cmd/protoc-gen-go: factor out internals into a new package

To permit the api-v1 branch to import the protoc-gen-go internals, move
them into a quasi-internal package.

Change-Id: I64e50ee299b99da1f648f7abb6cd0347a13e06e3
Reviewed-on: https://go-review.googlesource.com/137035
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/cmd/protoc-gen-go/internal_gengo/main.go b/cmd/protoc-gen-go/internal_gengo/main.go
new file mode 100644
index 0000000..518f427
--- /dev/null
+++ b/cmd/protoc-gen-go/internal_gengo/main.go
@@ -0,0 +1,915 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package internal_gengo is internal to the protobuf module.
+package internal_gengo
+
+import (
+	"bytes"
+	"compress/gzip"
+	"crypto/sha256"
+	"encoding/hex"
+	"flag"
+	"fmt"
+	"math"
+	"sort"
+	"strconv"
+	"strings"
+
+	"github.com/golang/protobuf/proto"
+	descpb "github.com/golang/protobuf/protoc-gen-go/descriptor"
+	"github.com/golang/protobuf/v2/protogen"
+	"github.com/golang/protobuf/v2/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() {
+	var flags flag.FlagSet
+	// TODO: Decide what to do for backwards compatibility with plugins=grpc.
+	flags.String("plugins", "", "")
+	opts := &protogen.Options{
+		ParamFunc: flags.Set,
+	}
+	protogen.Run(opts, func(gen *protogen.Plugin) error {
+		for _, f := range gen.Files {
+			if !f.Generate {
+				continue
+			}
+			genFile(gen, f)
+		}
+		return nil
+	})
+}
+
+type fileInfo struct {
+	*protogen.File
+	locationMap   map[string][]*descpb.SourceCodeInfo_Location
+	descriptorVar string // var containing the gzipped FileDescriptorProto
+	allEnums      []*protogen.Enum
+	allMessages   []*protogen.Message
+	allExtensions []*protogen.Extension
+}
+
+func genFile(gen *protogen.Plugin, file *protogen.File) {
+	f := &fileInfo{
+		File:        file,
+		locationMap: make(map[string][]*descpb.SourceCodeInfo_Location),
+	}
+	for _, loc := range file.Proto.GetSourceCodeInfo().GetLocation() {
+		key := pathKey(loc.Path)
+		f.locationMap[key] = append(f.locationMap[key], loc)
+	}
+
+	// The different order for enums and extensions is to match the output
+	// of the previous implementation.
+	//
+	// TODO: Eventually make this consistent.
+	f.allEnums = append(f.allEnums, f.File.Enums...)
+	walkMessages(f.Messages, func(message *protogen.Message) {
+		f.allMessages = append(f.allMessages, message)
+		f.allEnums = append(f.allEnums, message.Enums...)
+		f.allExtensions = append(f.allExtensions, message.Extensions...)
+	})
+	f.allExtensions = append(f.allExtensions, f.File.Extensions...)
+
+	// Determine the name of the var holding the file descriptor:
+	//
+	//     fileDescriptor_<hash of filename>
+	filenameHash := sha256.Sum256([]byte(f.Desc.Path()))
+	f.descriptorVar = fmt.Sprintf("fileDescriptor_%s", hex.EncodeToString(filenameHash[:8]))
+
+	g := gen.NewGeneratedFile(f.GeneratedFilenamePrefix+".pb.go", f.GoImportPath)
+	g.P("// Code generated by protoc-gen-go. DO NOT EDIT.")
+	if f.Proto.GetOptions().GetDeprecated() {
+		g.P("// ", f.Desc.Path(), " is a deprecated file.")
+	} else {
+		g.P("// source: ", f.Desc.Path())
+	}
+	g.P()
+	const filePackageField = 2 // FileDescriptorProto.package
+	genComment(g, f, []int32{filePackageField})
+	g.P()
+	g.P("package ", f.GoPackageName)
+	g.P()
+
+	// These references are not necessary, since we automatically add
+	// all necessary imports before formatting the generated file.
+	//
+	// This section exists to generate output more consistent with
+	// the previous version of protoc-gen-go, to make it easier to
+	// detect unintended variations.
+	//
+	// TODO: Eventually remove this.
+	g.P("// Reference imports to suppress errors if they are not otherwise used.")
