[Obs] Switch to peer instead of arbitrary macros

This adopts `@attached(peer)` for generating an observable type's
underscored storage when expanding the `@ObservationTracked` macro,
instead of using `arbitrary` with the member macro on the
observable type.

Author: natecook1000

lib/Macros/Sources/ObservationMacros/ObservableMacro.swift

@@ -225,15 +225,6 @@ extension ObservableMacro: MemberMacro {
     declaration.addIfNeeded(ObservableMacro.accessFunction(observableType), to: &declarations)
     declaration.addIfNeeded(ObservableMacro.withMutationFunction(observableType), to: &declarations)
 
-#if !OBSERVATION_SUPPORTS_PEER_MACROS
-    let storedInstanceVariables = declaration.definedVariables.filter { $0.isValidForObservation }
-    for property in storedInstanceVariables {
-       if property.hasMacroApplication(ObservableMacro.ignoredMacroName) { continue }
-       let storage = DeclSyntax(property.privatePrefixed("_", addingAttribute: ObservableMacro.ignoredAttribute))
-       declaration.addIfNeeded(storage, to: &declarations)
-    }
-#endif
-
     return declarations
   }
 }

stdlib/public/Observation/Sources/Observation/Observable.swift

@@ -40,11 +40,7 @@ public protocol Observable { }
 ///        }
 ///     }
 @available(SwiftStdlib 5.9, *)
-#if OBSERVATION_SUPPORTS_PEER_MACROS
 @attached(member, names: named(_$observationRegistrar), named(access), named(withMutation))
-#else
-@attached(member, names: named(_$observationRegistrar), named(access), named(withMutation), arbitrary)
-#endif
 @attached(memberAttribute)
 @attached(extension, conformances: Observable)
 public macro Observable() =
@@ -56,9 +52,7 @@ public macro Observable() =
 /// framework isn't necessary.
 @available(SwiftStdlib 5.9, *)
 @attached(accessor, names: named(init), named(get), named(set))
-#if OBSERVATION_SUPPORTS_PEER_MACROS
 @attached(peer, names: prefixed(_))
-#endif
 public macro ObservationTracked() =
   #externalMacro(module: "ObservationMacros", type: "ObservationTrackedMacro")
 

test/stdlib/Observation/Observable.swift

@@ -1,6 +1,6 @@
 // REQUIRES: swift_swift_parser, executable_test
 
-// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library -enable-experimental-feature InitAccessors -enable-experimental-feature Macros -enable-experimental-feature ExtensionMacros -Xfrontend -plugin-path -Xfrontend %swift-host-lib-dir/plugins)
+// RUN: %target-run-simple-swift( -Xfrontend -dump-macro-expansions -Xfrontend -disable-availability-checking -parse-as-library -enable-experimental-feature InitAccessors -enable-experimental-feature Macros -enable-experimental-feature ExtensionMacros -Xfrontend -plugin-path -Xfrontend %swift-host-lib-dir/plugins)
 
 // Run this test via the swift-plugin-server
 // RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -parse-as-library -enable-experimental-feature InitAccessors -enable-experimental-feature Macros -enable-experimental-feature ExtensionMacros -Xfrontend -external-plugin-path -Xfrontend %swift-host-lib-dir/plugins#%swift-plugin-server)
@@ -183,6 +183,10 @@ struct Validator {
 
 
     let suite = TestSuite("Observable")
+
+    suite.test("only instantiate") {
+      let test = MiddleNamePerson()
+    }
 
     suite.test("unobserved value changes") {
       let test = MiddleNamePerson()