reflect: support for struct tag use by multiple packages

Each package using struct field tags assumes that
it is the only package storing data in the tag.
This CL adds support in package reflect for sharing
tags between multiple packages.  In this scheme, the
tags must be of the form

        key:"value" key2:"value2"

(raw strings help when writing that tag in Go source).

reflect.StructField's Tag field now has type StructTag
(a string type), which has method Get(key string) string
that returns the associated value.

Clients of json and xml will need to be updated.
Code that says

        type T struct {
                X int "name"
        }

should become

        type T struct {
                X int `json:"name"`  // or `xml:"name"`
        }

Use govet to identify struct tags that need to be changed
to use the new syntax.

R=r, r, dsymonds, bradfitz, kevlar, fvbommel, n13m3y3r
CC=golang-dev
https://golang.org/cl/4645069

src/cmd/godoc/codewalk.go

@@ -74,7 +74,7 @@ func codewalk(w http.ResponseWriter, r *http.Request) {
 
 // A Codewalk represents a single codewalk read from an XML file.
 type Codewalk struct {
-	Title string "attr"
+	Title string `xml:"attr"`
 	File  []string
 	Step  []*Codestep
 }
@@ -83,9 +83,9 @@ type Codewalk struct {
 // A Codestep is a single step in a codewalk.
 type Codestep struct {
 	// Filled in from XML
-	Src   string "attr"
-	Title string "attr"
-	XML   string "innerxml"
+	Src   string `xml:"attr"`
+	Title string `xml:"attr"`
+	XML   string `xml:"innerxml"`
 
 	// Derived from Src; not in XML.
 	Err    os.Error

src/cmd/govet/govet.go

@@ -15,6 +15,7 @@ import (
 	"go/token"
 	"os"
 	"path/filepath"
+	"reflect"
 	"strconv"
 	"strings"
 	"utf8"
@@ -59,7 +60,7 @@ func main() {
 				var err os.Error
 				skip, err = strconv.Atoi(name[colon+1:])
 				if err != nil {
-					error(`illegal format for "Func:N" argument %q; %s`, name, err)
+					errorf(`illegal format for "Func:N" argument %q; %s`, name, err)
 				}
 				name = name[:colon]
 			}
@@ -93,7 +94,7 @@ func doFile(name string, reader io.Reader) {
 	fs := token.NewFileSet()
 	parsedFile, err := parser.ParseFile(fs, name, reader, 0)
 	if err != nil {
-		error("%s: %s", name, err)
+		errorf("%s: %s", name, err)
 		return
 	}
 	file := &File{fs.File(parsedFile.Pos())}
@@ -121,7 +122,7 @@ func walkDir(root string) {
 	done := make(chan bool)
 	go func() {
 		for e := range errors {
-			error("walk error: %s", e)
+			errorf("walk error: %s", e)
 		}
 		done <- true
 	}()
@@ -132,7 +133,7 @@ func walkDir(root string) {
 
 // error formats the error to standard error, adding program
 // identification and a newline
-func error(format string, args ...interface{}) {
+func errorf(format string, args ...interface{}) {
 	fmt.Fprintf(os.Stderr, "govet: "+format+"\n", args...)
 	setExit(2)
 }
@@ -185,15 +186,35 @@ func (f *File) checkFile(name string, file *ast.File) {
 
 // Visit implements the ast.Visitor interface.
 func (f *File) Visit(node ast.Node) ast.Visitor {
-	// TODO: could return nil for nodes that cannot contain a CallExpr -
-	// will shortcut traversal.  Worthwhile?
 	switch n := node.(type) {
 	case *ast.CallExpr:
 		f.checkCallExpr(n)
+	case *ast.Field:
+		f.checkFieldTag(n)
 	}
 	return f
 }
 
+// checkField checks a struct field tag.
+func (f *File) checkFieldTag(field *ast.Field) {
+	if field.Tag == nil {
+		return
+	}
+
+	tag, err := strconv.Unquote(field.Tag.Value)
+	if err != nil {
+		f.Warnf(field.Pos(), "unable to read struct tag %s", field.Tag.Value)
+		return
+	}
+
+	// Check tag for validity by appending
+	// new key:value to end and checking that
+	// the tag parsing code can find it.
+	if reflect.StructTag(tag+` _gofix:"_magic"`).Get("_gofix") != "_magic" {
+		f.Warnf(field.Pos(), "struct field tag %s not compatible with reflect.StructTag.Get", field.Tag.Value)
+		return
+	}
+}
 
 // checkCallExpr checks a call expression.
 func (f *File) checkCallExpr(call *ast.CallExpr) {
@@ -373,6 +394,10 @@ func BadFunctionUsedInTests() {
 	f.Warnf(0, "%s", "hello", 3) // wrong # %s in call to added function
 }
 
+type BadTypeUsedInTests struct {
+	X int "hello" // struct field not well-formed
+}
+
 // printf is used by the test.
 func printf(format string, args ...interface{}) {
 	panic("don't call - testing only")

src/pkg/asn1/asn1.go

@@ -707,7 +707,7 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 			if i == 0 && field.Type == rawContentsType {
 				continue
 			}
-			innerOffset, err = parseField(val.Field(i), innerBytes, innerOffset, parseFieldParameters(field.Tag))
+			innerOffset, err = parseField(val.Field(i), innerBytes, innerOffset, parseFieldParameters(field.Tag.Get("asn1")))
 			if err != nil {
 				return
 			}

src/pkg/asn1/asn1_test.go

@@ -299,11 +299,11 @@ type TestObjectIdentifierStruct struct {
 }
 
 type TestContextSpecificTags struct {
-	A int "tag:1"
+	A int `asn1:"tag:1"`
 }
 
 type TestContextSpecificTags2 struct {
-	A int "explicit,tag:1"
+	A int `asn1:"explicit,tag:1"`
 	B int
 }
 
