Rename protocol to proto applying

Author: Pouya Yarandi

Sources/SwiftProtobuf/ProtobufMirror.swift

@@ -138,6 +138,6 @@ public enum MessageModificationError: Error {
   case invalidFieldNumber
 }
 
-public protocol MessageModifier: Message {
+public protocol _ProtoApplying: Message {
   func applying(_ value: Any, for fieldNumber: Int) throws -> Self
 }

Sources/protoc-gen-swift/MessageGenerator.swift

@@ -196,7 +196,7 @@ class MessageGenerator {
   func generateRuntimeSupport(printer p: inout CodePrinter, file: FileGenerator, parent: MessageGenerator?) {
     p.print(
         "",
-        "extension \(swiftFullName): \(namer.swiftProtobufModulePrefix)Message, \(namer.swiftProtobufModulePrefix)_MessageImplementationBase, \(namer.swiftProtobufModulePrefix)_ProtoNameProviding, \(namer.swiftProtobufModulePrefix)MessageModifier {")
+        "extension \(swiftFullName): \(namer.swiftProtobufModulePrefix)Message, \(namer.swiftProtobufModulePrefix)_MessageImplementationBase, \(namer.swiftProtobufModulePrefix)_ProtoNameProviding, \(namer.swiftProtobufModulePrefix)_ProtoApplying {")
     p.withIndentation { p in
       if let parent = parent {
         p.print("\(visibility)static let protoMessageName: String = \(parent.swiftFullName).protoMessageName + \".\(descriptor.name)\"")
@@ -406,6 +406,13 @@ class MessageGenerator {
 
   private func generateMessageApplying(printer p: inout CodePrinter) {
     p.print("\(visibility)func applying(_ value: Any, for fieldNumber: Int) throws -> \(swiftFullName) {")
+    if fields.isEmpty {
+      p.withIndentation { p in
+        p.print("throw \(namer.swiftProtobufModulePrefix)MessageModificationError.invalidFieldNumber")
+      }
+      p.print("}")
+      return
+    }
     p.withIndentation { p in
       p.print("""
       func valueAs<T>(_ type: T.Type) throws -> T {