[Observation] Gate enabling of peer macros by flag

Author: phausler

lib/Macros/Sources/ObservationMacros/ObservableMacro.swift

@@ -219,6 +219,15 @@ 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

@@ -16,15 +16,21 @@
 #if $Macros && hasAttribute(attached)
 
 @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(conformance)
 public macro Observable() = 
   #externalMacro(module: "ObservationMacros", type: "ObservableMacro")
 
 @available(SwiftStdlib 5.9, *)
 @attached(accessor)
+#if OBSERVATION_SUPPORTS_PEER_MACROS
 @attached(peer, names: prefixed(_))
+#endif
 public macro ObservationTracked() =
   #externalMacro(module: "ObservationMacros", type: "ObservationTrackedMacro")