@@ -353,7 +353,7 @@ type Certificate struct {
 }
 
 type TBSCertificate struct {
-	Version            int "optional,explicit,default:0,tag:0"
+	Version            int `asn1:"optional,explicit,default:0,tag:0"`
 	SerialNumber       RawValue
 	SignatureAlgorithm AlgorithmIdentifier
 	Issuer             RDNSequence

src/pkg/asn1/marshal.go

@@ -413,7 +413,7 @@ func marshalBody(out *forkableWriter, value reflect.Value, params fieldParameter
 		for i := startingField; i < t.NumField(); i++ {
 			var pre *forkableWriter
 			pre, out = out.fork()
-			err = marshalField(pre, v.Field(i), parseFieldParameters(t.Field(i).Tag))
+			err = marshalField(pre, v.Field(i), parseFieldParameters(t.Field(i).Tag.Get("asn1")))
 			if err != nil {
 				return
 			}

src/pkg/asn1/marshal_test.go

@@ -30,23 +30,23 @@ type rawContentsStruct struct {
 }
 
 type implicitTagTest struct {
-	A int "implicit,tag:5"
+	A int `asn1:"implicit,tag:5"`
 }
 
 type explicitTagTest struct {
-	A int "explicit,tag:5"
+	A int `asn1:"explicit,tag:5"`
 }
 
 type ia5StringTest struct {
-	A string "ia5"
+	A string `asn1:"ia5"`
 }
 
 type printableStringTest struct {
-	A string "printable"
+	A string `asn1:"printable"`
 }
 
 type optionalRawValueTest struct {
-	A RawValue "optional"
+	A RawValue `asn1:"optional"`
 }
 
 type testSET []int

src/pkg/crypto/ocsp/ocsp.go

@@ -43,7 +43,7 @@ type certID struct {
 
 type responseASN1 struct {
 	Status   asn1.Enumerated
-	Response responseBytes "explicit,tag:0"
+	Response responseBytes `asn1:"explicit,tag:0"`
 }
 
 type responseBytes struct {
@@ -55,30 +55,30 @@ type basicResponse struct {
 	TBSResponseData    responseData
 	SignatureAlgorithm pkix.AlgorithmIdentifier
 	Signature          asn1.BitString
-	Certificates       []asn1.RawValue "explicit,tag:0,optional"
+	Certificates       []asn1.RawValue `asn1:"explicit,tag:0,optional"`
 }
 
 type responseData struct {
 	Raw           asn1.RawContent
-	Version       int              "optional,default:1,explicit,tag:0"
-	RequestorName pkix.RDNSequence "optional,explicit,tag:1"
-	KeyHash       []byte           "optional,explicit,tag:2"
+	Version       int              `asn1:"optional,default:1,explicit,tag:0"`
+	RequestorName pkix.RDNSequence `asn1:"optional,explicit,tag:1"`
+	KeyHash       []byte           `asn1:"optional,explicit,tag:2"`
 	ProducedAt    *time.Time
 	Responses     []singleResponse
 }
 
 type singleResponse struct {
 	CertID     certID
-	Good       asn1.Flag   "explicit,tag:0,optional"
-	Revoked    revokedInfo "explicit,tag:1,optional"
-	Unknown    asn1.Flag   "explicit,tag:2,optional"
+	Good       asn1.Flag   `asn1:"explicit,tag:0,optional"`
+	Revoked    revokedInfo `asn1:"explicit,tag:1,optional"`
+	Unknown    asn1.Flag   `asn1:"explicit,tag:2,optional"`
 	ThisUpdate *time.Time
-	NextUpdate *time.Time "explicit,tag:0,optional"
+	NextUpdate *time.Time `asn1:"explicit,tag:0,optional"`
 }
 
 type revokedInfo struct {
 	RevocationTime *time.Time
-	Reason         int "explicit,tag:0,optional"
+	Reason         int `asn1:"explicit,tag:0,optional"`
 }
 
 // This is the exposed reflection of the internal OCSP structures.

src/pkg/crypto/x509/pkix/pkix.go

@@ -16,7 +16,7 @@ import (
 // 5280, section 4.1.1.2.
 type AlgorithmIdentifier struct {
 	Algorithm  asn1.ObjectIdentifier
-	Parameters asn1.RawValue "optional"
+	Parameters asn1.RawValue `asn1:"optional"`
 }
 
 type RDNSequence []RelativeDistinguishedNameSET
@@ -32,7 +32,7 @@ type AttributeTypeAndValue struct {
 // 5280, section 4.2.
 type Extension struct {
 	Id       asn1.ObjectIdentifier
-	Critical bool "optional"
+	Critical bool `asn1:"optional"`
 	Value    []byte
 }
 
@@ -149,13 +149,13 @@ func (certList *CertificateList) HasExpired(currentTimeSeconds int64) bool {
 // 5280, section 5.1.
 type TBSCertificateList struct {
 	Raw                 asn1.RawContent
-	Version             int "optional,default:2"
+	Version             int `asn1:"optional,default:2"`
 	Signature           AlgorithmIdentifier
 	Issuer              RDNSequence
 	ThisUpdate          *time.Time
 	NextUpdate          *time.Time
-	RevokedCertificates []RevokedCertificate "optional"
-	Extensions          []Extension          "tag:0,optional,explicit"
+	RevokedCertificates []RevokedCertificate `asn1:"optional"`
+	Extensions          []Extension          `asn1:"tag:0,optional,explicit"`
 }
 
 // RevokedCertificate represents the ASN.1 structure of the same name. See RFC
@@ -163,5 +163,5 @@ type TBSCertificateList struct {
 type RevokedCertificate struct {
 	SerialNumber   *big.Int
 	RevocationTime *time.Time
-	Extensions     []Extension "optional"
+	Extensions     []Extension `asn1:"optional"`
 }

src/pkg/crypto/x509/x509.go

@@ -30,11 +30,11 @@ type pkcs1PrivateKey struct {
 	P       *big.Int
 	Q       *big.Int
 	// We ignore these values, if present, because rsa will calculate them.
-	Dp   *big.Int "optional"
-	Dq   *big.Int "optional"
-	Qinv *big.Int "optional"
+	Dp   *big.Int `asn1:"optional"`
+	Dq   *big.Int `asn1:"optional"`
+	Qinv *big.Int `asn1:"optional"`
 
