One of the goals of the Swift team with Swift’s concurrency features is to provide a model that allows developer to write safe code by default. This means that there’s a lot of time and energy invested into making sure that the Swift compiler helps developers detect, and prevent whole classes of bugs and concurrency issues altogether.
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In Swift, value types provide a lot of thread safety out of the box. When you pass a value type from one place to the next, a copy is created which means that each place that holds a copy of your value type can freely mutate its copy without affecting other parts of the code.
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Having this feature in our toolbox is incredibly useful when you’re slowly phasing Swift Concurrency into an existing project, but you’ll want to think twice, or maybe even three times, when you’re reaching for @unchecked Sendable. You should only use this feature when you’re really certain that your code is actually safe to be used in a concurrent environment.
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Lots of closures in Swift Concurrency are annotated with the @Sendable annotation. For example, here’s what the declaration for TaskGroup's addTask looks like:
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