+	g.P("var _ = ", protogen.GoIdent{GoImportPath: protoPackage, GoName: "Marshal"})
+	g.P("var _ = ", protogen.GoIdent{GoImportPath: "fmt", GoName: "Errorf"})
+	g.P("var _ = ", protogen.GoIdent{GoImportPath: "math", GoName: "Inf"})
+	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 i, imps := 0, f.Desc.Imports(); i < imps.Len(); i++ {
+		genImport(gen, g, f, imps.Get(i))
+	}
+	for _, enum := range f.allEnums {
+		genEnum(gen, g, f, enum)
+	}
+	for _, message := range f.allMessages {
+		genMessage(gen, g, f, message)
+	}
+	for _, extension := range f.Extensions {
+		genExtension(gen, g, f, extension)
+	}
+
+	genInitFunction(gen, g, f)
+
+	genFileDescriptor(gen, g, f)
+}
+
+// walkMessages calls f on each message and all of its descendants.
+func walkMessages(messages []*protogen.Message, f func(*protogen.Message)) {
+	for _, m := range messages {
+		f(m)
+		walkMessages(m.Messages, f)
+	}
+}
+
+func genImport(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, imp protoreflect.FileImport) {
+	impFile, ok := gen.FileByName(imp.Path())
+	if !ok {
+		return
+	}
+	if impFile.GoImportPath == f.GoImportPath {
+		// Don't generate imports or aliases for types in the same Go package.
+		return
+	}
+	// Generate imports for all dependencies, even if they are not
+	// referenced, because other code and tools depend on having the
+	// full transitive closure of protocol buffer types in the binary.
+	g.Import(impFile.GoImportPath)
+	if !imp.IsPublic {
+		return
+	}
+	var enums []*protogen.Enum
+	enums = append(enums, impFile.Enums...)
+	walkMessages(impFile.Messages, func(message *protogen.Message) {
+		enums = append(enums, message.Enums...)
+		g.P("// ", message.GoIdent.GoName, " from public import ", imp.Path())
+		g.P("type ", message.GoIdent.GoName, " = ", message.GoIdent)
+		for _, oneof := range message.Oneofs {
+			for _, field := range oneof.Fields {
+				typ := fieldOneofType(field)
+				g.P("type ", typ.GoName, " = ", typ)
+			}
+		}
+		g.P()
+	})
+	for _, enum := range enums {
+		g.P("// ", enum.GoIdent.GoName, " from public import ", imp.Path())
+		g.P("type ", enum.GoIdent.GoName, " = ", enum.GoIdent)
+		g.P("var ", enum.GoIdent.GoName, "_name = ", enum.GoIdent, "_name")
+		g.P("var ", enum.GoIdent.GoName, "_value = ", enum.GoIdent, "_value")
+		g.P()
+		for _, value := range enum.Values {
+			g.P("const ", value.GoIdent.GoName, " = ", enum.GoIdent.GoName, "(", value.GoIdent, ")")
+		}
+	}
+}
+
+func genFileDescriptor(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo) {
+	// Trim the source_code_info from the descriptor.
+	// Marshal and gzip it.
+	descProto := proto.Clone(f.Proto).(*descpb.FileDescriptorProto)
+	descProto.SourceCodeInfo = nil
+	b, err := proto.Marshal(descProto)
+	if err != nil {
+		gen.Error(err)
+		return
+	}
+	var buf bytes.Buffer
+	w, _ := gzip.NewWriterLevel(&buf, gzip.BestCompression)
+	w.Write(b)
+	w.Close()
+	b = buf.Bytes()
+
+	g.P("func init() { proto.RegisterFile(", strconv.Quote(f.Desc.Path()), ", ", f.descriptorVar, ") }")
+	g.P()
+	g.P("var ", f.descriptorVar, " = []byte{")
+	g.P("// ", len(b), " bytes of a gzipped FileDescriptorProto")
+	for len(b) > 0 {
+		n := 16
+		if n > len(b) {
+			n = len(b)
+		}
+
+		s := ""
+		for _, c := range b[:n] {
+			s += fmt.Sprintf("0x%02x,", c)
+		}
+		g.P(s)
+
+		b = b[n:]
+	}
+	g.P("}")
+	g.P()
+}
+
+func genEnum(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, enum *protogen.Enum) {
+	genComment(g, f, enum.Path)
+	g.P("type ", enum.GoIdent, " int32",
