cmd/protoc-gen-go: reference proto.ProtoPackageIsVersionX

Include the compile-time assertion that the generated file is compatible
with the proto package it is being compiled against.

Change-Id: Iefa27ee3a99a6669a93303b6674b0033794ba0be
Reviewed-on: https://go-review.googlesource.com/134995
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

cmd/protoc-gen-go/main.go

@@ -22,6 +22,12 @@ import (
 	"google.golang.org/proto/reflect/protoreflect"
 )
 
+// generatedCodeVersion indicates a version of the generated code.
+// It is incremented whenever an incompatibility between the generated code and
+// proto package is introduced; the generated code references
+// a constant, proto.ProtoPackageIsVersionN (where N is generatedCodeVersion).
+const generatedCodeVersion = 2
+
 const protoPackage = "github.com/golang/protobuf/proto"
 
 func main() {
@@ -74,6 +80,15 @@ func genFile(gen *protogen.Plugin, file *protogen.File) {
 	g.P()
 	g.P("package ", f.GoPackageName)
 	g.P()
+	g.P("// This is a compile-time assertion to ensure that this generated file")
+	g.P("// is compatible with the proto package it is being compiled against.")
+	g.P("// A compilation error at this line likely means your copy of the")
+	g.P("// proto package needs to be updated.")
+	g.P("const _ = ", protogen.GoIdent{
+		GoImportPath: protoPackage,
+		GoName:       fmt.Sprintf("ProtoPackageIsVersion%d", generatedCodeVersion),
+	}, "// please upgrade the proto package")
+	g.P()
 
 	for _, enum := range f.Enums {
 		genEnum(gen, g, f, enum)

cmd/protoc-gen-go/testdata/comments/comments.pb.go

@@ -5,6 +5,14 @@
 
 package proto2
 
+import proto "github.com/golang/protobuf/proto"
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
 // COMMENT: Message1
 type Message1 struct {
 	XXX_NoUnkeyedLiteral struct{} `json:"-"`

cmd/protoc-gen-go/testdata/fieldnames/fieldnames.pb.go

@@ -3,6 +3,14 @@
 
 package fieldnames
 
+import proto "github.com/golang/protobuf/proto"
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
 // Assorted edge cases in field name conflict resolution.
 //
 // Not all (or possibly any) of these behave in an easily-understood fashion.

cmd/protoc-gen-go/testdata/proto2/enum.pb.go

@@ -5,6 +5,12 @@ package proto2
 
 import proto "github.com/golang/protobuf/proto"
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
 // EnumType1 comment.
 type EnumType1 int32
 

cmd/protoc-gen-go/testdata/proto2/fields.pb.go

@@ -5,6 +5,12 @@ package proto2
 
 import proto "github.com/golang/protobuf/proto"
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
 type FieldTestMessage_Enum int32
 
 const (

cmd/protoc-gen-go/testdata/proto2/nested_messages.pb.go

@@ -3,6 +3,14 @@
 
 package proto2
 
+import proto "github.com/golang/protobuf/proto"
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
 type Layer1 struct {
 	L2                   *Layer1_Layer2        `protobuf:"bytes,1,opt,name=l2" json:"l2,omitempty"`
 	L3                   *Layer1_Layer2_Layer3 `protobuf:"bytes,2,opt,name=l3" json:"l3,omitempty"`

cmd/protoc-gen-go/testdata/proto2/proto2.pb.go

@@ -3,6 +3,14 @@
 
 package proto2
 
+import proto "github.com/golang/protobuf/proto"
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
 type Message struct {
 	I32                  *int32   `protobuf:"varint,1,opt,name=i32" json:"i32,omitempty"`
 	M                    *Message `protobuf:"bytes,2,opt,name=m" json:"m,omitempty"`

cmd/protoc-gen-go/testdata/proto3/enum.pb.go

@@ -5,6 +5,12 @@ package proto3
 
 import proto "github.com/golang/protobuf/proto"
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
 type Enum int32
 
 const (

cmd/protoc-gen-go/testdata/proto3/fields.pb.go

@@ -5,6 +5,12 @@ package proto3
 
 import proto "github.com/golang/protobuf/proto"
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
 type FieldTestMessage_Enum int32
 
 const (