1c5372858b
While here, fix some issues around implied isolated conformances (we could get into an inconsistent state). Also provide an educational note discussing isolated conformances and the kinds of errors one can see when they are used from outside of their isolation domain.
1.6 KiB
Isolated conformances
A protocol conformance can be isolated to a specific global actor, meaning that the conformance can only be used by code running on that actor. Isolated conformances are expressed by specifying the global actor on the conformance itself:
protocol P {
func f()
}
@MainActor
class MyType: @MainActor P {
/*@MainActor*/ func f() {
// must be called on the main actor
}
}
Swift will produce diagnostics if the conformance is directly accessed in code that isn't guaranteed to execute in the same global actor. For example:
func acceptP<T: P>(_ value: T) { }
/*nonisolated*/ func useIsolatedConformance(myType: MyType) {
acceptP(myType) // error: main actor-isolated conformance of 'MyType' to 'P' cannot be used in nonisolated context
}
To address this issue, mark the code as having the same global actor as the conformance it is trying to use. In this case, mark useIsolatedConformance as @MainActor so that the code is guaranteed to execute on the main actor.
An isolated conformance cannot be used together with a Sendable requirement, because doing so would allow the conformance to cross isolation boundaries and be used from outside the global actor. For example:
func acceptSendableP<T: P & Sendable>(_ value: T) { }
@MainActor func useIsolatedConformanceOnMainActor(myType: MyType) {
acceptSendableP(myType) // error: main-actor-isolated conformance of 'MyType' to 'P' cannot satisfy conformance requirement for 'Sendable' type parameter 'T'
}
These errors can be addressed by either making the conformance itself nonisolated or making the generic function not require Sendable.