+		deprecationComment(enumOptions(gen, enum).GetDeprecated()))
+	g.P("const (")
+	for _, value := range enum.Values {
+		genComment(g, f, value.Path)
+		g.P(value.GoIdent, " ", enum.GoIdent, " = ", value.Desc.Number(),
+			deprecationComment(enumValueOptions(gen, value).GetDeprecated()))
+	}
+	g.P(")")
+	g.P()
+	nameMap := enum.GoIdent.GoName + "_name"
+	g.P("var ", nameMap, " = map[int32]string{")
+	generated := make(map[protoreflect.EnumNumber]bool)
+	for _, value := range enum.Values {
+		duplicate := ""
+		if _, present := generated[value.Desc.Number()]; present {
+			duplicate = "// Duplicate value: "
+		}
+		g.P(duplicate, value.Desc.Number(), ": ", strconv.Quote(string(value.Desc.Name())), ",")
+		generated[value.Desc.Number()] = true
+	}
+	g.P("}")
+	g.P()
+	valueMap := enum.GoIdent.GoName + "_value"
+	g.P("var ", valueMap, " = map[string]int32{")
+	for _, value := range enum.Values {
+		g.P(strconv.Quote(string(value.Desc.Name())), ": ", value.Desc.Number(), ",")
+	}
+	g.P("}")
+	g.P()
+	if enum.Desc.Syntax() != protoreflect.Proto3 {
+		g.P("func (x ", enum.GoIdent, ") Enum() *", enum.GoIdent, " {")
+		g.P("p := new(", enum.GoIdent, ")")
+		g.P("*p = x")
+		g.P("return p")
+		g.P("}")
+		g.P()
+	}
+	g.P("func (x ", enum.GoIdent, ") String() string {")
+	g.P("return ", protogen.GoIdent{GoImportPath: protoPackage, GoName: "EnumName"}, "(", enum.GoIdent, "_name, int32(x))")
+	g.P("}")
+	g.P()
+
+	if enum.Desc.Syntax() != protoreflect.Proto3 {
+		g.P("func (x *", enum.GoIdent, ") UnmarshalJSON(data []byte) error {")
+		g.P("value, err := ", protogen.GoIdent{GoImportPath: protoPackage, GoName: "UnmarshalJSONEnum"}, "(", enum.GoIdent, `_value, data, "`, enum.GoIdent, `")`)
+		g.P("if err != nil {")
+		g.P("return err")
+		g.P("}")
+		g.P("*x = ", enum.GoIdent, "(value)")
+		g.P("return nil")
+		g.P("}")
+		g.P()
+	}
+
+	var indexes []string
+	for i := 1; i < len(enum.Path); i += 2 {
+		indexes = append(indexes, strconv.Itoa(int(enum.Path[i])))
+	}
+	g.P("func (", enum.GoIdent, ") EnumDescriptor() ([]byte, []int) {")
+	g.P("return ", f.descriptorVar, ", []int{", strings.Join(indexes, ","), "}")
+	g.P("}")
+	g.P()
+
+	genWellKnownType(g, enum.GoIdent, enum.Desc)
+}
+
+// enumRegistryName returns the name used to register an enum with the proto
+// package registry.
+//
+// Confusingly, this is <proto_package>.<go_ident>. This probably should have
+// been the full name of the proto enum type instead, but changing it at this
+// point would require thought.
+func enumRegistryName(enum *protogen.Enum) string {
+	// Find the FileDescriptor for this enum.
+	var desc protoreflect.Descriptor = enum.Desc
+	for {
+		p, ok := desc.Parent()
+		if !ok {
+			break
+		}
+		desc = p
+	}
+	fdesc := desc.(protoreflect.FileDescriptor)
+	return string(fdesc.Package()) + "." + enum.GoIdent.GoName
+}
+
+func genMessage(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, message *protogen.Message) {
+	if message.Desc.IsMapEntry() {
+		return
+	}
+
+	hasComment := genComment(g, f, message.Path)
+	if messageOptions(gen, message).GetDeprecated() {
+		if hasComment {
+			g.P("//")
+		}
+		g.P(deprecationComment(true))
+	}
+	g.P("type ", message.GoIdent, " struct {")
+	for _, field := range message.Fields {
+		if field.OneofType != nil {
+			// It would be a bit simpler to iterate over the oneofs below,
+			// but generating the field here keeps the contents of the Go
+			// struct in the same order as the contents of the source
+			// .proto file.
