Decorated non-escaping closures?

[Joe_Groff]

@za_creature

In fact, I don't see why your example is unsafe. Unexpected or weird maybe, but does it really violate the memory model? It's perfectly fine to capture the same value for mutation in two closures as long as they're isolated to the same context.

The problem is exclusivity. Two nonescaping closures can capture the same value for mutation, but the tradeoff is that those nonescaping closures must not possibly run at the same time. Since producer and consumer come from the same scope, they could potentially access the same mutable captures, and so you can't pass producer or a closure that captures producer as a parameter to consumer, since consumer will exclusively access the same capture scope as producer while it runs, so there would be no safe way for it to invoke producer, not without some new annotation to require the two closures have disjoint captures or something like that.

@tera

This compiles, BTW:

func intermediate(_ producer: Producer, _ consumer: Consumer) {
    let prod = { producer() + 1 }
    consumer(prod)
}

That's a bug.