Swift 6 / Strict Concurrency

When to invoke

• Starting a new Swift iOS / macOS App project and deciding the language mode.
• Writing Package.swift and setting swiftLanguageModes / SWIFT_STRICT_CONCURRENCY.
• Adding a new third-party dependency and hitting concurrency warnings / errors.
• User asks "how do I configure Swift 6 / complete concurrency / Sendable / actor / @preconcurrency".

Default decisions

• Swift 6 language mode + complete concurrency checking, applied from the first line of code.
• All in-house types are treated as needing `Sendable` by default; any type shared across actors must declare it explicitly (struct Foo: Sendable or final class Foo: Sendable — the latter only valid when all stored properties are Sendable and the class is non-inheritable; for classes with mutable state, prefer actor Foo or @unchecked Sendable with manual synchronisation).
• Protocols that cross actor boundaries (DI injection points) are always declared Sendable, with methods async throws.
• For third-party deps that don't support Swift 6 complete checking, in order of preference:

1. @preconcurrency import X to isolate the import
2. Switch packages
3. Defer adoption

Rationale

• A greenfield project has no legacy code to migrate, so the pain of complete checking gets amortised — solved once per actor / Sendable as you write — rather than as a one-shot big-bang migration later.
• Swift 6 is the long-term direction of the language; early adoption avoids technical debt.
• Complete checking catches data races at compile time, more than minimal / targeted can.

Deviation considerations

• Migrating existing Swift 5 code: switch to minimal or targeted concurrency checking and upgrade in stages; ramp up one file / module at a time, allowing @preconcurrency during transition.
• Significant third-party lag: if a critical dep doesn't support it, drop the whole project to targeted and patch module by module.
• Teaching / demo projects: if the goal is to demonstrate older API behaviour, keeping Swift 5 mode is fine.

Related skills

• apple-platform-targets: Xcode 16+ is the first toolchain that formally supports Swift 6 language mode; consider this alongside deployment target decisions.
• swiftpm-modularization: swiftLanguageModes: [.v6] sets the default for the whole package; individual targets can opt down with swiftSettings: [.swiftLanguageMode(.v5)] (available since swift-tools-version 6.0) — useful when migrating a legacy dependency without blocking the rest of the package.