swift-concurrency-expert

Fix Swift 6 data race errors by first identifying the actor context of the offending code, then applying the smallest safe fix: annotate UI-bound types with @MainActor, protect global state with the main actor or an actor, or add Sendable conformance to value types. Rebuild and run tests to confirm no regressions.

This error occurs when non-Sendable state crosses actor boundaries. In Swift 6.2, async functions stay on the caller's actor by default, which often resolves it; otherwise use an isolated conformance, move the work into a @concurrent function, or make the type Sendable.

Approachable concurrency is an opt-in Swift 6.2 mode that infers main actor isolation by default, keeping code single-threaded unless you opt out. Enable it under Swift Compiler - Concurrency in Xcode build settings or via SwiftSettings in a SwiftPM manifest.

Use @MainActor for UI-bound types and shared mutable state that must stay on the main thread. Use @concurrent on async functions performing CPU-intensive work that should run on the concurrent thread pool, keeping the calling actor responsive.

A nonisolated protocol requirement cannot access main-actor state, so the compiler rejects the conformance. Swift 6.2 supports isolated conformances: declare the conformance as 'extension Foo: @MainActor SomeProtocol' so it is only used on the main actor.

Avoid @unchecked Sendable unless you can prove the type is thread-safe, since it bypasses compiler verification of data-race safety. Prefer immutable value types or correct Sendable conformance, and use actors to guard genuinely mutable shared state.