[5.9] Observable protocol non-marker, @observable class only (#67196)

* 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.

* [Observation] Switch `Observable` to be a non-marker protocol (#66993)

With support for redundant conformance declarations via macros,
the `Observable` protocol can be a non-marker protocol, which
provides more flexibility for evolution in the future.

rdar://111463883

This change also switches to the new ExtensionMacro protocol,
the requirement for which includes information about whether the
conformance to the Observable protocol has already been added, either
in the declaration or in a superclass to the macro-attributed type.
This allows the @observable macro to be applied to subclasses of
observable types without redundant-conformance errors.

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

@@ -173,10 +173,12 @@ extension PatternBindingListSyntax {
 }
 
 extension VariableDeclSyntax {
-    func privatePrefixed(_ prefix: String, addingAttribute attribute: AttributeSyntax) -> VariableDeclSyntax {
-    VariableDeclSyntax(
+  func privatePrefixed(_ prefix: String, addingAttribute attribute: AttributeSyntax) -> VariableDeclSyntax {
+    let newAttributes = AttributeListSyntax(
+      (attributes.map(Array.init) ?? []) + [.attribute(attribute)])
+    return VariableDeclSyntax(
       leadingTrivia: leadingTrivia,
-      attributes: attributes?.appending(.attribute(attribute)) ?? [.attribute(attribute)],
+      attributes: newAttributes,
       modifiers: modifiers?.privatePrefixed(prefix) ?? ModifierListSyntax(keyword: .private),
       bindingKeyword: TokenSyntax(bindingKeyword.tokenKind, leadingTrivia: .space, trailingTrivia: .space, presence: .present),
       bindings: bindings.privatePrefixed(prefix),
@@ -206,11 +208,15 @@ 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]()
@@ -261,30 +267,27 @@ extension ObservableMacro: MemberAttributeMacro {
   }
 }
 
-extension ObservableMacro: ConformanceMacro {
-  public static func expansion<Declaration: DeclGroupSyntax, Context: MacroExpansionContext>(
+extension ObservableMacro: ExtensionMacro {
+  public static func expansion(
     of node: AttributeSyntax,
-    providingConformancesOf declaration: Declaration,
-    in context: Context
-  ) throws -> [(TypeSyntax, GenericWhereClauseSyntax?)] {
-    let inheritanceList: InheritedTypeListSyntax?
-    if let classDecl = declaration.as(ClassDeclSyntax.self) {
-      inheritanceList = classDecl.inheritanceClause?.inheritedTypeCollection
-    } else if let structDecl = declaration.as(StructDeclSyntax.self) {
-      inheritanceList = structDecl.inheritanceClause?.inheritedTypeCollection
-    } else {
-      inheritanceList = nil
-    }
-    
-    if let inheritanceList {
-      for inheritance in inheritanceList {
-        if inheritance.typeName.identifier == ObservableMacro.conformanceName {
-          return []
-        }
-      }
+    attachedTo declaration: some DeclGroupSyntax,
+    providingExtensionsOf type: some TypeSyntaxProtocol,
+    conformingTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [ExtensionDeclSyntax] {
+    // This method can be called twice - first with an empty `protocols` when
+    // no conformance is needed, and second with a `MissingTypeSyntax` instance.
+    if protocols.isEmpty {
+      return []
     }
-    
-    return [(ObservableMacro.observableConformanceType, nil)]
+
+    let decl: DeclSyntax = """
+      extension \(raw: type.trimmedDescription): \(raw: qualifiedConformanceName) {}
+      """
+
+    return [
+      decl.cast(ExtensionDeclSyntax.self)
+    ]
   }
 }
 

stdlib/public/Observation/Sources/Observation/CMakeLists.txt

@@ -24,6 +24,7 @@ add_swift_target_library(swiftObservation ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS
   SWIFT_COMPILE_FLAGS
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     "-enable-experimental-feature" "Macros"
+    "-enable-experimental-feature" "ExtensionMacros"
     -Xfrontend -disable-implicit-string-processing-module-import
 
   C_COMPILE_FLAGS

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

@@ -11,7 +11,7 @@
 
 
 @available(SwiftStdlib 5.9, *)
-@_marker public protocol Observable { }
+public protocol Observable { }
 
 #if $Macros && hasAttribute(attached)
 
@@ -22,7 +22,7 @@
 @attached(member, names: named(_$observationRegistrar), named(access), named(withMutation), arbitrary)
 #endif
 @attached(memberAttribute)
-@attached(conformance)
+@attached(extension, conformances: Observable)
 public macro Observable() =
   #externalMacro(module: "ObservationMacros", type: "ObservableMacro")
 

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

@@ -129,4 +129,29 @@ 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

@@ -1,6 +1,9 @@
 // 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 -Xfrontend -plugin-path -Xfrontend %swift-host-lib-dir/plugins)
+// 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 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)
 
 // Asserts is required for '-enable-experimental-feature InitAccessors'.
 // REQUIRES: asserts
@@ -19,29 +22,6 @@ 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
@@ -87,6 +67,31 @@ 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
@@ -153,9 +158,6 @@ class IsolatedInstance {
   var test = "hello"
 }
 
-@Observable
-struct StructHasExistingConformance: Observable { }
-
 @Observable
 class ClassHasExistingConformance: Observable { }
 
@@ -204,23 +206,6 @@ 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

test/stdlib/Observation/ObservableDidSetWillSet.swift

@@ -1,6 +1,6 @@
 // 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
+// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -enable-experimental-feature InitAccessors -enable-experimental-feature Macros -enable-experimental-feature ExtensionMacros -Xfrontend -plugin-path -Xfrontend %swift-host-lib-dir/plugins) | %FileCheck %s
 
 // Asserts is required for '-enable-experimental-feature InitAccessors'.
 // REQUIRES: asserts