[Macros] Bump the PluginMessage protocol version number for extension macros

and add a macro plugin test for extension macros.

Author: hborla

Examples/Sources/ExamplePlugin/ExamplePlugin.swift

@@ -10,6 +10,7 @@ struct ThePlugin: CompilerPlugin {
     PeerValueWithSuffixNameMacro.self,
     MemberDeprecatedMacro.self,
     EquatableConformanceMacro.self,
+    SendableExtensionMacro.self,
     DidSetPrintMacro.self,
     PrintAnyMacro.self,
   ]

Examples/Sources/ExamplePlugin/Macros.swift

@@ -86,6 +86,26 @@ struct EquatableConformanceMacro: ConformanceMacro {
   }
 }
 
+public struct SendableExtensionMacro: ExtensionMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    attachedTo: some DeclGroupSyntax,
+    providingExtensionsOf type: some TypeSyntaxProtocol,
+    in context: some MacroExpansionContext
+  ) throws -> [ExtensionDeclSyntax] {
+    let sendableExtension: DeclSyntax =
+      """
+      extension \(type.trimmed): Sendable {}
+      """
+
+    guard let extensionDecl = sendableExtension.as(ExtensionDeclSyntax.self) else {
+      return []
+    }
+
+    return [extensionDecl]
+  }
+}
+
 /// Add 'didSet' printing the new value.
 struct DidSetPrintMacro: AccessorMacro {
   static func expansion(

Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift

@@ -77,7 +77,7 @@ internal enum PluginToHostMessage: Codable {
 }
 
 /*namespace*/ internal enum PluginMessage {
-  static var PROTOCOL_VERSION_NUMBER: Int { 5 }  // Added 'expandMacroResult'.
+  static var PROTOCOL_VERSION_NUMBER: Int { 6 }  // Added 'expandMacroResult'.
 
   struct HostCapability: Codable {
     var protocolVersion: Int

lit_tests/compiler_plugin_basic.swift

@@ -33,12 +33,16 @@ macro MemberDeprecated() = #externalMacro(module: "ExamplePlugin", type: "Member
 @attached(conformance)
 macro Equatable() = #externalMacro(module: "ExamplePlugin", type: "EquatableConformanceMacro")
 
+@attached(extension, conformances: Sendable)
+macro AddSendbale() = #externalMacro(module: "ExamplePlugin", type: "SendableExtensionMacro")
+
 @attached(accessor)
 macro DidSetPrint() = #externalMacro(module: "ExamplePlugin", type: "DidSetPrintMacro")
 
 @Metadata
 @MemberDeprecated
 @Equatable
+@AddSendable
 @PeerWithSuffix
 struct MyStruct {
   @DidSetPrint
@@ -81,6 +85,11 @@ struct MyStruct {
 // CHECK-NEXT: extension MyStruct : Equatable  {}
 // CHECK-NEXT: ------------------------------
 
+// CHECK: @__swiftmacro_7TestApp8MyStruct9EquatablefMe_.swift
+// CHECK-NEXT: ------------------------------
+// CHECK-NEXT: extension MyStruct: Sendable  {}
+// CHECK-NEXT: ------------------------------
+
 // CHECK: @__swiftmacro_7TestApp8MyStructV5value11DidSetPrintfMa_.swift
 // CHECK-NEXT: ------------------------------
 // CHECK-NEXT: {