Source: stringer.go in package google.golang.org/protobuf/internal/descfmt

Package descfmt provides functionality to format descriptors.

package descfmt

import (
	"fmt"
	"io"
	"reflect"
	"strconv"
	"strings"

	"google.golang.org/protobuf/internal/detrand"
	"google.golang.org/protobuf/internal/pragma"
	pref "google.golang.org/protobuf/reflect/protoreflect"
)

type list interface {
	Len() int
	pragma.DoNotImplement
}

func FormatList(s fmt.State, r rune, vs list) {
	io.WriteString(s, formatListOpt(vs, true, r == 'v' && (s.Flag('+') || s.Flag('#'))))
}
func formatListOpt(vs list, isRoot, allowMulti bool) string {
	start, end := "[", "]"
	if isRoot {
		var name string
		switch vs.(type) {
		case pref.Names:
			name = "Names"
		case pref.FieldNumbers:
			name = "FieldNumbers"
		case pref.FieldRanges:
			name = "FieldRanges"
		case pref.EnumRanges:
			name = "EnumRanges"
		case pref.FileImports:
			name = "FileImports"
		case pref.Descriptor:
			name = reflect.ValueOf(vs).MethodByName("Get").Type().Out(0).Name() + "s"
		}
		start, end = name+"{", "}"
	}

	var ss []string
	switch vs := vs.(type) {
	case pref.Names:
		for i := 0; i < vs.Len(); i++ {
			ss = append(ss, fmt.Sprint(vs.Get(i)))
		}
		return start + joinStrings(ss, false) + end
	case pref.FieldNumbers:
		for i := 0; i < vs.Len(); i++ {
			ss = append(ss, fmt.Sprint(vs.Get(i)))
		}
		return start + joinStrings(ss, false) + end
	case pref.FieldRanges:
		for i := 0; i < vs.Len(); i++ {
			r := vs.Get(i)
			if r[0]+1 == r[1] {
				ss = append(ss, fmt.Sprintf("%d", r[0]))
			} else {
				ss = append(ss, fmt.Sprintf("%d:%d", r[0], r[1])) // enum ranges are end exclusive
			}
		}
		return start + joinStrings(ss, false) + end
	case pref.EnumRanges:
		for i := 0; i < vs.Len(); i++ {
			r := vs.Get(i)
			if r[0] == r[1] {
				ss = append(ss, fmt.Sprintf("%d", r[0]))
			} else {
				ss = append(ss, fmt.Sprintf("%d:%d", r[0], int64(r[1])+1)) // enum ranges are end inclusive
			}
		}
		return start + joinStrings(ss, false) + end
	case pref.FileImports:
		for i := 0; i < vs.Len(); i++ {
			var rs records
			rs.Append(reflect.ValueOf(vs.Get(i)), "Path", "Package", "IsPublic", "IsWeak")
			ss = append(ss, "{"+rs.Join()+"}")
		}
		return start + joinStrings(ss, allowMulti) + end
	default:
		_, isEnumValue := vs.(pref.EnumValueDescriptors)
		for i := 0; i < vs.Len(); i++ {
			m := reflect.ValueOf(vs).MethodByName("Get")
			v := m.Call([]reflect.Value{reflect.ValueOf(i)})[0].Interface()
			ss = append(ss, formatDescOpt(v.(pref.Descriptor), false, allowMulti && !isEnumValue))
		}
		return start + joinStrings(ss, allowMulti && isEnumValue) + end
	}
}

descriptorAccessors is a list of accessors to print for each descriptor. Do not print all accessors since some contain redundant information, while others are pointers that we do not want to follow since the descriptor is actually a cyclic graph. Using a list allows us to print the accessors in a sensible order.

