write-swift

Fix Swift 6 data-race errors by working down an ordered 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. Global and static variables are the most common source.

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 so callers decide where work runs.

Write some P by default because it preserves type relationships and enables compiler specialization. Change to any P only when you need heterogeneous storage or optionality of the underlying type, accepting that associated-type relationships are erased and calls become opaque to the optimizer.

Use Swift Testing for new tests, since @Test, #expect, and parameterized arguments cover most needs with parallel execution by default. XCTest remains required only for UI automation with XCUIApplication, performance metrics with XCTMetric, and Objective-C exception handling.

Use an actor only when you need to move mutable state off the main actor after profiling shows excessive main-actor hops. Actors guarantee mutual exclusion but not transactions, so mutate state in synchronous methods and re-check assumptions after every await.