Merge pull request swiftlang#20488 from atrick/changelog

ChangeLog addition: SR-7139: Support enforcement of exclusive memory

Author: atrick

CHANGELOG.md

@@ -24,6 +24,20 @@ CHANGELOG
 Swift 5.0
 ---------
 
+* [SR-7139][]:
+
+  Exclusive memory access is now enforced at runtime by default in
+  optimized (`-O`/`-Osize`) builds. Programs that violate exclusivity will
+  trap at runtime with an "overlapping access" diagnostic
+  message. This can be disabled via a command line flag:
+  `-enforce-exclusivity=unchecked`, but doing so may result in undefined
+  behavior.
+
+  Runtime violations of exclusivity typically result from
+  simultaneous access of class properties, global variables (including
+  variables in top-level code), or variables captured by escaping
+  closures.
+
 * [SE-0216][]:
 
   The `@dynamicCallable` attribute enables nominal types to be "callable" via a
@@ -7290,4 +7304,5 @@ Swift 1.0
 [SR-2394]: <https://bugs.swift.org/browse/SR-2394>
 [SR-2608]: <https://bugs.swift.org/browse/SR-2608>
 [SR-4248]: <https://bugs.swift.org/browse/SR-4248>
+[SR-7139]: <https://bugs.swift.org/browse/SR-7139>
 [SR-7251]: <https://bugs.swift.org/browse/SR-7251>