var descriptorAccessors = map[reflect.Type][]string{
	reflect.TypeOf((*pref.FileDescriptor)(nil)).Elem():      {"Path", "Package", "Imports", "Messages", "Enums", "Extensions", "Services"},
	reflect.TypeOf((*pref.MessageDescriptor)(nil)).Elem():   {"IsMapEntry", "Fields", "Oneofs", "ReservedNames", "ReservedRanges", "RequiredNumbers", "ExtensionRanges", "Messages", "Enums", "Extensions"},
	reflect.TypeOf((*pref.FieldDescriptor)(nil)).Elem():     {"Number", "Cardinality", "Kind", "HasJSONName", "JSONName", "HasPresence", "IsExtension", "IsPacked", "IsWeak", "IsList", "IsMap", "MapKey", "MapValue", "HasDefault", "Default", "ContainingOneof", "ContainingMessage", "Message", "Enum"},
	reflect.TypeOf((*pref.OneofDescriptor)(nil)).Elem():     {"Fields"}, // not directly used; must keep in sync with formatDescOpt
	reflect.TypeOf((*pref.EnumDescriptor)(nil)).Elem():      {"Values", "ReservedNames", "ReservedRanges"},
	reflect.TypeOf((*pref.EnumValueDescriptor)(nil)).Elem(): {"Number"},
	reflect.TypeOf((*pref.ServiceDescriptor)(nil)).Elem():   {"Methods"},
	reflect.TypeOf((*pref.MethodDescriptor)(nil)).Elem():    {"Input", "Output", "IsStreamingClient", "IsStreamingServer"},
}

func FormatDesc(s fmt.State, r rune, t pref.Descriptor) {
	io.WriteString(s, formatDescOpt(t, true, r == 'v' && (s.Flag('+') || s.Flag('#'))))
}
func formatDescOpt(t pref.Descriptor, isRoot, allowMulti bool) string {
	rv := reflect.ValueOf(t)
	rt := rv.MethodByName("ProtoType").Type().In(0)

	start, end := "{", "}"
	if isRoot {
		start = rt.Name() + "{"
	}

	_, isFile := t.(pref.FileDescriptor)
	rs := records{allowMulti: allowMulti}
	if t.IsPlaceholder() {
		if isFile {
			rs.Append(rv, "Path", "Package", "IsPlaceholder")
		} else {
			rs.Append(rv, "FullName", "IsPlaceholder")
		}
	} else {
		switch {
		case isFile:
			rs.Append(rv, "Syntax")
		case isRoot:
			rs.Append(rv, "Syntax", "FullName")
		default:
			rs.Append(rv, "Name")
		}
		switch t := t.(type) {
		case pref.FieldDescriptor:
			for _, s := range descriptorAccessors[rt] {
				switch s {
				case "MapKey":
					if k := t.MapKey(); k != nil {
						rs.recs = append(rs.recs, [2]string{"MapKey", k.Kind().String()})
					}
				case "MapValue":
					if v := t.MapValue(); v != nil {
						switch v.Kind() {
						case pref.EnumKind:
							rs.recs = append(rs.recs, [2]string{"MapValue", string(v.Enum().FullName())})
						case pref.MessageKind, pref.GroupKind:
							rs.recs = append(rs.recs, [2]string{"MapValue", string(v.Message().FullName())})
						default:
							rs.recs = append(rs.recs, [2]string{"MapValue", v.Kind().String()})
						}
					}
				case "ContainingOneof":
					if od := t.ContainingOneof(); od != nil {
						rs.recs = append(rs.recs, [2]string{"Oneof", string(od.Name())})
					}
				case "ContainingMessage":
					if t.IsExtension() {
						rs.recs = append(rs.recs, [2]string{"Extendee", string(t.ContainingMessage().FullName())})
					}
				case "Message":
					if !t.IsMap() {
						rs.Append(rv, s)
					}
				default:
					rs.Append(rv, s)
				}
			}
		case pref.OneofDescriptor:
			var ss []string
			fs := t.Fields()
			for i := 0; i < fs.Len(); i++ {
				ss = append(ss, string(fs.Get(i).Name()))
			}
			if len(ss) > 0 {
				rs.recs = append(rs.recs, [2]string{"Fields", "[" + joinStrings(ss, false) + "]"})
			}
		default:
			rs.Append(rv, descriptorAccessors[rt]...)
		}
		if rv.MethodByName("GoType").IsValid() {
			rs.Append(rv, "GoType")
		}
	}
	return start + rs.Join() + end
}

