Fix copy-on-write behavior for merged messages. (apple#327)

* Fix copy-on-write behavior for merged messages.
* Update docs for decodeField to warn about CoW semantics.

Author: allevato

Reference/google/protobuf/any_test.pb.swift

@@ -107,6 +107,7 @@ struct ProtobufUnittest_TestAny: SwiftProtobuf.Proto3Message, SwiftProtobuf._Mes
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/api.pb.swift

@@ -167,6 +167,7 @@ struct Google_Protobuf_Api: SwiftProtobuf.Proto3Message, SwiftProtobuf._MessageI
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/compiler/plugin.pb.swift

@@ -270,6 +270,7 @@ struct Google_Protobuf_Compiler_CodeGeneratorRequest: SwiftProtobuf.Proto2Messag
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/descriptor.pb.swift

@@ -285,6 +285,7 @@ struct Google_Protobuf_FileDescriptorProto: SwiftProtobuf.Proto2Message, SwiftPr
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -652,6 +653,7 @@ struct Google_Protobuf_DescriptorProto: SwiftProtobuf.Proto2Message, SwiftProtob
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -1083,6 +1085,7 @@ struct Google_Protobuf_FieldDescriptorProto: SwiftProtobuf.Proto2Message, SwiftP
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -1228,6 +1231,7 @@ struct Google_Protobuf_OneofDescriptorProto: SwiftProtobuf.Proto2Message, SwiftP
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -1342,6 +1346,7 @@ struct Google_Protobuf_EnumDescriptorProto: SwiftProtobuf.Proto2Message, SwiftPr
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -1466,6 +1471,7 @@ struct Google_Protobuf_EnumValueDescriptorProto: SwiftProtobuf.Proto2Message, Sw
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -1585,6 +1591,7 @@ struct Google_Protobuf_ServiceDescriptorProto: SwiftProtobuf.Proto2Message, Swif
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -1755,6 +1762,7 @@ struct Google_Protobuf_MethodDescriptorProto: SwiftProtobuf.Proto2Message, Swift
   }
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/map_lite_unittest.pb.swift

@@ -332,6 +332,7 @@ struct ProtobufUnittest_TestMapLite: SwiftProtobuf.Proto2Message, SwiftProtobuf.
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -625,6 +626,7 @@ struct ProtobufUnittest_TestArenaMapLite: SwiftProtobuf.Proto2Message, SwiftProt
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/map_unittest.pb.swift

@@ -259,6 +259,7 @@ struct ProtobufUnittest_TestMap: SwiftProtobuf.Proto3Message, SwiftProtobuf._Mes
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -418,6 +419,7 @@ struct ProtobufUnittest_TestMapSubmessage: SwiftProtobuf.Proto3Message, SwiftPro
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -745,6 +747,7 @@ struct ProtobufUnittest_TestArenaMap: SwiftProtobuf.Proto3Message, SwiftProtobuf
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/map_unittest_proto3.pb.swift

@@ -258,6 +258,7 @@ struct Proto3TestMap: SwiftProtobuf.Proto3Message, SwiftProtobuf._MessageImpleme
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -412,6 +413,7 @@ struct Proto3TestMapSubmessage: SwiftProtobuf.Proto3Message, SwiftProtobuf._Mess
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/struct.pb.swift

@@ -347,6 +347,7 @@ struct Google_Protobuf_Value: SwiftProtobuf.Proto3Message, SwiftProtobuf._Messag
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/test_messages_proto3.pb.swift

@@ -1460,6 +1460,7 @@ struct ProtobufTestMessages_Proto3_TestAllTypes: SwiftProtobuf.Proto3Message, Sw
     init() {}
 
     mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+      _ = _uniqueStorage()
       try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
         while let fieldNumber = try decoder.nextFieldNumber() {
           try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -1500,6 +1501,7 @@ struct ProtobufTestMessages_Proto3_TestAllTypes: SwiftProtobuf.Proto3Message, Sw
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)

Reference/google/protobuf/type.pb.swift

@@ -175,6 +175,7 @@ struct Google_Protobuf_Type: SwiftProtobuf.Proto3Message, SwiftProtobuf._Message
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -623,6 +624,7 @@ struct Google_Protobuf_Enum: SwiftProtobuf.Proto3Message, SwiftProtobuf._Message
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)
@@ -791,6 +793,7 @@ struct Google_Protobuf_Option: SwiftProtobuf.Proto3Message, SwiftProtobuf._Messa
   init() {}
 
   mutating func _protobuf_generated_decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
+    _ = _uniqueStorage()
     try withExtendedLifetime(_storage) { (_storage: _StorageClass) in
       while let fieldNumber = try decoder.nextFieldNumber() {
         try decodeField(decoder: &decoder, fieldNumber: fieldNumber)