[Macros] Add a plugin message to clear cached syntax

Author: rintaro

Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift

@@ -195,6 +195,10 @@ public class CompilerPluginMessageHandler<Provider: PluginProvider> {
         )
       }
       return .loadPluginLibraryResult(loaded: diags.isEmpty, diagnostics: diags)
+
+    case .clearCache:
+      syntaxRegistry.clear()
+      return .clearCacheResult
     }
   }
 }

Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift

@@ -12,6 +12,8 @@
 
 // NOTE: Types in this file should be self-contained and should not depend on any non-stdlib types.
 
+private var _MESSAGE_PROTOCOL_VERSION_NUMBER: Int { 8 } // Added 'clearCache'.
+
 @_spi(PluginMessage)
 public enum HostToPluginMessage: Codable {
   /// Send capability of the host, and get capability of the plugin.
@@ -48,6 +50,9 @@ public enum HostToPluginMessage: Codable {
     libraryPath: String,
     moduleName: String
   )
+
+  /// Signal to clear any caches.
+  case clearCache
 }
 
 @_spi(PluginMessage)
@@ -78,11 +83,13 @@ public enum PluginToHostMessage: Codable {
     loaded: Bool,
     diagnostics: [PluginMessage.Diagnostic]
   )
+
+  case clearCacheResult
 }
 
 @_spi(PluginMessage)
 public enum PluginMessage {
-  public static var PROTOCOL_VERSION_NUMBER: Int { 7 }  // Pass extension protocol list
+  public static var PROTOCOL_VERSION_NUMBER: Int { _MESSAGE_PROTOCOL_VERSION_NUMBER }
 
   public struct HostCapability: Codable {
     var protocolVersion: Int