[Release Notes] Swift 5.10 closes all known holes in compile-time strict

concurrency checking.

Author: hborla

CHANGELOG.md

@@ -61,6 +61,54 @@
 
 ## Swift 5.10
 
+* Swift 5.10 closes all known static data-race safey holes in complete strict
+concurrency checking.
+
+When writing code against `-strict-concurrency=complete`, Swift 5.10 will
+diagnose all potential for data races at compile time unless an explicit unsafe
+opt out, such as `nonisolated(unsafe)` or `@unchecked Sendable`, is used.
+
+For example, in Swift 5.9, the following code crashes at runtime due to a
+`@MainActor`-isolated initializer being evaluated outside the actor, but it was
+not diagnosed under `-strict-concurrency=complete`:
+
+```swift
+@MainActor
+class MyModel {
+  init() {
+    MainActor.assertIsolated()
+  }
+
+  static let shared = MyModel()
+}
+
+func useShared() async {
+  let model = MyModel.shared
+}
+
+await useShared()
+```
+
+The above code admits data races because a `@MainActor`-isolated static
+variable, which evaluates a `@MainActor`-isolated initial value upon first
+access, is accessed synchronously from a `nonisolated` context. In Swift 5.10,
+compiling the code with `-strict-concurrency=complete` produces a warning that
+the access must be done asynchronously:
+
+```
+warning: expression is 'async' but is not marked with 'await'
+  let model = MyModel.shared
+              ^~~~~~~~~~~~~~
+              await
+```
+
+Swift 5.10 fixed numerous other bugs in `Sendable` and actor isolation checking
+to strengthen the guarantees of complete concurrency checking.
+
+Note that the complete concurrency model in Swift 5.10 is conservative. Several
+Swift Evolution proposals are in active development to improve the usability of
+strict concurrency checking ahead of Swift 6.
+
 * [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.
@@ -9998,4 +10046,4 @@ using the `.dynamicType` member to retrieve the type of an expression should mig
 [#57225]: <https://github.com/apple/swift/issues/57225>
 [#56139]: <https://github.com/apple/swift/issues/56139>
 [#70065]: <https://github.com/apple/swift/pull/70065>
-[swift-syntax]: https://github.com/apple/swift-syntax
+[swift-syntax]: https://github.com/apple/swift-syntax