swift

# Swift Standards Default load: this file only; pull refs (see References) on demand. Swift owns language-level correctness only: optionals, errors, concurrency semantics, value/reference types, protocols and generics, ARC, naming, access control. Platform and app-shape concerns — scenes/windows, SwiftUI architecture, sandboxing, distribution, persistence, localization — live in `standards/macos/`. Universal rules stay in `standards/global/`. **Scope:** these rules apply in full to new targets and modules. In existing code, match established conventions; propose migrations (language mode, `ObservableObject` → `@Observable`, deployment targets, dependency swaps) as separate explicit tasks — never as a side effect of a feature change. ## Priority: P0 — Language Correctness ### Language Mode, Isolation & Concurrency P0 rules → `refs/concurrency.md` — load for any concurrency, isolation, or language-mode task. ### Availability - Gate every API newer than the deployment target with `if #available` / `@available` and a real fallback path. Annotate declarations rather than sprinkling runtime checks. Never raise the deployment target to dodge a check. ### Optionals - Never force-unwrap (`!`) or force-cast (`as!`) in production paths. If non-nil is a true invariant, use `guard let x else { preconditionFailure("why") }` so failure carries a message. - No implicitly unwrapped optionals (`T!`) in new pure-Swift code; acceptable only for UI-lifecycle objects (`@IBOutlet`) and Obj-...