-	AdditionalPrimes []pkcs1AdditionalRSAPrime "optional"
+	AdditionalPrimes []pkcs1AdditionalRSAPrime `asn1:"optional"`
 }
 
 type pkcs1AdditionalRSAPrime struct {
@@ -136,16 +136,16 @@ type certificate struct {
 
 type tbsCertificate struct {
 	Raw                asn1.RawContent
-	Version            int "optional,explicit,default:1,tag:0"
+	Version            int `asn1:"optional,explicit,default:1,tag:0"`
 	SerialNumber       *big.Int
 	SignatureAlgorithm pkix.AlgorithmIdentifier
 	Issuer             pkix.RDNSequence
 	Validity           validity
 	Subject            pkix.RDNSequence
 	PublicKey          publicKeyInfo
-	UniqueId           asn1.BitString   "optional,tag:1"
-	SubjectUniqueId    asn1.BitString   "optional,tag:2"
-	Extensions         []pkix.Extension "optional,explicit,tag:3"
+	UniqueId           asn1.BitString   `asn1:"optional,tag:1"`
+	SubjectUniqueId    asn1.BitString   `asn1:"optional,tag:2"`
+	Extensions         []pkix.Extension `asn1:"optional,explicit,tag:3"`
 }
 
 type dsaAlgorithmParameters struct {
@@ -168,7 +168,7 @@ type publicKeyInfo struct {
 
 // RFC 5280,  4.2.1.1
 type authKeyId struct {
-	Id []byte "optional,tag:0"
+	Id []byte `asn1:"optional,tag:0"`
 }
 
 type SignatureAlgorithm int
@@ -480,8 +480,8 @@ func (h UnhandledCriticalExtension) String() string {
 }
 
 type basicConstraints struct {
-	IsCA       bool "optional"
-	MaxPathLen int  "optional"
+	IsCA       bool `asn1:"optional"`
+	MaxPathLen int  `asn1:"optional"`
 }
 
 type rsaPublicKey struct {
@@ -497,14 +497,14 @@ type policyInformation struct {
 
 // RFC 5280, 4.2.1.10
 type nameConstraints struct {
-	Permitted []generalSubtree "optional,tag:0"
-	Excluded  []generalSubtree "optional,tag:1"
+	Permitted []generalSubtree `asn1:"optional,tag:0"`
+	Excluded  []generalSubtree `asn1:"optional,tag:1"`
 }
 
 type generalSubtree struct {
-	Name string "tag:2,optional,ia5"
-	Min  int    "optional,tag:0"
-	Max  int    "optional,tag:1"
+	Name string `asn1:"tag:2,optional,ia5"`
+	Min  int    `asn1:"optional,tag:0"`
+	Max  int    `asn1:"optional,tag:1"`
 }
 
 func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{}, os.Error) {

src/pkg/go/types/testdata/exports.go

@@ -38,7 +38,7 @@ type (
 	T9  struct {
 		a    int
 		b, c float32
-		d    []string "tag"
+		d    []string `go:"tag"`
 	}
 	T10 struct {
 		T8

src/pkg/json/decode.go

@@ -482,7 +482,7 @@ func (d *decodeState) object(v reflect.Value) {
 			if isValidTag(key) {
 				for i := 0; i < sv.NumField(); i++ {
 					f = st.Field(i)
-					if f.Tag == key {
+					if f.Tag.Get("json") == key {
 						ok = true
 						break
 					}

src/pkg/json/decode_test.go

@@ -42,8 +42,9 @@ var (
 
 type badTag struct {
 	X string
-	Y string "y"
-	Z string "@#*%(#@"
+	Y string `json:"y"`
+	Z string `x:"@#*%(#@"`
+	W string `json:"@#$@#$"`
 }
 
 type unmarshalTest struct {
@@ -68,7 +69,7 @@ var unmarshalTests = []unmarshalTest{
 	{`{"x": 1}`, new(tx), tx{}, &UnmarshalFieldError{"x", txType, txType.Field(0)}},
 
 	// skip invalid tags
-	{`{"X":"a", "y":"b", "Z":"c"}`, new(badTag), badTag{"a", "b", "c"}, nil},
+	{`{"X":"a", "y":"b", "Z":"c", "W":"d"}`, new(badTag), badTag{"a", "b", "c", "d"}, nil},
 
 	// syntax errors
 	{`{"X": "foo", "Y"}`, nil, nil, &SyntaxError{"invalid character '}' after object key", 17}},
@@ -250,7 +251,7 @@ type All struct {
 	Float32 float32
 	Float64 float64
 
-	Foo string "bar"
+	Foo string `json:"bar"`
 
 	PBool    *bool
 	PInt     *int

src/pkg/json/encode.go

@@ -36,11 +36,13 @@ import (
 // Array and slice values encode as JSON arrays, except that
 // []byte encodes as a base64-encoded string.
 //
-// Struct values encode as JSON objects.  Each struct field becomes
-// a member of the object.  By default the object's key name is the
-// struct field name.  If the struct field has a non-empty tag consisting
-// of only Unicode letters, digits, and underscores, that tag will be used
-// as the name instead.  Only exported fields will be encoded.
+// Struct values encode as JSON objects.  Each exported struct field
+// becomes a member of the object.  By default the object's key string
+// is the struct field name.  If the struct field's tag has a "json" key with a
+// value that is a non-empty string consisting of only Unicode letters,
+// digits, and underscores, that value will be used as the object key.
+// For example, the field tag `json:"myName"` says to use "myName"
+// as the object key.
 //
 // Map values encode as JSON objects.
 // The map's key type must be string; the object keys are used directly
@@ -236,8 +238,8 @@ func (e *encodeState) reflectValue(v reflect.Value) {
 			} else {
 				e.WriteByte(',')
 			}
-			if isValidTag(f.Tag) {
-				e.string(f.Tag)
+			if tag := f.Tag.Get("json"); tag != "" && isValidTag(tag) {
+				e.string(tag)
 			} else {
 				e.string(f.Name)
 			}

