[Experiment] [Concurrency] Loosen isolation checking for overrides/witnesses to ObjC. #35275
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When overriding a method or witnessing a requirement that comes from Objective-C and has no actor annotation, allow the overriding method or witness to specify a global actor (any global actor, but probably almost always the main actor) without triggering the actor-isolation errors one would normally get if both entities were written in Swift. This opens up a hole in actor-isolation checking, because nothing guarantees that code won't call these methods through the superclass or protocol from the wrong actor. On the other hand, it's a very convenient hole, because it allows us to state when we "know" that an Objective-C framework will only call a method on (e.g.) the main actor, and make that known to Swift to be enforced everywhere else in Swift. If this is a good idea, it's plausible to introduce runtime assertions of some form to tell the user when such an annotation is wrong.
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DougGregor
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When overriding a method or witnessing a requirement that comes from
Objective-C and has no actor annotation, allow the overriding method
or witness to specify a global actor (any global actor, but probably
almost always the main actor) without triggering the actor-isolation
errors one would normally get if both entities were written in Swift.
This opens up a hole in actor-isolation checking, because nothing
guarantees that code won't call these methods through the superclass
or protocol from the wrong actor. On the other hand, it's a very
convenient hole, because it allows us to state when we "know" that an
Objective-C framework will only call a method on (e.g.) the main
actor, and make that known to Swift to be enforced everywhere else in
Swift. If this is a good idea, it's plausible to introduce runtime
assertions of some form to tell the user when such an annotation is
wrong.