1Your app module still builds in Swift 5 mode — which concurrency flags do you turn on first, and how do you get to Swift 6 without a dead week?Medium
2A reviewer finds nonisolated(unsafe) in your pull request — what exactly are you promising, and when is it the honest answer rather than a silencer?Medium
3Your class has nothing but let properties and the compiler still refuses to let it cross an actor boundary — what is it objecting to?Medium
4static var shared compiled fine for years and is now an error in Swift 6 — what are the legal shapes for a piece of shared mutable state?Medium
5You hand a plain non-Sendable model object to an actor method and it compiles — what did the compiler prove, and what makes the same line start failing?Medium
6What can a sending parameter express that @Sendable cannot, and which compiler error does only sending fix?Medium
7Your @MainActor view model has to conform to Equatable and the compiler rejects the conformance — what are your options?Hard
8The build goes green after you add @preconcurrency to an import and to a conformance — what did each one silence, and what risk survives the silence?Hard
9You write a @globalActor for the storage layer, and a type that needs both it and @MainActor will not compile — what does a global actor buy you, and where does the serial-queue intuition break?Hard
10A thread-hostile C library has to be called from one context and nowhere else — how do you make an actor run its work there instead of on the cooperative pool?Hard
11The vendor documents that its callback arrives on the main thread but the method is nonisolated — is MainActor.assumeIsolated the right tool, and what makes it crash?Hard
12nonisolated on a stored property, on a synchronous method and on an async method — what does each promise, and which of them puts work in the background?Hard
13What do Approachable Concurrency and Default Actor Isolation: MainActor change in an app target, and why would you not ship a shared framework with them on?Hard
14A vendor's Objective-C SDK calls your delegate back on whatever thread it likes — how do you get that into a Swift 6 module without @unchecked Sendable on everything?Hard
15Your module compiles clean in Swift 6 language mode and still corrupts data under load — which races is the isolation checker structurally unable to see?Hard
16Thread Sanitizer reports a race in a target that compiles clean under Swift 6 — is the compiler wrong, and how do you track the bug down?Hard