src/pkg/net/dnsmsg.go

@@ -93,7 +93,7 @@ const (
 
 // DNS queries.
 type dnsQuestion struct {
-	Name   string "domain-name" // "domain-name" specifies encoding; see packers below
+	Name   string `net:"domain-name"` // `net:"domain-name"` specifies encoding; see packers below
 	Qtype  uint16
 	Qclass uint16
 }
@@ -102,7 +102,7 @@ type dnsQuestion struct {
 // There are many types of messages,
 // but they all share the same header.
 type dnsRR_Header struct {
-	Name     string "domain-name"
+	Name     string `net:"domain-name"`
 	Rrtype   uint16
 	Class    uint16
 	Ttl      uint32
@@ -121,7 +121,7 @@ type dnsRR interface {
 
 type dnsRR_CNAME struct {
 	Hdr   dnsRR_Header
-	Cname string "domain-name"
+	Cname string `net:"domain-name"`
 }
 
 func (rr *dnsRR_CNAME) Header() *dnsRR_Header {
@@ -140,7 +140,7 @@ func (rr *dnsRR_HINFO) Header() *dnsRR_Header {
 
 type dnsRR_MB struct {
 	Hdr dnsRR_Header
-	Mb  string "domain-name"
+	Mb  string `net:"domain-name"`
 }
 
 func (rr *dnsRR_MB) Header() *dnsRR_Header {
@@ -149,7 +149,7 @@ func (rr *dnsRR_MB) Header() *dnsRR_Header {
 
 type dnsRR_MG struct {
 	Hdr dnsRR_Header
-	Mg  string "domain-name"
+	Mg  string `net:"domain-name"`
 }
 
 func (rr *dnsRR_MG) Header() *dnsRR_Header {
@@ -158,8 +158,8 @@ func (rr *dnsRR_MG) Header() *dnsRR_Header {
 
 type dnsRR_MINFO struct {
 	Hdr   dnsRR_Header
-	Rmail string "domain-name"
-	Email string "domain-name"
+	Rmail string `net:"domain-name"`
+	Email string `net:"domain-name"`
 }
 
 func (rr *dnsRR_MINFO) Header() *dnsRR_Header {
@@ -168,7 +168,7 @@ func (rr *dnsRR_MINFO) Header() *dnsRR_Header {
 
 type dnsRR_MR struct {
 	Hdr dnsRR_Header
-	Mr  string "domain-name"
+	Mr  string `net:"domain-name"`
 }
 
 func (rr *dnsRR_MR) Header() *dnsRR_Header {
@@ -178,7 +178,7 @@ func (rr *dnsRR_MR) Header() *dnsRR_Header {
 type dnsRR_MX struct {
 	Hdr  dnsRR_Header
 	Pref uint16
-	Mx   string "domain-name"
+	Mx   string `net:"domain-name"`
 }
 
 func (rr *dnsRR_MX) Header() *dnsRR_Header {
@@ -187,7 +187,7 @@ func (rr *dnsRR_MX) Header() *dnsRR_Header {
 
 type dnsRR_NS struct {
 	Hdr dnsRR_Header
-	Ns  string "domain-name"
+	Ns  string `net:"domain-name"`
 }
 
 func (rr *dnsRR_NS) Header() *dnsRR_Header {
@@ -196,7 +196,7 @@ func (rr *dnsRR_NS) Header() *dnsRR_Header {
 
 type dnsRR_PTR struct {
 	Hdr dnsRR_Header
-	Ptr string "domain-name"
+	Ptr string `net:"domain-name"`
 }
 
 func (rr *dnsRR_PTR) Header() *dnsRR_Header {
@@ -205,8 +205,8 @@ func (rr *dnsRR_PTR) Header() *dnsRR_Header {
 
 type dnsRR_SOA struct {
 	Hdr     dnsRR_Header
-	Ns      string "domain-name"
-	Mbox    string "domain-name"
+	Ns      string `net:"domain-name"`
+	Mbox    string `net:"domain-name"`
 	Serial  uint32
 	Refresh uint32
 	Retry   uint32
@@ -232,7 +232,7 @@ type dnsRR_SRV struct {
 	Priority uint16
 	Weight   uint16
 	Port     uint16
-	Target   string "domain-name"
+	Target   string `net:"domain-name"`
 }
 
 func (rr *dnsRR_SRV) Header() *dnsRR_Header {
@@ -241,7 +241,7 @@ func (rr *dnsRR_SRV) Header() *dnsRR_Header {
 
 type dnsRR_A struct {
 	Hdr dnsRR_Header
-	A   uint32 "ipv4"
+	A   uint32 `net:"ipv4"`
 }
 
 func (rr *dnsRR_A) Header() *dnsRR_Header {
@@ -250,7 +250,7 @@ func (rr *dnsRR_A) Header() *dnsRR_Header {
 
 type dnsRR_AAAA struct {
 	Hdr  dnsRR_Header
-	AAAA [16]byte "ipv6"
+	AAAA [16]byte `net:"ipv6"`
 }
 
 func (rr *dnsRR_AAAA) Header() *dnsRR_Header {
@@ -435,7 +435,7 @@ func packStructValue(val reflect.Value, msg []byte, off int) (off1 int, ok bool)
 			default:
 				fmt.Fprintf(os.Stderr, "net: dns: unknown string tag %v", f.Tag)
 				return len(msg), false
-			case "domain-name":
+			case `net:"domain-name"`:
 				off, ok = packDomainName(s, msg, off)
 				if !ok {
 					return len(msg), false
@@ -506,7 +506,7 @@ func unpackStructValue(val reflect.Value, msg []byte, off int) (off1 int, ok boo
 			default:
 				fmt.Fprintf(os.Stderr, "net: dns: unknown string tag %v", f.Tag)
 				return len(msg), false
-			case "domain-name":
+			case `net:"domain-name"`:
 				s, off, ok = unpackDomainName(msg, off)
 				if !ok {
 					return len(msg), false
@@ -536,9 +536,9 @@ func unpackStruct(any interface{}, msg []byte, off int) (off1 int, ok bool) {
 }
 