+			if field == field.OneofType.Fields[0] {
+				genOneofField(gen, g, f, message, field.OneofType)
+			}
+			continue
+		}
+		genComment(g, f, field.Path)
+		goType, pointer := fieldGoType(g, field)
+		if pointer {
+			goType = "*" + goType
+		}
+		tags := []string{
+			fmt.Sprintf("protobuf:%q", fieldProtobufTag(field)),
+			fmt.Sprintf("json:%q", fieldJSONTag(field)),
+		}
+		if field.Desc.IsMap() {
+			key := field.MessageType.Fields[0]
+			val := field.MessageType.Fields[1]
+			tags = append(tags,
+				fmt.Sprintf("protobuf_key:%q", fieldProtobufTag(key)),
+				fmt.Sprintf("protobuf_val:%q", fieldProtobufTag(val)),
+			)
+		}
+		g.P(field.GoName, " ", goType, " `", strings.Join(tags, " "), "`",
+			deprecationComment(fieldOptions(gen, field).GetDeprecated()))
+	}
+	g.P("XXX_NoUnkeyedLiteral struct{} `json:\"-\"`")
+
+	if message.Desc.ExtensionRanges().Len() > 0 {
+		var tags []string
+		if messageOptions(gen, message).GetMessageSetWireFormat() {
+			tags = append(tags, `protobuf_messageset:"1"`)
+		}
+		tags = append(tags, `json:"-"`)
+		g.P(protogen.GoIdent{
+			GoImportPath: protoPackage,
+			GoName:       "XXX_InternalExtensions",
+		}, " `", strings.Join(tags, " "), "`")
+	}
+	// TODO XXX_InternalExtensions
+	g.P("XXX_unrecognized []byte `json:\"-\"`")
+	g.P("XXX_sizecache int32 `json:\"-\"`")
+	g.P("}")
+	g.P()
+
+	// Reset
+	g.P("func (m *", message.GoIdent, ") Reset() { *m = ", message.GoIdent, "{} }")
+	// String
+	g.P("func (m *", message.GoIdent, ") String() string { return ", protogen.GoIdent{
+		GoImportPath: protoPackage,
+		GoName:       "CompactTextString",
+	}, "(m) }")
+	// ProtoMessage
+	g.P("func (*", message.GoIdent, ") ProtoMessage() {}")
+	// Descriptor
+	var indexes []string
+	for i := 1; i < len(message.Path); i += 2 {
+		indexes = append(indexes, strconv.Itoa(int(message.Path[i])))
+	}
+	g.P("func (*", message.GoIdent, ") Descriptor() ([]byte, []int) {")
+	g.P("return ", f.descriptorVar, ", []int{", strings.Join(indexes, ","), "}")
+	g.P("}")
+	g.P()
+
+	// ExtensionRangeArray
+	if extranges := message.Desc.ExtensionRanges(); extranges.Len() > 0 {
+		if messageOptions(gen, message).GetMessageSetWireFormat() {
+			g.P("func (m *", message.GoIdent, ") MarshalJSON() ([]byte, error) {")
+			g.P("return ", protogen.GoIdent{
+				GoImportPath: protoPackage,
+				GoName:       "MarshalMessageSetJSON",
+			}, "(&m.XXX_InternalExtensions)")
+			g.P("}")
+			g.P("func (m *", message.GoIdent, ") UnmarshalJSON(buf []byte) error {")
+			g.P("return ", protogen.GoIdent{
+				GoImportPath: protoPackage,
+				GoName:       "UnmarshalMessageSetJSON",
+			}, "(buf, &m.XXX_InternalExtensions)")
+			g.P("}")
+			g.P()
+		}
+
+		protoExtRange := protogen.GoIdent{
+			GoImportPath: protoPackage,
+			GoName:       "ExtensionRange",
+		}
+		extRangeVar := "extRange_" + message.GoIdent.GoName
+		g.P("var ", extRangeVar, " = []", protoExtRange, " {")
+		for i := 0; i < extranges.Len(); i++ {
+			r := extranges.Get(i)
+			g.P("{Start:", r[0], ", End:", r[1]-1 /* inclusive */, "},")
+		}
+		g.P("}")
+		g.P()
+		g.P("func (*", message.GoIdent, ") ExtensionRangeArray() []", protoExtRange, " {")
+		g.P("return ", extRangeVar)
+		g.P("}")
+		g.P()
+	}
+
+	// Table-driven proto support.
+	//
+	// TODO: It does not scale to keep adding another method for every
+	// operation on protos that we want to switch over to using the
+	// table-driven approach. Instead, we should only add a single method
+	// that allows getting access to the *InternalMessageInfo struct and then
+	// calling Unmarshal, Marshal, Merge, Size, and Discard directly on that.
