write-swift

Fix Swift 6 data-race errors by working down a priority list: first stop sharing the object by giving each concurrent job its own instance, then make it a Sendable value type, then isolate it to an actor, and only finally use Mutex or @unchecked Sendable as a last resort.

In Swift 6.2, marking a function async no longer moves it off the current actor; it runs where it was called from. Use @concurrent to guarantee background execution for CPU-heavy work and nonisolated as the right default for library APIs.

Use Swift Testing for new tests by default. XCTest remains required only for UI automation with XCUIApplication, performance metrics with XCTMetric, and tests written in Objective-C or catching Objective-C exceptions.

Write some P by default since it preserves type relationships and enables compiler specialization. Change to any P only when you need heterogeneous storage, optionality of the underlying type, or to fully hide the abstraction behind a type-erased box.

Actors guarantee mutual exclusion, not transactions, so state can change between awaits due to reentrancy. Mutate actor state in synchronous methods, keep async methods thin, and re-check assumptions after every await, such as re-validating a cache after a download.

Avoid concurrency until profiling with Instruments shows an actual hang, since task allocation and scheduling carry real costs. Start single-threaded on the main actor, add async only to hide latency, and never spawn tasks for trivial work like reading a UserDefaults value.