You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: legacy_extension.go
    path of file: ./repos/docker-ce/components/cli/vendor/google.golang.org/protobuf/internal/impl
    
    the code of the file until where you have to start completion: // 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 impl

import (
	"reflect"

	"google.golang.org/protobuf/internal/descopts"
	"google.golang.org/protobuf/internal/encoding/messageset"
	ptag "google.golang.org/protobuf/internal/encoding/tag"
	"google.golang.org/protobuf/internal/filedesc"
	"google.golang.org/protobuf/internal/pragma"
	pref "google.golang.org/protobuf/reflect/protoreflect"
	preg "google.golang.org/protobuf/reflect/protoregistry"
	piface "google.golang.org/protobuf/runtime/protoiface"
)

func (xi *ExtensionInfo) initToLegacy() {
	xd := xi.desc
	var parent piface.MessageV1
	messageName := xd.ContainingMessage().FullName()
	if mt, _ := preg.GlobalTypes.FindMessageByName(messageName); mt != nil {
		// Create a new parent message and unwrap it if possible.
		mv := mt.New().Interface()
		t := reflect.TypeOf(mv)
		if mv, ok := mv.(unwrapper); ok {
			t = reflect.TypeOf(mv.protoUnwrap())
		}

		// Check whether the message implements the legacy v1 Message interface.
		mz := reflect.Zero(t).Interface()
		if mz, ok := mz.(piface.MessageV1); ok {
			parent = mz
		}
	}

	// Determine the v1 extension type, which is unfortunately not the same as
	// the v2 ExtensionType.GoType.
	extType := xi.goType
	switch extType.Kind() {
	case reflect.Bool, reflect.Int32, reflect.Int64, reflect.Uint32, reflect.Uint64, reflect.Float32, reflect.Float64, reflect.String:
		extType = reflect.PtrTo(extType) // T -> *T for singular scalar fields
	}

	// Reconstruct the legacy enum full name.
	var enumName string
	if xd.Kind() == pref.EnumKind {
		enumName = legacyEnumName(xd.Enum())
	}

	// Derive the proto file that the extension was declared within.
	var filename string
	if fd := xd.ParentFile(); fd !
    
func (xi *ExtensionInfo) initFromLegacy() {
	// The v1 API returns "type incomplete" descriptors where only the
	// field number is specified. In such a case, use a placeholder.
	if xi.ExtendedType == nil || xi.ExtensionType == nil {
		xd := placeholderExtension{
			name:   pref.FullName(xi.Name),
			number: pref.FieldNumber(xi.Field),
		}
		xi.desc = extensionTypeDescriptor{xd, xi}
		return
	}

	// Resolve enum or message dependencies.
	var ed pref.EnumDescriptor
	var md pref.MessageDescriptor
	t := reflect.TypeOf(xi.ExtensionType)
	isOptional := t.Kind() == reflect.Ptr && t.Elem().Kind() != reflect.Struct
	isRepeated := t.Kind() == reflect.Slice && t.Elem().Kind() != reflect.Uint8
	if isOptional || isRepeated {
		t = t.Elem()
	}
	switch v := reflect.Zero(t).Interface().(type) {
	case pref.Enum:
		ed = v.Descriptor()
	case enumV1:
		ed = LegacyLoadEnumDesc(t)
	case pref.ProtoMessage:
		md = v.ProtoReflect().Descriptor()
	case messageV1:
		md = LegacyLoadMessageDesc(t)
	}

	// Derive basic field information from the struct tag.
	var evs pref.EnumValueDescriptors
	if ed != nil {
		evs = ed.Values()
	}
	fd := ptag.Unmarshal(xi.Tag, t, evs).(*filedesc.Field)

	// Construct a v2 ExtensionType.
	xd := &filedesc.Extension{L2: new(filedesc.ExtensionL2)}
	xd.L0.ParentFile = filedesc.SurrogateProto2
	xd.L0.FullName = pref.FullName(xi.Name)
	xd.L1.Number = pref.FieldNumber(xi.Field)
	xd.L1.Cardinality = fd.L1.Cardinality
	xd.L1.Kind = fd.L1.Kind
	xd.L2.IsPacked = fd.L1.IsPacked
	xd.L2.Default = fd.L1.Default
	xd.L1.Extendee = Export{}.MessageDescriptorOf(xi.ExtendedType)
	xd.L2.Enum = ed
	xd.L2.Message = md

	// Derive real extension field name for MessageSets.
	if messageset.IsMessageSet(xd.L1.Extendee) && md.FullName() == xd.L0.FullName {
		xd.L0.FullName = xd.L0.FullName.Append(messageset.ExtensionName)
	}

	tt := reflect.TypeOf(xi.ExtensionType)
	if isOptional {
		tt = tt.Elem()
	}
	xi.goType = tt
	xi.desc = extensionTypeDescriptor{xd, xi}
}