swift6-concurrencylisted

# 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; earl