Add ability to include a build tag

Makefile

@@ -64,6 +64,16 @@ gen-testproto: get-grpc-testproto gen-wkt-testproto install
 		testproto/proto2/scalars.proto \
 		testproto/unsafe/unsafe.proto \
 		|| exit 1;
+	$(PROTOBUF_ROOT)/src/protoc \
+		--proto_path=testproto \
+		--proto_path=include \
+		--go_out=. --plugin protoc-gen-go="${GOBIN}/protoc-gen-go" \
+		--go-vtproto_opt=paths=source_relative \
+		--go-vtproto_opt=buildTag=vtprotobuf \
+		--go-vtproto_out=allow-empty=true:./testproto/buildtag --plugin protoc-gen-go-vtproto="${GOBIN}/protoc-gen-go-vtproto" \
+		-I$(PROTOBUF_ROOT)/src \
+		testproto/empty/empty.proto \
+		|| exit 1;
 
 get-grpc-testproto: install
 	$(PROTOBUF_ROOT)/src/protoc \

README.md

@@ -108,10 +108,20 @@ The following features can be generated:
             --go-vtproto_opt=pool=vitess.io/vitess/go/vt/proto/query.Row \
             --go-vtproto_opt=pool=vitess.io/vitess/go/vt/proto/binlogdata.VStreamRowsResponse \
     ```
+6. (Optional) if you want to selectively compile the generate `vtprotobuf` files, the `--vtproto_opt=buildTag=<tag>` can be used.
 
-6. Compile the `.proto` files in your project. You should see `_vtproto.pb.go` files next to the `.pb.go` and `_grpc.pb.go` files that were already being generated.
+    When using this option, the generated code will only be compiled in if a build tag is provided.
 
-7. (Optional) Switch your RPC framework to use the optimized helpers (see following sections)
+    It is recommended, but not required, to use `vtprotobuf` as the build tag if this is desired, especially if your project is imported by others.
+    This will reduce the number of build tags a user will need to configure if they are importing multiple libraries following this pattern.
+
+    When using this option, it is strongly recommended to make your code compile with and without the build tag.
+    This can be done with type assertions before using `vtprotobuf` generated methods.
+    The `grpc.Codec{}` object (discussed below) shows an example.
+
+7. Compile the `.proto` files in your project. You should see `_vtproto.pb.go` files next to the `.pb.go` and `_grpc.pb.go` files that were already being generated.
+
+8. (Optional) Switch your RPC framework to use the optimized helpers (see following sections)
 
 ## `vtprotobuf` package and well-known types
 

cmd/protoc-gen-go-vtproto/main.go

@@ -26,6 +26,7 @@ func main() {
 	f.Var(&cfg.Poolable, "pool", "use memory pooling for this object")
 	f.BoolVar(&cfg.Wrap, "wrap", false, "generate wrapper types")
 	f.StringVar(&features, "features", "all", "list of features to generate (separated by '+')")
+	f.StringVar(&cfg.BuildTag, "buildTag", "", "the go:build tag to set on generated files")
 
 	protogen.Options{ParamFunc: f.Set}.Run(func(plugin *protogen.Plugin) error {
 		gen, err := generator.NewGenerator(plugin, strings.Split(features, "+"), &cfg)

generator/generator.go

@@ -37,9 +37,10 @@ func (o ObjectSet) Set(s string) error {
 }
 
 type Config struct {
-	Poolable       ObjectSet
-	Wrap           bool
-	AllowEmpty     bool
+	Poolable   ObjectSet
+	Wrap       bool
+	AllowEmpty bool
+	BuildTag   string
 }
 
 type Generator struct {
@@ -97,6 +98,11 @@ func (gen *Generator) generateFile(gf *protogen.GeneratedFile, file *protogen.Fi
 		LocalPackages: gen.local,
 	}
 
+	if p.Config.BuildTag != "" {
+		// Support both forms of tags for maximum compatibility
+		p.P("//go:build ", p.Config.BuildTag)
+		p.P("// +build ", p.Config.BuildTag)
+	}
 	p.P("// Code generated by protoc-gen-go-vtproto. DO NOT EDIT.")
 	if bi, ok := debug.ReadBuildInfo(); ok {
 		p.P("// protoc-gen-go-vtproto version: ", bi.Main.Version)