What is Sendable and why is it needed?
The essence
Sendable is a marker protocol with no requirements: "a value of this type can be safely passed across an isolation boundary (between actors, tasks, threads) without data races". The compiler checks it at every boundary: arguments and results of actor calls, captures in Task { }, @Sendable closures. In Swift 6 a violation is a compile error; in Swift 5 with -strict-concurrency=complete it is a warning.
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