Add SE-0338 to the ChangeLog now that it's implemented

Author: DougGregor

CHANGELOG.md

@@ -5,6 +5,39 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 
 ## Swift 5.7
 
+* [SE-0338][]:
+
+  Non-isolated async functions now always execute on the global concurrent pool,
+  so calling a non-isolated async function from actor-isolated code will leave
+  the actor. For example:
+
+  ```swift
+  class C { }
+
+  func f(_: C) async { /* always executes on the global concurrent pool */ }
+
+  actor A {
+    func g(c: C) async {
+      /* always executes on the actor */
+      print("on the actor")
+
+      await f(c)
+    }
+  }
+  ```
+
+  Prior to this change, the call from `f` to `g` might have started execution of
+  `g` on the actor, which could lead to actors being busy longer than strictly
+  necessary. Now, the non-isolated async function will always hop to the global
+  cooperative pool, not run on the actor. This can result in a behavior change
+  for programs that assumed that a non-isolated async function called from a
+  `@MainActor` context will be executed on the main actor, although such
+  programs were already technically incorrect.
+
+  Additionally, when leaving an actor to execution on the global cooperative
+  pool, `Sendable` checking will be performed, so the compiler will emit a
+  diagnostic in the call to `f` if `c` is not of `Sendable` type.
+
 * [SE-0329][]:
   New types representing time and clocks were introduced. This includes a protocol `Clock` defining clocks which allow for defining a concept of now and a way to wake up after a given instant. Additionally a new protocol `InstantProtocol` for defining instants in time was added. Furthermore a new protocol `DurationProtocol` was added to define an elapsed duration between two given `InstantProtocol` types. Most commonly the `Clock` types for general use are the `SuspendingClock` and `ContinuousClock` which represent the most fundamental clocks for the system. The `SuspendingClock` type does not progress while the machine is suspended whereas the `ContinuousClock` progresses no matter the state of the machine. 
 
@@ -9396,6 +9429,7 @@ Swift 1.0
 [SE-0335]: <https://github.com/apple/swift-evolution/blob/main/proposals/0335-existential-any.md>
 [SE-0336]: <https://github.com/apple/swift-evolution/blob/main/proposals/0336-distributed-actor-isolation.md>
 [SE-0337]: <https://github.com/apple/swift-evolution/blob/main/proposals/0337-support-incremental-migration-to-concurrency-checking.md>
+[SE-0338]: <https://github.com/apple/swift-evolution/blob/main/proposals/0338-clarify-execution-non-actor-async.md>
 [SE-0340]: <https://github.com/apple/swift-evolution/blob/main/proposals/0340-swift-noasync.md>
 [SE-0341]: <https://github.com/apple/swift-evolution/blob/main/proposals/0341-opaque-parameters.md>
 [SE-0343]: <https://github.com/apple/swift-evolution/blob/main/proposals/0343-top-level-concurrency.md>