encoding/xml: fix 'unsupported type' error on interface{} attributes

src/encoding/xml/marshal.go

@@ -555,6 +555,15 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 
 // marshalAttr marshals an attribute with the given name and value, adding to start.Attr.
 func (p *printer) marshalAttr(start *StartElement, name Name, val reflect.Value) error {
+	// Dereference or skip nil pointer, interface values.
+	switch val.Kind() {
+	case reflect.Pointer, reflect.Interface:
+		if val.IsNil() {
+			return nil
+		}
+		val = val.Elem()
+	}
+
 	if val.CanInterface() && val.Type().Implements(marshalerAttrType) {
 		attr, err := val.Interface().(MarshalerAttr).MarshalXMLAttr(name)
 		if err != nil {
@@ -601,15 +610,6 @@ func (p *printer) marshalAttr(start *StartElement, name Name, val reflect.Value)
 		}
 	}
 
-	// Dereference or skip nil pointer, interface values.
-	switch val.Kind() {
-	case reflect.Pointer, reflect.Interface:
-		if val.IsNil() {
-			return nil
-		}
-		val = val.Elem()
-	}
-
 	// Walk slices.
 	if val.Kind() == reflect.Slice && val.Type().Elem().Kind() != reflect.Uint8 {
 		n := val.Len()

src/encoding/xml/marshal_test.go

@@ -343,6 +343,10 @@ type MarshalerStruct struct {
 	Foo MyMarshalerAttrTest `xml:",attr"`
 }
 
+type IMarshalerStruct struct {
+	Foo interface{} `xml:",attr"`
+}
+
 type InnerStruct struct {
 	XMLName Name `xml:"testns outer"`
 }
@@ -1252,6 +1256,11 @@ var marshalTests = []struct {
 		ExpectXML: `<MarshalerStruct Foo="hello world"></MarshalerStruct>`,
 		Value:     &MarshalerStruct{},
 	},
+	{
+		ExpectXML:   `<IMarshalerStruct Foo="hello world"></IMarshalerStruct>`,
+		Value:       &IMarshalerStruct{Foo: &MyMarshalerAttrTest{}},
+		MarshalOnly: true,
+	},
 	{
 		ExpectXML: `<outer xmlns="testns" int="10"></outer>`,
 		Value:     &OuterStruct{IntAttr: 10},