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

Author: sophiapoirier

CHANGELOG.md

@@ -5,6 +5,31 @@
 
 ## 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.
+
+  ```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