+	messageInfoVar := "xxx_messageInfo_" + message.GoIdent.GoName
+	// XXX_Unmarshal
+	g.P("func (m *", message.GoIdent, ") XXX_Unmarshal(b []byte) error {")
+	g.P("return ", messageInfoVar, ".Unmarshal(m, b)")
+	g.P("}")
+	// XXX_Marshal
+	g.P("func (m *", message.GoIdent, ") XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {")
+	g.P("return ", messageInfoVar, ".Marshal(b, m, deterministic)")
+	g.P("}")
+	// XXX_Merge
+	g.P("func (m *", message.GoIdent, ") XXX_Merge(src proto.Message) {")
+	g.P(messageInfoVar, ".Merge(m, src)")
+	g.P("}")
+	// XXX_Size
+	g.P("func (m *", message.GoIdent, ") XXX_Size() int {")
+	g.P("return ", messageInfoVar, ".Size(m)")
+	g.P("}")
+	// XXX_DiscardUnknown
+	g.P("func (m *", message.GoIdent, ") XXX_DiscardUnknown() {")
+	g.P(messageInfoVar, ".DiscardUnknown(m)")
+	g.P("}")
+	g.P()
+	g.P("var ", messageInfoVar, " ", protogen.GoIdent{
+		GoImportPath: protoPackage,
+		GoName:       "InternalMessageInfo",
+	})
+	g.P()
+
+	// Constants and vars holding the default values of fields.
+	for _, field := range message.Fields {
+		if !field.Desc.HasDefault() {
+			continue
+		}
+		defVarName := "Default_" + message.GoIdent.GoName + "_" + field.GoName
+		def := field.Desc.Default()
+		switch field.Desc.Kind() {
+		case protoreflect.StringKind:
+			g.P("const ", defVarName, " string = ", strconv.Quote(def.String()))
+		case protoreflect.BytesKind:
+			g.P("var ", defVarName, " []byte = []byte(", strconv.Quote(string(def.Bytes())), ")")
+		case protoreflect.EnumKind:
+			enum := field.EnumType
+			evalue := enum.Values[enum.Desc.Values().ByNumber(def.Enum()).Index()]
+			g.P("const ", defVarName, " ", field.EnumType.GoIdent, " = ", evalue.GoIdent)
+		case protoreflect.FloatKind, protoreflect.DoubleKind:
+			// Floating point numbers need extra handling for -Inf/Inf/NaN.
+			f := field.Desc.Default().Float()
+			goType := "float64"
+			if field.Desc.Kind() == protoreflect.FloatKind {
+				goType = "float32"
+			}
+			// funcCall returns a call to a function in the math package,
+			// possibly converting the result to float32.
+			funcCall := func(fn, param string) string {
+				s := g.QualifiedGoIdent(protogen.GoIdent{
+					GoImportPath: "math",
+					GoName:       fn,
+				}) + param
+				if goType != "float64" {
+					s = goType + "(" + s + ")"
+				}
+				return s
+			}
+			switch {
+			case math.IsInf(f, -1):
+				g.P("var ", defVarName, " ", goType, " = ", funcCall("Inf", "(-1)"))
+			case math.IsInf(f, 1):
+				g.P("var ", defVarName, " ", goType, " = ", funcCall("Inf", "(1)"))
+			case math.IsNaN(f):
+				g.P("var ", defVarName, " ", goType, " = ", funcCall("NaN", "()"))
+			default:
+				g.P("const ", defVarName, " ", goType, " = ", f)
+			}
+		default:
+			goType, _ := fieldGoType(g, field)
+			g.P("const ", defVarName, " ", goType, " = ", def.Interface())
+		}
+	}
+	g.P()
+
+	// Getters.
+	for _, field := range message.Fields {
+		if field.OneofType != nil {
+			if field == field.OneofType.Fields[0] {
+				genOneofTypes(gen, g, f, message, field.OneofType)
+			}
+		}
+		goType, pointer := fieldGoType(g, field)
+		defaultValue := fieldDefaultValue(g, message, field)
+		if fieldOptions(gen, field).GetDeprecated() {
+			g.P(deprecationComment(true))
+		}
+		g.P("func (m *", message.GoIdent, ") Get", field.GoName, "() ", goType, " {")
+		if field.OneofType != nil {
+			g.P("if x, ok := m.Get", field.OneofType.GoName, "().(*", fieldOneofType(field), "); ok {")
+			g.P("return x.", field.GoName)
+			g.P("}")
+		} else {
+			if field.Desc.Syntax() == protoreflect.Proto3 || defaultValue == "nil" {
+				g.P("if m != nil {")
+			} else {
+				g.P("if m != nil && m.", field.GoName, " != nil {")
+			}
+			star := ""
+			if pointer {
+				star = "*"
+			}
+			g.P("return ", star, " m.", field.GoName)
+			g.P("}")
+		}
+		g.P("return ", defaultValue)
+		g.P("}")
+		g.P()
+	}
+
+	genWellKnownType(g, message.GoIdent, message.Desc)
+
+	if len(message.Oneofs) > 0 {
+		genOneofFuncs(gen, g, f, message)
+	}
+	for _, extension := range message.Extensions {
+		genExtension(gen, g, f, extension)
+	}
+}
+
+// fieldGoType returns the Go type used for a field.
