Does Swift's MainActor annotation actually do anything at runtime to ensure code is called from the main thread, or is it entirely a compile-time sanity check that might not play well across opaque binary/framework boundaries?
@stroughtonsmith it emits a check on entry to a synchronous, actor-isolated function that ensures it’s running on the right actor. The check itself is fairly cheap, but could add up in a. Tight loop. The Swift 6 implementation should have lower overhead, because it can limit checks to those places where it is interfacing with code that isn’t statically checked.
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