Merge pull request #66731 from DougGregor/observable-willset-didset-5.9

[Macros] Add test using observer accessors (didSet/willSet) with Observable

Author: DougGregor

test/stdlib/Observation/ObservableDidSetWillSet.swift

@@ -0,0 +1,44 @@
+// REQUIRES: swift_swift_parser, executable_test
+
+// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -enable-experimental-feature InitAccessors -enable-experimental-feature Macros -Xfrontend -plugin-path -Xfrontend %swift-host-lib-dir/plugins) | %FileCheck %s
+
+// Asserts is required for '-enable-experimental-feature InitAccessors'.
+// REQUIRES: asserts
+
+// REQUIRES: observation
+// REQUIRES: concurrency
+// REQUIRES: objc_interop
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+import _Observation
+
+@Observable
+public class Model {
+  public enum State {
+    case initializing
+    case running
+    case complete
+  }
+
+  public var state: State = .initializing {
+    willSet {
+      print("new state=\(String(describing: newValue))")
+    }
+
+    didSet {
+      guard oldValue != state else { return }
+      print("old state=\(String(describing: oldValue))")
+    }
+  }
+}
+
+
+let m = Model()
+
+// CHECK: new state=running
+// CHECK: old state=initializing
+m.state = .running
+// CHECK: new state=complete
+// CHECK: old state=running
+m.state = .complete