encoding/xml: Marshal ",any" fields

Fixes #3559.

This makes Marshal handle fields marked ",any" instead of ignoring
them. That makes Marshal more symmetrical with Unmarshal, which seems
to have been a design goal.

Note some test cases were changed, because this patch changes
marshalling behavior. I think the previous behavior was buggy, but
there's still a backward-compatibility question to consider.

R=rsc
CC=golang-dev, n13m3y3r
https://golang.org/cl/6938068

src/pkg/encoding/xml/marshal.go

@@ -273,7 +273,7 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 	s := parentStack{printer: p}
 	for i := range tinfo.fields {
 		finfo := &tinfo.fields[i]
-		if finfo.flags&(fAttr|fAny) != 0 {
+		if finfo.flags&(fAttr) != 0 {
 			continue
 		}
 		vf := finfo.value(val)
@@ -340,7 +340,7 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 				continue
 			}
 
-		case fElement:
+		case fElement, fElement | fAny:
 			s.trim(finfo.parents)
 			if len(finfo.parents) > len(s.stack) {
 				if vf.Kind() != reflect.Ptr && vf.Kind() != reflect.Interface || !vf.IsNil() {

src/pkg/encoding/xml/marshal_test.go

@@ -188,6 +188,18 @@ type AnyTest struct {
 	AnyField AnyHolder `xml:",any"`
 }
 
+type AnyOmitTest struct {
+	XMLName  struct{}   `xml:"a"`
+	Nested   string     `xml:"nested>value"`
+	AnyField *AnyHolder `xml:",any,omitempty"`
+}
+
+type AnySliceTest struct {
+	XMLName  struct{}    `xml:"a"`
+	Nested   string      `xml:"nested>value"`
+	AnyField []AnyHolder `xml:",any"`
+}
+
 type AnyHolder struct {
 	XMLName Name
 	XML     string `xml:",innerxml"`
@@ -652,12 +664,43 @@ var marshalTests = []struct {
 				XML:     "<sub>unknown</sub>",
 			},
 		},
-		UnmarshalOnly: true,
 	},
 	{
-		Value:       &AnyTest{Nested: "known", AnyField: AnyHolder{XML: "<unknown/>"}},
+		Value: &AnyTest{Nested: "known",
-		ExpectXML:   `<a><nested><value>known</value></nested></a>`,
+			AnyField: AnyHolder{
-		MarshalOnly: true,
+				XML:     "<unknown/>",
+				XMLName: Name{Local: "AnyField"},
+			},
+		},
+		ExpectXML: `<a><nested><value>known</value></nested><AnyField><unknown/></AnyField></a>`,
+	},
+	{
+		ExpectXML: `<a><nested><value>b</value></nested></a>`,
+		Value: &AnyOmitTest{
+			Nested: "b",
+		},
+	},
+	{
+		ExpectXML: `<a><nested><value>b</value></nested><c><d>e</d></c><g xmlns="f"><h>i</h></g></a>`,
+		Value: &AnySliceTest{
+			Nested: "b",
+			AnyField: []AnyHolder{
+				{
+					XMLName: Name{Local: "c"},
+					XML:     "<d>e</d>",
+				},
+				{
+					XMLName: Name{Space: "f", Local: "g"},
+					XML:     "<h>i</h>",
+				},
+			},
+		},
+	},
+	{
+		ExpectXML: `<a><nested><value>b</value></nested></a>`,
+		Value: &AnySliceTest{
+			Nested: "b",
+		},
 	},
 
 	// Test recursive types.
@@ -690,15 +733,17 @@ var marshalTests = []struct {
 
 	// Test escaping.
 	{
-		ExpectXML: `<a><nested><value>dquote: &#34;; squote: &#39;; ampersand: &amp;; less: &lt;; greater: &gt;;</value></nested></a>`,
+		ExpectXML: `<a><nested><value>dquote: &#34;; squote: &#39;; ampersand: &amp;; less: &lt;; greater: &gt;;</value></nested><empty></empty></a>`,
 		Value: &AnyTest{
-			Nested: `dquote: "; squote: '; ampersand: &; less: <; greater: >;`,
+			Nested:   `dquote: "; squote: '; ampersand: &; less: <; greater: >;`,
+			AnyField: AnyHolder{XMLName: Name{Local: "empty"}},
 		},
 	},
 	{
-		ExpectXML: `<a><nested><value>newline: &#xA;; cr: &#xD;; tab: &#x9;;</value></nested></a>`,
+		ExpectXML: `<a><nested><value>newline: &#xA;; cr: &#xD;; tab: &#x9;;</value></nested><AnyField></AnyField></a>`,
 		Value: &AnyTest{
-			Nested: "newline: \n; cr: \r; tab: \t;",
+			Nested:   "newline: \n; cr: \r; tab: \t;",
+			AnyField: AnyHolder{XMLName: Name{Local: "AnyField"}},
 		},
 	},
 	{

src/pkg/encoding/xml/read.go

@@ -279,7 +279,7 @@ func (p *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
 					saveComment = finfo.value(sv)
 				}
 
-			case fAny:
+			case fAny, fAny | fElement:
 				if !saveAny.IsValid() {
 					saveAny = finfo.value(sv)
 				}

src/pkg/encoding/xml/typeinfo.go

@@ -154,6 +154,9 @@ func structFieldInfo(typ reflect.Type, f *reflect.StructField) (*fieldInfo, erro
 			// This will also catch multiple modes in a single field.
 			valid = false
 		}
+		if finfo.flags&fMode == fAny {
+			finfo.flags |= fElement
+		}
 		if finfo.flags&fOmitEmpty != 0 && finfo.flags&(fElement|fAttr) == 0 {
 			valid = false
 		}