Merge pull request #39827 from etcwilde/ewilde/cherry-pick-async-main-changelog

[NFC]: Update changelog to include SE-0323

Author: etcwilde

CHANGELOG.md

@@ -84,6 +84,21 @@ Swift 5.5
 
 ### 2021-09-20 (Xcode 13.0)
 
+* [SE-0323][]:
+
+  The main function is executed with `MainActor` isolation applied, so functions
+  and variables with `MainActor` isolation may be called and modified
+  synchronously from the main function. If the main function is annotated with a
+  global actor explicitly, it must be the main actor or an error is emitted. If
+  no global actor annotation is present, the main function is implicitly run on
+  the main actor.
+
+  The main function is executed synchronously up to the first suspension point.
+  Any tasks enqueued by initializers in Objective-C or C++ will run after the
+  main function runs to the first suspension point. At the suspension point, the
+  main function suspends and the tasks are executed according to the Swift
+  concurrency mechanisms.
+
 * [SE-0313][]:
 
   Parameters of actor type can be declared as `isolated`, which means that they
@@ -8745,6 +8760,7 @@ Swift 1.0
 [SE-0315]: <https://github.com/apple/swift-evolution/blob/main/proposals/0315-placeholder-types.md>
 [SE-0316]: <https://github.com/apple/swift-evolution/blob/main/proposals/0316-global-actors.md>
 [SE-0324]: <https://github.com/apple/swift-evolution/blob/main/proposals/0324-c-lang-pointer-arg-conversion.md>
+[SE-0323]: <https://github.com/apple/swift-evolution/blob/main/proposals/0323-async-main-semantics.md>
 
 [SR-75]: <https://bugs.swift.org/browse/SR-75>
 [SR-106]: <https://bugs.swift.org/browse/SR-106>