[Pre-Pitch] Disconnected Properties and Variables, and Opaque enums

Evolution Discussion
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12

As far as I can tell, Mutex suffers the same data races:

import Synchronization

class NS { var a = 0 }
let mutex = Mutex<NS>(NS())

Task.detached {
    let a = mutex.withLock {
        $0
    }
    a.a += 1
}

Task.detached {
    let a = mutex.withLock {
        $0
    }
    a.a += 1
}

It is not clear to me, whether it is truly valid to use { $0 } for a (inout sending T) -> sending R closure. The source of being unsafe is that although $0 was in a disconnected region inside the closure, after it got returned, the value could be merged into any task-isolated regions later, so multiple invocations could definitely race.

The above code has racing accesses on a, the code compiles fine with Swift 6.1 & 6.2-dev, but was rejected (correctly?) by Swift 6.0.

I believe both Mutex and your Disconnected are inherently safe, the problem is the compiler fails to detect some erroneous usages in the client code.

PS: found related discussions:

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