftPM command-line invocation

-strict-concurrency=complete can be passed in a Swift package manager command-line invocation using the -Xswiftc flag:

~ swift build -Xswiftc -strict-concurrency=complete
~ swift test -Xswiftc -strict-concurrency=complete

This can be useful to gauge the amount of concurrency warnings before adding the flag permanently in the package manifest as described in the following section.

In a SwiftPM package manifest

To enable complete concurrency checking for a target in a Swift package using Swift 5.9 or Swift 5.10 tools, use SwiftSetting.enableExperimentalFeature in the Swift settings for the given target:

.target(
  name: "MyTarget",
  swiftSettings: [
    .enableExperimentalFeature("StrictConcurrency")
  ]
)

When using Swift 6.0 tools or later, use SwiftSetting.enableUpcomingFeature in the Swift settings for the given target:

.target(
  name: "MyTarget",
  swiftSettings: [
    .enableUpcomingFeature("StrictConcurrency")
  ]
)

Using Xcode

To enable complete concurrency checking in an Xcode project, set the “Strict Concurrency Checking” setting to “Complete” in the Xcode build settings. Alternatively, you can set SWIFT_STRICT_CONCURRENCY to complete in an xcconfig file:

// In a Settings.xcconfig

SWIFT_STRICT_CONCURRENCY = complete;