diff --git a/lib/ASTGen/Sources/ASTGen/Macros.swift b/lib/ASTGen/Sources/ASTGen/Macros.swift
index 52751fa704d16..7ac35ce30cab1 100644
--- a/lib/ASTGen/Sources/ASTGen/Macros.swift
+++ b/lib/ASTGen/Sources/ASTGen/Macros.swift
@@ -584,7 +584,7 @@ func expandAttachedMacroIPC(
     macro: .init(moduleName: macro.moduleName, typeName: macro.typeName, name: macroName),
     macroRole: macroRole,
     discriminator: discriminator,
-    customAttributeSyntax: customAttributeSyntax,
+    attributeSyntax: customAttributeSyntax,
     declSyntax: declSyntax,
     parentDeclSyntax: parentDeclSyntax)
   do {
diff --git a/lib/ASTGen/Sources/ASTGen/PluginMessages.swift b/lib/ASTGen/Sources/ASTGen/PluginMessages.swift
index 971c52ff7f185..de5e44f0cfa74 100644
--- a/lib/ASTGen/Sources/ASTGen/PluginMessages.swift
+++ b/lib/ASTGen/Sources/ASTGen/PluginMessages.swift
@@ -10,37 +10,44 @@
 //
 //===----------------------------------------------------------------------===//
 
+// NOTE: This file should be synced between swift and swift-syntax repository.
+// NOTE: Types in this file should be self-contained and should not depend on any non-stdlib types.
+
 internal enum HostToPluginMessage: Codable {
   case getCapability
 
   case expandFreestandingMacro(
     macro: PluginMessage.MacroReference,
     discriminator: String,
-    syntax: PluginMessage.Syntax)
+    syntax: PluginMessage.Syntax
+  )
 
   case expandAttachedMacro(
     macro: PluginMessage.MacroReference,
     macroRole: PluginMessage.MacroRole,
     discriminator: String,
-    customAttributeSyntax: PluginMessage.Syntax,
+    attributeSyntax: PluginMessage.Syntax,
     declSyntax: PluginMessage.Syntax,
-    parentDeclSyntax: PluginMessage.Syntax?)
+    parentDeclSyntax: PluginMessage.Syntax?
+  )
 }
 
 internal enum PluginToHostMessage: Codable {
   case expandFreestandingMacroResult(
     expandedSource: String?,
-    diagnostics: [PluginMessage.Diagnostic])
+    diagnostics: [PluginMessage.Diagnostic]
+  )
 
   case expandAttachedMacroResult(
     expandedSources: [String]?,
-    diagnostics: [PluginMessage.Diagnostic])
+    diagnostics: [PluginMessage.Diagnostic]
+  )
 
   case getCapabilityResult(capability: PluginMessage.PluginCapability)
 }
 
 /*namespace*/ internal enum PluginMessage {
-  static var PROTOCOL_VERSION_NUMBER: Int { 2 } // Added 'MacroRole.conformance'
+  static var PROTOCOL_VERSION_NUMBER: Int { 3 }  // Renamed 'customAttributeSyntax' to 'attributeSyntax'.
 
   struct PluginCapability: Codable {
     var protocolVersion: Int
@@ -69,9 +76,17 @@ internal enum PluginToHostMessage: Codable {
   }
 
   struct SourceLocation: Codable {
+    /// A file ID consisting of the module name and file name (without full path),
+    /// as would be generated by the macro expansion `#fileID`.
     var fileID: String
+
+    /// A full path name as would be generated by the macro expansion `#filePath`,
+    /// e.g., `/home/taylor/alison.swift`.
     var fileName: String
+
+    /// UTF-8 offset of the location in the file.
     var offset: Int
+
     var line: Int
     var column: Int
   }
@@ -84,12 +99,23 @@ internal enum PluginToHostMessage: Codable {
     }
     struct Position: Codable {
       var fileName: String
+      /// UTF-8 offset in the file.
       var offset: Int
+
+      static var invalid: Self {
+        .init(fileName: "", offset: 0)
+      }
     }
     struct PositionRange: Codable {
       var fileName: String
+      /// UTF-8 offset of the start of the range in the file.
       var startOffset: Int
+      /// UTF-8 offset of the end of the range in the file.
       var endOffset: Int
+
+      static var invalid: Self {
+        .init(fileName: "", startOffset: 0, endOffset: 0)
+      }
     }
     struct Note: Codable {
       var position: Position