Add compatibility markers to proto generated code.

This asserts at build time that the generated code is compatible with
the proto package that it is linked to.

jsonpb/jsonpb_test_proto/more_test_objects.pb.go

diff --git a/jsonpb/jsonpb_test_proto/more_test_objects.pb.go b/jsonpb/jsonpb_test_proto/more_test_objects.pb.go
index 8e3e97c..5548ae4 100644
--- a/jsonpb/jsonpb_test_proto/more_test_objects.pb.go
+++ b/jsonpb/jsonpb_test_proto/more_test_objects.pb.go
@@ -31,6 +31,10 @@
 var _ = fmt.Errorf
 var _ = math.Inf
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+const _ = proto.ProtoPackageIsVersion1
+
 type Simple3 struct {
 	Dub float64 `protobuf:"fixed64,1,opt,name=dub" json:"dub,omitempty"`
 }

proto/lib.go

diff --git a/proto/lib.go b/proto/lib.go
index 6837512..42a58c6 100644
--- a/proto/lib.go
+++ b/proto/lib.go
@@ -887,3 +887,7 @@
 	}
 	return false
 }
+
+// ProtoPackageIsVersion1 is referenced from generated protocol buffer files
+// to assert that that code is compatible with this version of the proto package.
+const ProtoPackageIsVersion1 = true

proto/testdata/test.pb.go

diff --git a/proto/testdata/test.pb.go b/proto/testdata/test.pb.go
index d8ab627..e3c83fc 100644
--- a/proto/testdata/test.pb.go
+++ b/proto/testdata/test.pb.go
@@ -50,6 +50,10 @@
 var _ = fmt.Errorf
 var _ = math.Inf
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+const _ = proto.ProtoPackageIsVersion1
+
 type FOO int32
 
 const (

protoc-gen-go/descriptor/descriptor.pb.go

diff --git a/protoc-gen-go/descriptor/descriptor.pb.go b/protoc-gen-go/descriptor/descriptor.pb.go
index a537d5e..6a3be28 100644
--- a/protoc-gen-go/descriptor/descriptor.pb.go
+++ b/protoc-gen-go/descriptor/descriptor.pb.go
@@ -39,6 +39,10 @@
 var _ = fmt.Errorf
 var _ = math.Inf
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+const _ = proto.ProtoPackageIsVersion1
+
 type FieldDescriptorProto_Type int32
 
 const (

protoc-gen-go/generator/generator.go

diff --git a/protoc-gen-go/generator/generator.go b/protoc-gen-go/generator/generator.go
index 776a116..c2c96bf 100644
--- a/protoc-gen-go/generator/generator.go
+++ b/protoc-gen-go/generator/generator.go
@@ -58,6 +58,12 @@
 	plugin "github.com/golang/protobuf/protoc-gen-go/plugin"
 )
 
+// 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 = 1
+
 // A Plugin provides functionality to add to the output during Go code generation,
 // such as to produce RPC stubs.
 type Plugin interface {
@@ -1122,6 +1128,14 @@
 	g.file = g.FileOf(file.FileDescriptorProto)
 	g.usedPackages = make(map[string]bool)
 
+	if g.file.index == 0 {
+		// For one file in the package, assert version compatibility.
+		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("const _ = ", g.Pkg["proto"], ".ProtoPackageIsVersion", generatedCodeVersion)
+		g.P()
+	}
+
 	for _, td := range g.file.imp {
 		g.generateImported(td)
 	}

protoc-gen-go/plugin/plugin.pb.go

diff --git a/protoc-gen-go/plugin/plugin.pb.go b/protoc-gen-go/plugin/plugin.pb.go
index c1f49fd..af31f73 100644
--- a/protoc-gen-go/plugin/plugin.pb.go
+++ b/protoc-gen-go/plugin/plugin.pb.go
@@ -24,6 +24,10 @@
 var _ = fmt.Errorf
 var _ = math.Inf
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+const _ = proto.ProtoPackageIsVersion1
+
 // An encoded CodeGeneratorRequest is written to the plugin's stdin.
 type CodeGeneratorRequest struct {
 	// The .proto files that were explicitly listed on the command-line.  The

protoc-gen-go/testdata/my_test/test.pb.go

diff --git a/protoc-gen-go/testdata/my_test/test.pb.go b/protoc-gen-go/testdata/my_test/test.pb.go
index d63f58f..b0375f0 100644
--- a/protoc-gen-go/testdata/my_test/test.pb.go
+++ b/protoc-gen-go/testdata/my_test/test.pb.go
@@ -31,6 +31,10 @@
 var _ = fmt.Errorf
 var _ = math.Inf
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+const _ = proto.ProtoPackageIsVersion1
+
 type HatType int32
 
 const (

protoc-gen-go/testdata/my_test/test.pb.go.golden

diff --git a/protoc-gen-go/testdata/my_test/test.pb.go.golden b/protoc-gen-go/testdata/my_test/test.pb.go.golden
index d63f58f..b0375f0 100644
--- a/protoc-gen-go/testdata/my_test/test.pb.go.golden
+++ b/protoc-gen-go/testdata/my_test/test.pb.go.golden
@@ -31,6 +31,10 @@
 var _ = fmt.Errorf
 var _ = math.Inf
 
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+const _ = proto.ProtoPackageIsVersion1
+
 type HatType int32
 
 const (