 // Generic struct printer.
-// Doesn't care about the string tag "domain-name",
-// but does look for an "ipv4" tag on uint32 variables
-// and the "ipv6" tag on array variables,
+// Doesn't care about the string tag `net:"domain-name"`,
+// but does look for an `net:"ipv4"` tag on uint32 variables
+// and the `net:"ipv6"` tag on array variables,
 // printing them as IP addresses.
 func printStructValue(val reflect.Value) string {
 	s := "{"
@@ -553,10 +553,10 @@ func printStructValue(val reflect.Value) string {
 		fval := val.Field(i)
 		if fv := fval; fv.Kind() == reflect.Struct {
 			s += printStructValue(fv)
-		} else if fv := fval; (fv.Kind() == reflect.Uint || fv.Kind() == reflect.Uint8 || fv.Kind() == reflect.Uint16 || fv.Kind() == reflect.Uint32 || fv.Kind() == reflect.Uint64 || fv.Kind() == reflect.Uintptr) && f.Tag == "ipv4" {
+		} else if fv := fval; (fv.Kind() == reflect.Uint || fv.Kind() == reflect.Uint8 || fv.Kind() == reflect.Uint16 || fv.Kind() == reflect.Uint32 || fv.Kind() == reflect.Uint64 || fv.Kind() == reflect.Uintptr) && f.Tag == `net:"ipv4"` {
 			i := fv.Uint()
 			s += IPv4(byte(i>>24), byte(i>>16), byte(i>>8), byte(i)).String()
-		} else if fv := fval; fv.Kind() == reflect.Array && f.Tag == "ipv6" {
+		} else if fv := fval; fv.Kind() == reflect.Array && f.Tag == `net:"ipv6"` {
 			i := fv.Interface().([]byte)
 			s += IP(i).String()
 		} else {

src/pkg/reflect/all_test.go

@@ -127,17 +127,17 @@ var typeTests = []pair{
 	},
 	{struct {
 		x struct {
-			a int8 "hi there"
+			a int8 `reflect:"hi there"`
 		}
 	}{},
-		`struct { a int8 "hi there" }`,
+		`struct { a int8 "reflect:\"hi there\"" }`,
 	},
 	{struct {
 		x struct {
-			a int8 "hi \x00there\t\n\"\\"
+			a int8 `reflect:"hi \x00there\t\n\"\\"`
 		}
 	}{},
-		`struct { a int8 "hi \x00there\t\n\"\\" }`,
+		`struct { a int8 "reflect:\"hi \\x00there\\t\\n\\\"\\\\\"" }`,
 	},
 	{struct {
 		x struct {
@@ -423,7 +423,7 @@ func TestAll(t *testing.T) {
 
 	// make sure tag strings are not part of element type
 	typ = TypeOf(struct {
-		d []uint32 "TAG"
+		d []uint32 `reflect:"TAG"`
 	}{}).Field(0).Type
 	testType(t, 14, typ, "[]uint32")
 }
@@ -1544,3 +1544,23 @@ func TestVariadic(t *testing.T) {
 		t.Errorf("after Fprintf CallSlice: %q != %q", b.String(), "hello 42 world")
 	}
 }
+
+var tagGetTests = []struct {
+	Tag   StructTag
+	Key   string
+	Value string
+}{
+	{`protobuf:"PB(1,2)"`, `protobuf`, `PB(1,2)`},
+	{`protobuf:"PB(1,2)"`, `foo`, ``},
+	{`protobuf:"PB(1,2)"`, `rotobuf`, ``},
+	{`protobuf:"PB(1,2)" json:"name"`, `json`, `name`},
+	{`protobuf:"PB(1,2)" json:"name"`, `protobuf`, `PB(1,2)`},
+}
+
+func TestTagGet(t *testing.T) {
+	for _, tt := range tagGetTests {
+		if v := tt.Tag.Get(tt.Key); v != tt.Value {
+			t.Errorf("StructTag(%#q).Get(%#q) = %#q, want %#q", tt.Tag, tt.Key, v, tt.Value)
+		}
+	}
+}

src/pkg/reflect/type.go

@@ -274,7 +274,7 @@ const (
 
 // arrayType represents a fixed array type.
 type arrayType struct {
-	commonType "array"
+	commonType `reflect:"array"`
 	elem       *runtime.Type
 	slice      *runtime.Type
 	len        uintptr
@@ -282,14 +282,14 @@ type arrayType struct {
 
 // chanType represents a channel type.
 type chanType struct {
-	commonType "chan"
+	commonType `reflect:"chan"`
 	elem       *runtime.Type
 	dir        uintptr
 }
 
 // funcType represents a function type.
 type funcType struct {
-	commonType "func"
+	commonType `reflect:"func"`
 	dotdotdot  bool
 	in         []*runtime.Type
 	out        []*runtime.Type
@@ -304,26 +304,26 @@ type imethod struct {
 
 // interfaceType represents an interface type.
 type interfaceType struct {
-	commonType "interface"
+	commonType `reflect:"interface"`
 	methods    []imethod
 }
 
 // mapType represents a map type.
 type mapType struct {
-	commonType "map"
+	commonType `reflect:"map"`
 	key        *runtime.Type
 	elem       *runtime.Type
 }
 
 // ptrType represents a pointer type.
 type ptrType struct {
-	commonType "ptr"
+	commonType `reflect:"ptr"`
 	elem       *runtime.Type
 }
 
 // sliceType represents a slice type.
 type sliceType struct {
-	commonType "slice"
+	commonType `reflect:"slice"`
 	elem       *runtime.Type
 }
 
@@ -338,7 +338,7 @@ type structField struct {
 
 // structType represents a struct type.
 type structType struct {
-	commonType "struct"
+	commonType `reflect:"struct"`
 	fields     []structField
 }
 
