Swift ObservableMacro to implement ExtensionMacro

This change switch to implement 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/ObservableMacro.swift

@@ -267,30 +267,25 @@ 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] {
+    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

@@ -46,7 +46,7 @@ public protocol Observable { }
 @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")
 

test/stdlib/Observation/Observable.swift

@@ -1,9 +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 -Xfrontend -external-plugin-path -Xfrontend %swift-host-lib-dir/plugins#%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

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