+//
+// If it returns pointer=true, the struct field is a pointer to the type.
+func fieldGoType(g *protogen.GeneratedFile, field *protogen.Field) (goType string, pointer bool) {
+	pointer = true
+	switch field.Desc.Kind() {
+	case protoreflect.BoolKind:
+		goType = "bool"
+	case protoreflect.EnumKind:
+		goType = g.QualifiedGoIdent(field.EnumType.GoIdent)
+	case protoreflect.Int32Kind, protoreflect.Sint32Kind, protoreflect.Sfixed32Kind:
+		goType = "int32"
+	case protoreflect.Uint32Kind, protoreflect.Fixed32Kind:
+		goType = "uint32"
+	case protoreflect.Int64Kind, protoreflect.Sint64Kind, protoreflect.Sfixed64Kind:
+		goType = "int64"
+	case protoreflect.Uint64Kind, protoreflect.Fixed64Kind:
+		goType = "uint64"
+	case protoreflect.FloatKind:
+		goType = "float32"
+	case protoreflect.DoubleKind:
+		goType = "float64"
+	case protoreflect.StringKind:
+		goType = "string"
+	case protoreflect.BytesKind:
+		goType = "[]byte"
+		pointer = false
+	case protoreflect.MessageKind, protoreflect.GroupKind:
+		if field.Desc.IsMap() {
+			keyType, _ := fieldGoType(g, field.MessageType.Fields[0])
+			valType, _ := fieldGoType(g, field.MessageType.Fields[1])
+			return fmt.Sprintf("map[%v]%v", keyType, valType), false
+		}
+		goType = "*" + g.QualifiedGoIdent(field.MessageType.GoIdent)
+		pointer = false
+	}
+	if field.Desc.Cardinality() == protoreflect.Repeated {
+		goType = "[]" + goType
+		pointer = false
+	}
+	if field.Desc.Syntax() == protoreflect.Proto3 {
+		pointer = false
+	}
+	return goType, pointer
+}
+
+func fieldProtobufTag(field *protogen.Field) string {
+	var tag []string
+	// wire type
+	tag = append(tag, wireTypes[field.Desc.Kind()])
+	// field number
+	tag = append(tag, strconv.Itoa(int(field.Desc.Number())))
+	// cardinality
+	switch field.Desc.Cardinality() {
+	case protoreflect.Optional:
+		tag = append(tag, "opt")
+	case protoreflect.Required:
+		tag = append(tag, "req")
+	case protoreflect.Repeated:
+		tag = append(tag, "rep")
+	}
+	if field.Desc.IsPacked() {
+		tag = append(tag, "packed")
+	}
+	// TODO: packed
+	// name
+	name := string(field.Desc.Name())
+	if field.Desc.Kind() == protoreflect.GroupKind {
+		// The name of the FieldDescriptor for a group field is
+		// lowercased. To find the original capitalization, we
+		// look in the field's MessageType.
+		name = string(field.MessageType.Desc.Name())
+	}
+	tag = append(tag, "name="+name)
+	// JSON name
+	if jsonName := field.Desc.JSONName(); jsonName != "" && jsonName != name {
+		tag = append(tag, "json="+jsonName)
+	}
+	// proto3
+	if field.Desc.Syntax() == protoreflect.Proto3 {
+		tag = append(tag, "proto3")
+	}
+	// enum
+	if field.Desc.Kind() == protoreflect.EnumKind {
+		tag = append(tag, "enum="+enumRegistryName(field.EnumType))
+	}
+	// oneof
+	if field.Desc.OneofType() != nil {
+		tag = append(tag, "oneof")
+	}
+	// default value
+	// This must appear last in the tag, since commas in strings aren't escaped.
