SE-0470: Global-actor isolated conformances

[Douglas_Gregor]

@Nickolas_Pohilets

Our app still targets iOS 15, so it cannot use DispatchSerialQueue. Instead there are custom executor implementations that attempt to wrap DispatchQueue (non-main). Since underlying Dispatch API implementing isIsolatingCurrentContext() is private, custom implementations won't be able to implement it. They can only implement checkIsolated(). But even if implemented, checkIsolated() won't be called in pre-iOS 18 runtimes.

Generally speaking, we try to keep back-deployment considerations out of the design of language features, because they are vendor-specific and if taken to the extreme, can negatively affect the long-term language design.

@Nickolas_Pohilets

There is still one problematic case. I don't think we should let conditional casts silently pass, but not being able to use force casts can become a blocker.

The dynamic casting machinery is part of the runtime, and "force cast" is effectively a conditional cast + a force unwrap. We can encode the requirements of an isolated conformance in a manner that they will cause an older runtime to always reject the conformance (so it never works dynamically) or always accept the conformance (so it will always work dynamically). The implementation currently does the latter, which admits data races. The only alternative I can think of that wasn't taken would be to inject an otherwise-unused assumeIsolated {} into each of the functions that satisfies a requirement in an isolated conformance, but only do so when back-deploying. That way the cast (whether conditional or forced) would succeed (ugh), but you'd get a fatal error at runtime rather than a data race.

@Nickolas_Pohilets

I've found a problem with down casting, but it is not specific to global-actor isolated conformances, but rather a general problem with marker protocols:

Normally it is not possible to cast to Sendable:

print(x as? Sendable) // error: marker protocol 'Sendable' cannot be used in a conditional cast

But if you ask nicely, it becomes possible

Sneaky! The prototype implementation does not implement it yet, but the last paragraph on data-race safety in the proposal indicates that we will need to communicate the Sendable-ness of an existential type down to the runtime so it can refuse to use an isolated conformance for a cast to (e.g.)any P & Sendable. I believe that covers this case.

Doug