SE-0343 Update changelog for async top-level

Author: etcwilde

CHANGELOG.md

@@ -5,6 +5,23 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 
 ## Swift 5.7
 
+* [SE-0343][]:
+
+Top-level scripts support asynchronous calls.
+
+Using an `await` by calling an asynchronous function or accessing an isolated
+variable transitions the top-level to an asynchronous context. As an
+asynchronous context, top-level variables are `@MainActor`-isolated and the
+top-level is run on the `@MainActor`.
+
+Note that the transition affects function overload resolution and starts an
+implicit run loop to drive the concurrency machinery.
+
+Unmodified scripts are not affected by this change unless `-warn-concurrency` is
+passed to the compiler invocation. With `-warn-concurrency`, variables in the
+top-level are isolated to the main actor and the top-level context is isolated
+to the main actor, but is not an asynchronous context.
+
 * [SE-0336][]:
 
 It is now possible to declare `distributed actor` and `distributed func`s inside of them.
@@ -9065,6 +9082,7 @@ Swift 1.0
 [SE-0335]: <https://github.com/apple/swift-evolution/blob/main/proposals/0335-existential-any.md>
 [SE-0341]: <https://github.com/apple/swift-evolution/blob/main/proposals/0341-opaque-parameters.md>
 [SE-0336]: <https://github.com/apple/swift-evolution/blob/main/proposals/0336-distributed-actor-isolation.md>
+[SE-0343]: <https://github.com/apple/swift-evolution/blob/main/proposals/0343-top-level-concurrency.md>
 
 [SR-75]: <https://bugs.swift.org/browse/SR-75>
 [SR-106]: <https://bugs.swift.org/browse/SR-106>