Make the @Observable macro class only (#67033)

* Make ObservationRegistrar Codable/Hashable

These conformances enable automatic Codable synthesis for Observable
types, and smooth the runway for structs being supported by the
Observable macro in the future.

* Limit Observable macro to classes

This removes the ability for the Observable macro to apply to structs,
and adds diagnostic tests for the three disallowed declaration kinds.

Author: natecook1000

lib/Macros/Sources/ObservationMacros/Extensions.swift

@@ -275,4 +275,8 @@ extension DeclGroupSyntax {
   var isEnum: Bool {
     return self.is(EnumDeclSyntax.self)
   }
+  
+  var isStruct: Bool {
+    return self.is(StructDeclSyntax.self)
+  }
 }

lib/Macros/Sources/ObservationMacros/ObservableMacro.swift

@@ -206,11 +206,15 @@ extension ObservableMacro: MemberMacro {
 
     if declaration.isEnum {
       // enumerations cannot store properties
-      throw DiagnosticsError(syntax: node, message: "@Observable cannot be applied to enumeration type \(observableType.text)", id: .invalidApplication)
+      throw DiagnosticsError(syntax: node, message: "'@Observable' cannot be applied to enumeration type '\(observableType.text)'", id: .invalidApplication)
+    }
+    if declaration.isStruct {
+      // structs are not yet supported; copying/mutation semantics tbd
+      throw DiagnosticsError(syntax: node, message: "'@Observable' cannot be applied to struct type '\(observableType.text)'", id: .invalidApplication)
     }
     if declaration.isActor {
       // actors cannot yet be supported for their isolation
-      throw DiagnosticsError(syntax: node, message: "@Observable cannot be applied to actor type \(observableType.text)", id: .invalidApplication) 
+      throw DiagnosticsError(syntax: node, message: "'@Observable' cannot be applied to actor type '\(observableType.text)'", id: .invalidApplication)
     }
 
     var declarations = [DeclSyntax]()

stdlib/public/Observation/Sources/Observation/ObservationRegistrar.swift

@@ -162,4 +162,29 @@ public struct ObservationRegistrar: Sendable {
     defer { didSet(subject, keyPath: keyPath) }
     return try mutation()
   }
-}
+}
+
+@available(SwiftStdlib 5.9, *)
+extension ObservationRegistrar: Codable {
+  public init(from decoder: any Decoder) throws {
+    self.init()
+  }
+  
+  public func encode(to encoder: any Encoder) {
+    // Don't encode a registrar's transient state.
+  }
+}
+
+@available(SwiftStdlib 5.9, *)
+extension ObservationRegistrar: Hashable {
+  public static func == (lhs: Self, rhs: Self) -> Bool {
+    // A registrar should be ignored for the purposes of determining its
+    // parent type's equality.
+    return true
+  }
+  
+  public func hash(into hasher: inout Hasher) {
+    // Don't include a registrar's transient state in its parent type's
+    // hash value.
+  }
+}

test/stdlib/Observation/Observable.swift

@@ -24,29 +24,6 @@ func _blackHole<T>(_ value: T) { }
 @Observable
 class ContainsNothing { }
 
-@Observable
-struct Structure {
-  var field: Int = 0
-}
-
-@Observable
-struct MemberwiseInitializers {
-  var field: Int
-}
-
-func validateMemberwiseInitializers() {
-  _ = MemberwiseInitializers(field: 3)
-}
-
-@Observable
-struct DefiniteInitialization {
-  var field: Int
-
-  init(field: Int) {
-    self.field = field
-  }
-}
-
 @Observable
 class ContainsWeak {
   weak var obj: AnyObject? = nil
@@ -92,6 +69,31 @@ struct NonObservableContainer {
   }
 }
 
+@Observable
+final class SendableClass: Sendable {
+  var field: Int = 3
+}
+
+@Observable
+class CodableClass: Codable {
+  var field: Int = 3
+}
+
+@Observable
+final class HashableClass {
+  var field: Int = 3
+}
+
+extension HashableClass: Hashable {
+  static func == (lhs: HashableClass, rhs: HashableClass) -> Bool {
+    lhs.field == rhs.field
+  }
+
+  func hash(into hasher: inout Hasher) {
+    hasher.combine(field)
+  }
+}
+
 @Observable
 class ImplementsAccessAndMutation {
   var field = 3
@@ -158,9 +160,6 @@ class IsolatedInstance {
   var test = "hello"
 }
 
-@Observable
-struct StructHasExistingConformance: Observable { }
-
 @Observable
 class ClassHasExistingConformance: Observable { }
 
@@ -209,23 +208,6 @@ struct Validator {
       expectEqual(changed.state, false)
     }
 
-    suite.test("tracking structure changes") {
-      let changed = CapturedState(state: false)
-      
-      var test = Structure()
-      withObservationTracking {
-        _blackHole(test.field)
-      } onChange: {
-        changed.state = true
-      }
-      
-      test.field = 4
-      expectEqual(changed.state, true)
-      changed.state = false
-      test.field = 5
-      expectEqual(changed.state, false)
-    }
-
     suite.test("conformance") {
       func testConformance<O: Observable>(_ o: O) -> Bool {
         return true