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🍒 [SE-0412] changelog note about strict concurrency for global variables #70863

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Jan 17, 2024
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🍒 [SE-0412] changelog note about strict concurrency for global variables #70863

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26 changes: 26 additions & 0 deletions CHANGELOG.md
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## Swift 5.10

* [SE-0412][]:

Under strict concurrency checking, every global or static variable must be either isolated to a global actor or be both immutable and of `Sendable` type.
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Can you show an example of some code that previously could exhibit data races at runtime that is now diagnosed under the proposal?


```swift
var mutableGlobal = 1
// warning: var 'mutableGlobal' is not concurrency-safe because it is non-isolated global shared mutable state
// (unless it is top-level code which implicitly isolates to @MainActor)

final class NonsendableType {
init() {}
}

struct S {
static let immutableNonsendable = NonsendableType()
// warning: static property 'immutableNonsendable' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor
}
```

The attribute `nonisolated(unsafe)` can be used to annotate a global variable (or any form of storage) to disable static checking of data isolation, but note that without correct implementation of a synchronization mechanism to achieve data isolation, dynamic run-time analysis from exclusivity enforcement or tools such as Thread Sanitizer could still identify failures.

```swift
nonisolated(unsafe) var global: String
```

* [SE-0411][]:

Default value expressions can now have the same isolation as the enclosing
Expand Down Expand Up @@ -9871,6 +9896,7 @@ using the `.dynamicType` member to retrieve the type of an expression should mig
[SE-0397]: https://github.com/apple/swift-evolution/blob/main/proposals/0397-freestanding-declaration-macros.md
[SE-0407]: https://github.com/apple/swift-evolution/blob/main/proposals/0407-member-macro-conformances.md
[SE-0411]: https://github.com/apple/swift-evolution/blob/main/proposals/0411-isolated-default-values.md
[SE-0412]: https://github.com/apple/swift-evolution/blob/main/proposals/0412-strict-concurrency-for-global-variables.md
[#64927]: <https://github.com/apple/swift/issues/64927>
[#42697]: <https://github.com/apple/swift/issues/42697>
[#42728]: <https://github.com/apple/swift/issues/42728>
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