+	if field.Desc.HasDefault() {
+		var def string
+		switch field.Desc.Kind() {
+		case protoreflect.BoolKind:
+			if field.Desc.Default().Bool() {
+				def = "1"
+			} else {
+				def = "0"
+			}
+		case protoreflect.BytesKind:
+			def = string(field.Desc.Default().Bytes())
+		case protoreflect.FloatKind, protoreflect.DoubleKind:
+			f := field.Desc.Default().Float()
+			switch {
+			case math.IsInf(f, -1):
+				def = "-inf"
+			case math.IsInf(f, 1):
+				def = "inf"
+			case math.IsNaN(f):
+				def = "nan"
+			default:
+				def = fmt.Sprint(f)
+			}
+		default:
+			def = fmt.Sprint(field.Desc.Default().Interface())
+		}
+		tag = append(tag, "def="+def)
+	}
+	return strings.Join(tag, ",")
+}
+
+func fieldDefaultValue(g *protogen.GeneratedFile, message *protogen.Message, field *protogen.Field) string {
+	if field.Desc.Cardinality() == protoreflect.Repeated {
+		return "nil"
+	}
+	if field.Desc.HasDefault() {
+		defVarName := "Default_" + message.GoIdent.GoName + "_" + field.GoName
+		if field.Desc.Kind() == protoreflect.BytesKind {
+			return "append([]byte(nil), " + defVarName + "...)"
+		}
+		return defVarName
+	}
+	switch field.Desc.Kind() {
+	case protoreflect.BoolKind:
+		return "false"
+	case protoreflect.StringKind:
+		return `""`
+	case protoreflect.MessageKind, protoreflect.GroupKind, protoreflect.BytesKind:
+		return "nil"
+	case protoreflect.EnumKind:
+		return g.QualifiedGoIdent(field.EnumType.Values[0].GoIdent)
+	default:
+		return "0"
+	}
+}
+
+var wireTypes = map[protoreflect.Kind]string{
+	protoreflect.BoolKind:     "varint",
+	protoreflect.EnumKind:     "varint",
+	protoreflect.Int32Kind:    "varint",
+	protoreflect.Sint32Kind:   "zigzag32",
+	protoreflect.Uint32Kind:   "varint",
+	protoreflect.Int64Kind:    "varint",
+	protoreflect.Sint64Kind:   "zigzag64",
+	protoreflect.Uint64Kind:   "varint",
+	protoreflect.Sfixed32Kind: "fixed32",
+	protoreflect.Fixed32Kind:  "fixed32",
+	protoreflect.FloatKind:    "fixed32",
+	protoreflect.Sfixed64Kind: "fixed64",
+	protoreflect.Fixed64Kind:  "fixed64",
+	protoreflect.DoubleKind:   "fixed64",
+	protoreflect.StringKind:   "bytes",
+	protoreflect.BytesKind:    "bytes",
+	protoreflect.MessageKind:  "bytes",
+	protoreflect.GroupKind:    "group",
+}
+
+func fieldJSONTag(field *protogen.Field) string {
+	return string(field.Desc.Name()) + ",omitempty"
+}
+
+func genExtension(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, extension *protogen.Extension) {
+	// Special case for proto2 message sets: If this extension is extending
+	// proto2.bridge.MessageSet, and its final name component is "message_set_extension",
+	// then drop that last component.
+	//
+	// TODO: This should be implemented in the text formatter rather than the generator.
+	// In addition, the situation for when to apply this special case is implemented
+	// differently in other languages:
+	// https://github.com/google/protobuf/blob/aff10976/src/google/protobuf/text_format.cc#L1560
+	name := extension.Desc.FullName()
+	if isExtensionMessageSetElement(gen, extension) {
+		name = name.Parent()
+	}
+
+	g.P("var ", extensionVar(f, extension), " = &", protogen.GoIdent{
+		GoImportPath: protoPackage,
+		GoName:       "ExtensionDesc",
+	}, "{")
+	g.P("ExtendedType: (*", extension.ExtendedType.GoIdent, ")(nil),")
+	goType, pointer := fieldGoType(g, extension)
+	if pointer {
+		goType = "*" + goType
+	}
+	g.P("ExtensionType: (", goType, ")(nil),")
+	g.P("Field: ", extension.Desc.Number(), ",")
+	g.P("Name: ", strconv.Quote(string(name)), ",")
+	g.P("Tag: ", strconv.Quote(fieldProtobufTag(extension)), ",")
+	g.P("Filename: ", strconv.Quote(f.Desc.Path()), ",")
+	g.P("}")
+	g.P()
+}
+
+func isExtensionMessageSetElement(gen *protogen.Plugin, extension *protogen.Extension) bool {
+	return extension.ParentMessage != nil &&
+		messageOptions(gen, extension.ExtendedType).GetMessageSetWireFormat() &&
+		extension.Desc.Name() == "message_set_extension"
+}
+
+// extensionVar returns the var holding the ExtensionDesc for an extension.
+func extensionVar(f *fileInfo, extension *protogen.Extension) protogen.GoIdent {
+	name := "E_"
+	if extension.ParentMessage != nil {
+		name += extension.ParentMessage.GoIdent.GoName + "_"
+	}
+	name += extension.GoName
+	return protogen.GoIdent{
+		GoImportPath: f.GoImportPath,
+		GoName:       name,
+	}
+}
+
+// genInitFunction generates an init function that registers the types in the
+// generated file with the proto package.
