encoding/xml: support ignoring fields with "-"

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5564045

src/pkg/encoding/xml/marshal_test.go

@@ -188,6 +188,10 @@ type PresenceTest struct {
 	Exists *struct{}
 }
 
+type IgnoreTest struct {
+	PublicSecret string `xml:"-"`
+}
+
 type MyBytes []byte
 
 type Data struct {
@@ -592,6 +596,22 @@ var marshalTests = []struct {
 		},
 		ExpectXML: `<RecurseA><A>a1</A><B><A><A>a2</A></A><B>b1</B></B></RecurseA>`,
 	},
+
+	// Test ignoring fields via "-" tag
+	{
+		ExpectXML: `<IgnoreTest></IgnoreTest>`,
+		Value:     &IgnoreTest{},
+	},
+	{
+		ExpectXML:   `<IgnoreTest></IgnoreTest>`,
+		Value:       &IgnoreTest{PublicSecret: "can't tell"},
+		MarshalOnly: true,
+	},
+	{
+		ExpectXML:     `<IgnoreTest><PublicSecret>ignore me</PublicSecret></IgnoreTest>`,
+		Value:         &IgnoreTest{},
+		UnmarshalOnly: true,
+	},
 }
 
 func TestMarshal(t *testing.T) {

src/pkg/encoding/xml/typeinfo.go

@@ -37,7 +37,6 @@ const (
 	fAny
 
 	// TODO:
-	//fIgnore
 	//fOmitEmpty
 
 	fMode = fElement | fAttr | fCharData | fInnerXml | fComment | fAny
@@ -62,7 +61,7 @@ func getTypeInfo(typ reflect.Type) (*typeInfo, error) {
 		n := typ.NumField()
 		for i := 0; i < n; i++ {
 			f := typ.Field(i)
-			if f.PkgPath != "" {
+			if f.PkgPath != "" || f.Tag.Get("xml") == "-" {
 				continue // Private field
 			}