When a large Swift codebase is migrated to Swift 6, developers are often surprised that code which worked reliably for years suddenly fails to compile. This can feel like Swift 6 is breaking backward compatibility.
In reality, Swift 6 does not intentionally break old code. It stops hiding issues related to concurrency, threading assumptions, and unsafe shared state that previously went unchecked.
In other words, Swift 6 does not create new bugs. It exposes old ones by making previously undefined runtime behavior explicit.
From best effort to compiler-enforced correctness
Swift 5 followed a best-effort philosophy where the compiler attempted to let code run even when safety could not be fully proven.
Swift 6 prioritizes correctness. If the compiler cannot prove safety through the type system and concurrency model, compilation fails.
The compiler no longer guesses execution context
Swift 5 often implicitly assumed execution on the main thread. Swift 6 requires execution context to be explicit and verifiable.
Shared mutable state is no longer acceptable
Shared mutable state is a primary source of concurrency bugs. Swift 6 enforces protection through actors or immutability.
Sendable becomes mandatory
In Swift 6, any data crossing concurrency boundaries must conform to Sendable, forcing clearer ownership models.
Swift 6 does not make code harder – it makes it more honest
Swift 6 does not introduce new complexity. It forces code to reflect its true runtime behavior rather than rely on unsafe assumptions.
Swift 6 migration is an architectural migration
Migrating to Swift 6 provides an opportunity to reduce technical debt, clarify concurrency boundaries, and improve overall architecture.