@@ -696,12 +696,72 @@ type StructField struct {
 	PkgPath   string // empty for uppercase Name
 	Name      string
 	Type      Type
-	Tag       string
+	Tag       StructTag
 	Offset    uintptr
 	Index     []int
 	Anonymous bool
 }
 
+// A StructTag is the tag string in a struct field.
+//
+// By convention, tag strings are a concatenation of
+// optionally space-separated key:"value" pairs.
+// Each key is a non-empty string consisting of non-control
+// characters other than space (U+0020 ' '), quote (U+0022 '"'),
+// and colon (U+003A ':').  Each value is quoted using U+0022 '"'
+// characters and Go string literal syntax.
+type StructTag string
+
+// Get returns the value associated with key in the tag string.
+// If there is no such key in the tag, Get returns the empty string.
+// If the tag does not have the conventional format, the value
+// returned by Get is unspecified, 
+func (tag StructTag) Get(key string) string {
+	for tag != "" {
+		// skip leading space
+		i := 0
+		for i < len(tag) && tag[i] == ' ' {
+			i++
+		}
+		tag = tag[i:]
+		if tag == "" {
+			break
+		}
+
+		// scan to colon.
+		// a space or a quote is a syntax error
+		i = 0
+		for i < len(tag) && tag[i] != ' ' && tag[i] != ':' && tag[i] != '"' {
+			i++
+		}
+		if i+1 >= len(tag) || tag[i] != ':' || tag[i+1] != '"' {
+			break
+		}
+		name := string(tag[:i])
+		tag = tag[i+1:]
+
+		// scan quoted string to find value
+		i = 1
+		for i < len(tag) && tag[i] != '"' {
+			if tag[i] == '\\' {
+				i++
+			}
+			i++
+		}
+		if i >= len(tag) {
+			break
+		}
+		qvalue := string(tag[:i+1])
+		tag = tag[i+1:]
+
+		if key == name {
+			value, _ := strconv.Unquote(qvalue)
+			return value
+		}
+	}
+	return ""
+}
+
 // Field returns the i'th struct field.
 func (t *structType) Field(i int) (f StructField) {
 	if i < 0 || i >= len(t.fields) {
@@ -723,7 +783,7 @@ func (t *structType) Field(i int) (f StructField) {
 		f.PkgPath = *p.pkgPath
 	}
 	if p.tag != nil {
-		f.Tag = *p.tag
+		f.Tag = StructTag(*p.tag)
 	}
 	f.Offset = p.offset
 	f.Index = []int{i}

src/pkg/rpc/jsonrpc/all_test.go

@@ -51,9 +51,9 @@ func init() {
 
 func TestServer(t *testing.T) {
 	type addResp struct {
-		Id     interface{} "id"
-		Result Reply       "result"
-		Error  interface{} "error"
+		Id     interface{} `json:"id"`
+		Result Reply       `json:"result"`
+		Error  interface{} `json:"error"`
 	}
 
 	cli, srv := net.Pipe()

src/pkg/rpc/jsonrpc/client.go

@@ -44,9 +44,9 @@ func NewClientCodec(conn io.ReadWriteCloser) rpc.ClientCodec {
 }
 
 type clientRequest struct {
-	Method string         "method"
-	Params [1]interface{} "params"
-	Id     uint64         "id"
+	Method string         `json:"method"`
+	Params [1]interface{} `json:"params"`
+	Id     uint64         `json:"id"`
 }
 
 func (c *clientCodec) WriteRequest(r *rpc.Request, param interface{}) os.Error {
@@ -60,9 +60,9 @@ func (c *clientCodec) WriteRequest(r *rpc.Request, param interface{}) os.Error {
 }
 
 type clientResponse struct {
-	Id     uint64           "id"
-	Result *json.RawMessage "result"
-	Error  interface{}      "error"
+	Id     uint64           `json:"id"`
+	Result *json.RawMessage `json:"result"`
+	Error  interface{}      `json:"error"`
 }
 
 func (r *clientResponse) reset() {

src/pkg/rpc/jsonrpc/server.go

@@ -43,9 +43,9 @@ func NewServerCodec(conn io.ReadWriteCloser) rpc.ServerCodec {
 }
 
 type serverRequest struct {
-	Method string           "method"
-	Params *json.RawMessage "params"
-	Id     *json.RawMessage "id"
+	Method string           `json:"method"`
+	Params *json.RawMessage `json:"params"`
+	Id     *json.RawMessage `json:"id"`
 }
 
 func (r *serverRequest) reset() {
@@ -59,9 +59,9 @@ func (r *serverRequest) reset() {
 }
 
 type serverResponse struct {
-	Id     *json.RawMessage "id"
-	Result interface{}      "result"
-	Error  interface{}      "error"
+	Id     *json.RawMessage `json:"id"`
+	Result interface{}      `json:"result"`
+	Error  interface{}      `json:"error"`
 }
 
 func (c *serverCodec) ReadRequestHeader(r *rpc.Request) os.Error {

src/pkg/xml/embed_test.go

@@ -12,14 +12,14 @@ type C struct {
 }
 
 type A struct {
-	XMLName Name "http://domain a"
+	XMLName Name `xml:"http://domain a"`
 	C
 	B      B
 	FieldA string
 }
 
 type B struct {
-	XMLName Name "b"
+	XMLName Name `xml:"b"`
 	C
 	FieldB string
 }
@@ -65,7 +65,7 @@ func TestEmbedded1(t *testing.T) {
 }
 
 type A2 struct {
-	XMLName Name "http://domain a"
+	XMLName Name `xml:"http://domain a"`
 	XY      string
 	Xy      string
 }
@@ -92,7 +92,7 @@ func TestEmbedded2(t *testing.T) {
 }
 
 type A3 struct {
-	XMLName Name "http://domain a"
+	XMLName Name `xml:"http://domain a"`
 	xy      string
 }
 