+func genInitFunction(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo) {
+	if len(f.allMessages) == 0 && len(f.allEnums) == 0 && len(f.allExtensions) == 0 {
+		return
+	}
+
+	g.P("func init() {")
+	for _, enum := range f.allEnums {
+		name := enum.GoIdent.GoName
+		g.P(protogen.GoIdent{
+			GoImportPath: protoPackage,
+			GoName:       "RegisterEnum",
+		}, fmt.Sprintf("(%q, %s_name, %s_value)", enumRegistryName(enum), name, name))
+	}
+	for _, message := range f.allMessages {
+		if message.Desc.IsMapEntry() {
+			continue
+		}
+
+		for _, extension := range message.Extensions {
+			genRegisterExtension(gen, g, f, extension)
+		}
+
+		name := message.GoIdent.GoName
+		g.P(protogen.GoIdent{
+			GoImportPath: protoPackage,
+			GoName:       "RegisterType",
+		}, fmt.Sprintf("((*%s)(nil), %q)", name, message.Desc.FullName()))
+
+		// Types of map fields, sorted by the name of the field message type.
+		var mapFields []*protogen.Field
+		for _, field := range message.Fields {
+			if field.Desc.IsMap() {
+				mapFields = append(mapFields, field)
+			}
+		}
+		sort.Slice(mapFields, func(i, j int) bool {
+			ni := mapFields[i].MessageType.Desc.FullName()
+			nj := mapFields[j].MessageType.Desc.FullName()
+			return ni < nj
+		})
+		for _, field := range mapFields {
+			typeName := string(field.MessageType.Desc.FullName())
+			goType, _ := fieldGoType(g, field)
+			g.P(protogen.GoIdent{
+				GoImportPath: protoPackage,
+				GoName:       "RegisterMapType",
+			}, fmt.Sprintf("((%v)(nil), %q)", goType, typeName))
+		}
+	}
+	for _, extension := range f.Extensions {
+		genRegisterExtension(gen, g, f, extension)
+	}
+	g.P("}")
+	g.P()
+}
+
+func genRegisterExtension(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, extension *protogen.Extension) {
+	g.P(protogen.GoIdent{
+		GoImportPath: protoPackage,
+		GoName:       "RegisterExtension",
+	}, "(", extensionVar(f, extension), ")")
+	if isExtensionMessageSetElement(gen, extension) {
+		goType, pointer := fieldGoType(g, extension)
+		if pointer {
+			goType = "*" + goType
+		}
+		g.P(protogen.GoIdent{
+			GoImportPath: protoPackage,
+			GoName:       "RegisterMessageSetType",
+		}, "((", goType, ")(nil), ", extension.Desc.Number(), ",", strconv.Quote(string(extension.Desc.FullName().Parent())), ")")
+	}
+}
+
+func genComment(g *protogen.GeneratedFile, f *fileInfo, path []int32) (hasComment bool) {
+	for _, loc := range f.locationMap[pathKey(path)] {
+		if loc.LeadingComments == nil {
+			continue
+		}
+		for _, line := range strings.Split(strings.TrimSuffix(loc.GetLeadingComments(), "\n"), "\n") {
+			hasComment = true
+			g.P("//", line)
+		}
+		break
+	}
+	return hasComment
+}
+
+// deprecationComment returns a standard deprecation comment if deprecated is true.
+func deprecationComment(deprecated bool) string {
+	if !deprecated {
+		return ""
+	}
+	return "// Deprecated: Do not use."
+}
+
+// pathKey converts a location path to a string suitable for use as a map key.
+func pathKey(path []int32) string {
+	var buf []byte
+	for i, x := range path {
+		if i != 0 {
+			buf = append(buf, ',')
+		}
+		buf = strconv.AppendInt(buf, int64(x), 10)
+	}
+	return string(buf)
+}
+
+func genWellKnownType(g *protogen.GeneratedFile, ident protogen.GoIdent, desc protoreflect.Descriptor) {
+	if wellKnownTypes[desc.FullName()] {
+		g.P("func (", ident, `) XXX_WellKnownType() string { return "`, desc.Name(), `" }`)
+		g.P()
+	}
+}
+
+// Names of messages and enums for which we will generate XXX_WellKnownType methods.
+var wellKnownTypes = map[protoreflect.FullName]bool{
+	"google.protobuf.NullValue": true,
+}