write-swift

Work down an ordered list: first don't share 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 as a last resort use Mutex or @unchecked Sendable. Most real errors are fixed by the first step.

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 default for library APIs so callers decide where work runs.

Write some P by default since it preserves type relationships and enables compiler specialization. Switch to any P only when you need heterogeneous storage, optionality of the underlying type, or full type erasure, accepting the runtime cost of dynamic dispatch.

Use Swift Testing for new tests; XCTest remains required only for UI automation with XCUIApplication, performance metrics with XCTMetric, and Objective-C tests. Both frameworks coexist in one target, so you can migrate incrementally.

Actors guarantee mutual exclusion, not transactions, and await is a suspension point where other work runs. Mutate actor state in synchronous methods, keep async methods thin, and re-check assumptions after every await to avoid check-then-act bugs.

Stay single-threaded on the main actor until profiling with Instruments shows a hang. Don't spawn tasks for trivial work, don't make functions async that have nothing to await, and always try algorithmic optimizations before adding concurrency.