@@ -108,7 +108,7 @@ func TestEmbedded3(t *testing.T) {
 }
 
 type A4 struct {
-	XMLName Name "http://domain a"
+	XMLName Name `xml:"http://domain a"`
 	Any     string
 }
 

src/pkg/xml/marshal.go

@@ -108,7 +108,7 @@ func (p *printer) marshalValue(val reflect.Value, name string) os.Error {
 	xmlns := ""
 	if kind == reflect.Struct {
 		if f, ok := typ.FieldByName("XMLName"); ok {
-			if tag := f.Tag; tag != "" {
+			if tag := f.Tag.Get("xml"); tag != "" {
 				if i := strings.Index(tag, " "); i >= 0 {
 					xmlns, name = tag[:i], tag[i+1:]
 				} else {
@@ -132,7 +132,7 @@ func (p *printer) marshalValue(val reflect.Value, name string) os.Error {
 		}
 
 		for i, n := 0, typ.NumField(); i < n; i++ {
-			if f := typ.Field(i); f.PkgPath == "" && f.Tag == "attr" {
+			if f := typ.Field(i); f.PkgPath == "" && f.Tag.Get("xml") == "attr" {
 				if f.Type.Kind() == reflect.String {
 					if str := val.Field(i).String(); str != "" {
 						p.WriteByte(' ')
@@ -173,7 +173,7 @@ func (p *printer) marshalValue(val reflect.Value, name string) os.Error {
 		for i, n := 0, val.NumField(); i < n; i++ {
 			if f := typ.Field(i); f.Name != "XMLName" && f.PkgPath == "" {
 				name := f.Name
-				switch tag := f.Tag; tag {
+				switch tag := f.Tag.Get("xml"); tag {
 				case "":
 				case "chardata":
 					if tk := f.Type.Kind(); tk == reflect.String {

src/pkg/xml/marshal_test.go

@@ -22,18 +22,18 @@ const (
 )
 
 type Passenger struct {
-	Name   []string "name"
-	Weight float32  "weight"
+	Name   []string `xml:"name"`
+	Weight float32  `xml:"weight"`
 }
 
 type Ship struct {
-	XMLName Name "spaceship"
+	XMLName Name `xml:"spaceship"`
 
-	Name      string       "attr"
-	Pilot     string       "attr"
-	Drive     DriveType    "drive"
-	Age       uint         "age"
-	Passenger []*Passenger "passenger"
+	Name      string       `xml:"attr"`
+	Pilot     string       `xml:"attr"`
+	Drive     DriveType    `xml:"drive"`
+	Age       uint         `xml:"age"`
+	Passenger []*Passenger `xml:"passenger"`
 	secret    string
 }
 
@@ -46,22 +46,22 @@ func (rx RawXML) MarshalXML() ([]byte, os.Error) {
 type NamedType string
 
 type Port struct {
-	XMLName Name   "port"
-	Type    string "attr"
-	Number  string "chardata"
+	XMLName Name   `xml:"port"`
+	Type    string `xml:"attr"`
+	Number  string `xml:"chardata"`
 }
 
 type Domain struct {
-	XMLName Name   "domain"
-	Country string "attr"
-	Name    []byte "chardata"
+	XMLName Name   `xml:"domain"`
+	Country string `xml:"attr"`
+	Name    []byte `xml:"chardata"`
 }
 
 type SecretAgent struct {
-	XMLName   Name   "agent"
-	Handle    string "attr"
+	XMLName   Name   `xml:"agent"`
+	Handle    string `xml:"attr"`
 	Identity  string
-	Obfuscate string "innerxml"
+	Obfuscate string `xml:"innerxml"`
 }
 
 var nilStruct *Ship

src/pkg/xml/read.go

@@ -31,16 +31,16 @@ import (
 // For example, given these definitions:
 //
 //	type Email struct {
-//		Where string "attr"
+//		Where string `xml:"attr"`
 //		Addr  string
 //	}
 //
 //	type Result struct {
-//		XMLName xml.Name "result"
+//		XMLName xml.Name `xml:"result"`
 //		Name	string
 //		Phone	string
 //		Email	[]Email
-//		Groups  []string "group>value"
+//		Groups  []string `xml:"group>value"`
 //	}
 //
 //	result := Result{Name: "name", Phone: "phone", Email: nil}
@@ -79,11 +79,13 @@ import (
 // Groups was assigned considering the element path provided in the
 // field tag.
 //
-// Because Unmarshal uses the reflect package, it can only
-// assign to upper case fields.  Unmarshal uses a case-insensitive
+// Because Unmarshal uses the reflect package, it can only assign
+// to exported (upper case) fields.  Unmarshal uses a case-insensitive
 // comparison to match XML element names to struct field names.
 //
-// Unmarshal maps an XML element to a struct using the following rules:
+// Unmarshal maps an XML element to a struct using the following rules.
+// In the rules, the tag of a field refers to the value associated with the
+// key 'xml' in the struct field's tag (see the example above).
 //
 //   * If the struct has a field of type []byte or string with tag "innerxml",
 //      Unmarshal accumulates the raw XML nested inside the element
@@ -92,9 +94,9 @@ import (
 //   * If the struct has a field named XMLName of type xml.Name,
 //      Unmarshal records the element name in that field.
 //
-//   * If the XMLName field has an associated tag string of the form
-//      "tag" or "namespace-URL tag", the XML element must have
-//      the given tag (and, optionally, name space) or else Unmarshal
+//   * If the XMLName field has an associated tag of the form
+//      "name" or "namespace-URL name", the XML element must have
+//      the given name (and, optionally, name space) or else Unmarshal
 //      returns an error.
 //
 //   * If the XML element has an attribute whose name matches a
@@ -112,14 +114,14 @@ import (
 //      field, the comments are discarded.
 //
 //   * If the XML element contains a sub-element whose name matches
-//      the prefix of a struct field tag formatted as "a>b>c", unmarshal
+//      the prefix of a tag formatted as "a>b>c", unmarshal
 //      will descend into the XML structure looking for elements with the
 //      given names, and will map the innermost elements to that struct field.
-//      A struct field tag starting with ">" is equivalent to one starting
+//      A tag starting with ">" is equivalent to one starting
 //      with the field name followed by ">".
 //
 //   * If the XML element contains a sub-element whose name
-//      matches a struct field whose tag is neither "attr" nor "chardata",
+//      matches a field whose tag is neither "attr" nor "chardata",
 //      Unmarshal maps the sub-element to that struct field.
 //      Otherwise, if the struct has a field named Any, unmarshal
 //      maps the sub-element to that struct field.
@@ -297,8 +299,7 @@ func (p *Parser) unmarshal(val reflect.Value, start *StartElement) os.Error {
 		// Assign name.
 		if f, ok := typ.FieldByName("XMLName"); ok {
 			// Validate element name.
-			if f.Tag != "" {
-				tag := f.Tag
+			if tag := f.Tag.Get("xml"); tag != "" {
 				ns := ""
 				i := strings.LastIndex(tag, " ")
 				if i >= 0 {
@@ -330,7 +331,7 @@ func (p *Parser) unmarshal(val reflect.Value, start *StartElement) os.Error {
 		// Also, determine whether we need to save character data or comments.
 		for i, n := 0, typ.NumField(); i < n; i++ {
 			f := typ.Field(i)
-			switch f.Tag {
+			switch f.Tag.Get("xml") {
 			case "attr":
 				strv := sv.FieldByIndex(f.Index)
 				// Look for attribute.
@@ -366,15 +367,15 @@ func (p *Parser) unmarshal(val reflect.Value, start *StartElement) os.Error {
 				}
 
