Strict Concurrency Checking is one of the most controversial changes when migrating to Swift 6. It is the mechanism the compiler uses to rigorously validate concurrency and thread-safety assumptions at compile time.
If actor isolation defines the safety boundary, strict concurrency checking acts as the gatekeeper, ensuring no code bypasses that boundary accidentally or due to legacy habits.
What is Strict Concurrency Checking?
Strict concurrency checking is a set of rules enforced by Swift 6 to reject code that cannot prove its safety in concurrent environments. What used to be warnings are now errors.
This mechanism forces developers to be explicit about execution context, data ownership, and async boundaries.
Why Swift 6 needs strict checking
In Swift 5, many concurrency bugs surfaced randomly on user devices. The compiler lacked enough information to prevent them.
Swift 6 sacrifices permissiveness in favor of safety, requiring every assumption to be proven.
Common errors when enabling strict concurrency
When strict concurrency checking is enabled, developers often encounter errors related to unsafe shared state, closure captures, and missing Sendable conformance.
These errors are not new bugs; they are existing issues that are now clearly exposed.
Strategies for handling strict concurrency during migration
The most effective approach is to enable strict concurrency early, fix issues incrementally, and treat compiler errors as architectural guidance.
Instead of disabling strict checking, code should be refactored to align with the new concurrency model.
Swift 6 does not make strict checking painful – it makes code safe
Strict concurrency checking may feel painful initially, but it eliminates an entire class of hard-to-reproduce and costly production bugs.
Once code passes strict checking, developers can trust the concurrent behavior of the system with greater confidence.
Conclusion
Strict concurrency checking is a key tool enabling Swift 6 to deliver compile-time concurrency safety. Embracing it is essential for a successful migration.