type records struct {
	recs       [][2]string
	allowMulti bool
}

func (rs *records) Append(v reflect.Value, accessors ...string) {
	for _, a := range accessors {
		var rv reflect.Value
		if m := v.MethodByName(a); m.IsValid() {
			rv = m.Call(nil)[0]
		}
		if v.Kind() == reflect.Struct && !rv.IsValid() {
			rv = v.FieldByName(a)
		}
		if !rv.IsValid() {
			panic(fmt.Sprintf("unknown accessor: %v.%s", v.Type(), a))
		}
		if _, ok := rv.Interface().(pref.Value); ok {
			rv = rv.MethodByName("Interface").Call(nil)[0]
			if !rv.IsNil() {
				rv = rv.Elem()
			}
		}

Ignore zero values.

		var isZero bool
		switch rv.Kind() {
		case reflect.Interface, reflect.Slice:
			isZero = rv.IsNil()
		case reflect.Bool:
			isZero = rv.Bool() == false
		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
			isZero = rv.Int() == 0
		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
			isZero = rv.Uint() == 0
		case reflect.String:
			isZero = rv.String() == ""
		}
		if n, ok := rv.Interface().(list); ok {
			isZero = n.Len() == 0
		}
		if isZero {
			continue
		}

Format the value.

		var s string
		v := rv.Interface()
		switch v := v.(type) {
		case list:
			s = formatListOpt(v, false, rs.allowMulti)
		case pref.FieldDescriptor, pref.OneofDescriptor, pref.EnumValueDescriptor, pref.MethodDescriptor:
			s = string(v.(pref.Descriptor).Name())
		case pref.Descriptor:
			s = string(v.FullName())
		case string:
			s = strconv.Quote(v)
		case []byte:
			s = fmt.Sprintf("%q", v)
		default:
			s = fmt.Sprint(v)
		}
		rs.recs = append(rs.recs, [2]string{a, s})
	}
}

func (rs *records) Join() string {
	var ss []string

In single line mode, simply join all records with commas.

	if !rs.allowMulti {
		for _, r := range rs.recs {
			ss = append(ss, r[0]+formatColon(0)+r[1])
		}
		return joinStrings(ss, false)
	}

In allowMulti line mode, align single line records for more readable output.

	var maxLen int
	flush := func(i int) {
		for _, r := range rs.recs[len(ss):i] {
			ss = append(ss, r[0]+formatColon(maxLen-len(r[0]))+r[1])
		}
		maxLen = 0
	}
	for i, r := range rs.recs {
		if isMulti := strings.Contains(r[1], "\n"); isMulti {
			flush(i)
			ss = append(ss, r[0]+formatColon(0)+strings.Join(strings.Split(r[1], "\n"), "\n\t"))
		} else if maxLen < len(r[0]) {
			maxLen = len(r[0])
		}
	}
	flush(len(rs.recs))
	return joinStrings(ss, true)
}

Deliberately introduce instability into the debug output to discourage users from performing string comparisons. This provides us flexibility to change the output in the future.

	if detrand.Bool() {
		return ":" + strings.Repeat(" ", 1+padding) // use non-breaking spaces (U+00a0)
	} else {
		return ":" + strings.Repeat(" ", 1+padding) // use regular spaces (U+0020)
	}
}

func joinStrings(ss []string, isMulti bool) string {
	if len(ss) == 0 {
		return ""
	}
	if isMulti {
		return "\n\t" + strings.Join(ss, "\n\t") + "\n"
	}
	return strings.Join(ss, ", ")