 			default:
-				if strings.Contains(f.Tag, ">") {
+				if tag := f.Tag.Get("xml"); strings.Contains(tag, ">") {
 					if fieldPaths == nil {
 						fieldPaths = make(map[string]pathInfo)
 					}
-					path := strings.ToLower(f.Tag)
-					if strings.HasPrefix(f.Tag, ">") {
+					path := strings.ToLower(tag)
+					if strings.HasPrefix(tag, ">") {
 						path = strings.ToLower(f.Name) + path
 					}
-					if strings.HasSuffix(f.Tag, ">") {
+					if strings.HasSuffix(tag, ">") {
 						path = path[:len(path)-1]
 					}
 					err := addFieldPath(sv, fieldPaths, path, f.Index)
@@ -574,7 +575,7 @@ func tagError(sv reflect.Value, idx1 []int, idx2 []int) os.Error {
 	t := sv.Type()
 	f1 := t.FieldByIndex(idx1)
 	f2 := t.FieldByIndex(idx2)
-	return &TagPathError{t, f1.Name, f1.Tag, f2.Name, f2.Tag}
+	return &TagPathError{t, f1.Name, f1.Tag.Get("xml"), f2.Name, f2.Tag.Get("xml")}
 }
 
 // unmarshalPaths walks down an XML structure looking for

src/pkg/xml/read_test.go

@@ -78,7 +78,7 @@ not being used from outside intra_region_diff.py.
 </summary></entry></feed> 	   `
 
 type Feed struct {
-	XMLName Name "http://www.w3.org/2005/Atom feed"
+	XMLName Name `xml:"http://www.w3.org/2005/Atom feed"`
 	Title   string
 	Id      string
 	Link    []Link
@@ -97,20 +97,20 @@ type Entry struct {
 }
 
 type Link struct {
-	Rel  string "attr"
-	Href string "attr"
+	Rel  string `xml:"attr"`
+	Href string `xml:"attr"`
 }
 
 type Person struct {
 	Name     string
 	URI      string
 	Email    string
-	InnerXML string "innerxml"
+	InnerXML string `xml:"innerxml"`
 }
 
 type Text struct {
-	Type string "attr"
-	Body string "chardata"
+	Type string `xml:"attr"`
+	Body string `xml:"chardata"`
 }
 
 type Time string
@@ -255,18 +255,18 @@ type PathTestItem struct {
 }
 
 type PathTestA struct {
-	Items         []PathTestItem ">item1"
+	Items         []PathTestItem `xml:">item1"`
 	Before, After string
 }
 
 type PathTestB struct {
-	Other         []PathTestItem "items>Item1"
+	Other         []PathTestItem `xml:"items>Item1"`
 	Before, After string
 }
 
 type PathTestC struct {
-	Values1       []string "items>item1>value"
-	Values2       []string "items>item2>value"
+	Values1       []string `xml:"items>item1>value"`
+	Values2       []string `xml:"items>item2>value"`
 	Before, After string
 }
 
@@ -275,7 +275,7 @@ type PathTestSet struct {
 }
 
 type PathTestD struct {
-	Other         PathTestSet "items>"
+	Other         PathTestSet `xml:"items>"`
 	Before, After string
 }
 
@@ -299,15 +299,15 @@ func TestUnmarshalPaths(t *testing.T) {
 }
 
 type BadPathTestA struct {
-	First  string "items>item1"
-	Other  string "items>item2"
-	Second string "items>"
+	First  string `xml:"items>item1"`
+	Other  string `xml:"items>item2"`
+	Second string `xml:"items>"`
 }
 
 type BadPathTestB struct {
-	Other  string "items>item2>value"
-	First  string "items>item1"
-	Second string "items>item1>value"
+	Other  string `xml:"items>item2>value"`
+	First  string `xml:"items>item1"`
+	Second string `xml:"items>item1>value"`
 }
 
 var badPathTests = []struct {
@@ -342,13 +342,13 @@ type AttrTest struct {
 }
 
 type Test1 struct {
-	Int   int     "attr"
-	Float float64 "attr"
-	Uint8 uint8   "attr"
+	Int   int     `xml:"attr"`
+	Float float64 `xml:"attr"`
+	Uint8 uint8   `xml:"attr"`
 }
 
 type Test2 struct {
-	Bool bool "attr"
+	Bool bool `xml:"attr"`
 }
 
 const attrString = `