swift-concurrency

# Swift Concurrency ## Fast Path Before proposing a fix: 1. Analyze `Package.swift` or `.pbxproj` to determine Swift language mode, strict concurrency level, default isolation, and upcoming features. Do this always, not only for migration work. 2. Capture the exact diagnostic and offending symbol. 3. Determine the isolation boundary: `@MainActor`, custom actor, actor instance isolation, or `nonisolated`. 4. Confirm whether the code is UI-bound or intended to run off the main actor. Project settings that change concurrency behavior: | Setting | SwiftPM (`Package.swift`) | Xcode (`.pbxproj`) | |---|---|---| | Language mode | `swiftLanguageVersions` or `-swift-version` (`// swift-tools-version:` is not a reliable proxy) | Swift Language Version | | Strict concurrency | `.enableExperimentalFeature("StrictConcurrency=targeted")` | `SWIFT_STRICT_CONCURRENCY` | | Default isolation | `.defaultIsolation(MainActor.self)` | `SWIFT_DEFAULT_ACTOR_ISOLATION` | | Upcoming features | `.enableUpcomingFeature("NonisolatedNonsendingByDefault")` | `SWIFT_UPCOMING_FEATURE_*` | If any of these are unknown, ask the developer to confirm them before giving migration-sensitive guidance. Do not guess. Guardrails: - Do not recommend `@MainActor` as a blanket fix. Justify why the code is truly UI-bound. - Prefer structured concurrency over unstructured tasks. Use `Task.detached` only with a clear reason. - If recommending `@preconcurrency`, `@unchecked Sendable`, or `nonisolated(unsafe)`, require...