encoding/xml: add, support Marshaler interface

See golang.org/s/go12xml for design.

Repeat of CL 12603044, which was submitted accidentally
and then rolled back.

Fixes #2771.
Fixes #4169.
Fixes #5975.
Fixes #6125.

R=golang-dev
CC=golang-dev
https://golang.org/cl/12919043

api/go1.1.txt

@@ -92,7 +92,6 @@ pkg encoding/json, method (Number) String() string
 pkg encoding/json, type Number string
 pkg encoding/xml, func EscapeText(io.Writer, []uint8) error
 pkg encoding/xml, method (*Encoder) Indent(string, string)
-pkg encoding/xml, method (Encoder) ReadFrom(io.Reader) (int64, error)
 pkg encoding/xml, type Decoder struct, DefaultSpace string
 pkg go/ast, func NewCommentMap(*token.FileSet, Node, []*CommentGroup) CommentMap
 pkg go/ast, method (CommentMap) Comments() []*CommentGroup

api/go1.txt

@@ -2425,13 +2425,6 @@ pkg encoding/xml, method (*UnsupportedTypeError) Error() string
 pkg encoding/xml, method (CharData) Copy() CharData
 pkg encoding/xml, method (Comment) Copy() Comment
 pkg encoding/xml, method (Directive) Copy() Directive
-pkg encoding/xml, method (Encoder) Available() int
-pkg encoding/xml, method (Encoder) Buffered() int
-pkg encoding/xml, method (Encoder) Flush() error
-pkg encoding/xml, method (Encoder) Write([]uint8) (int, error)
-pkg encoding/xml, method (Encoder) WriteByte(uint8) error
-pkg encoding/xml, method (Encoder) WriteRune(int32) (int, error)
-pkg encoding/xml, method (Encoder) WriteString(string) (int, error)
 pkg encoding/xml, method (ProcInst) Copy() ProcInst
 pkg encoding/xml, method (StartElement) Copy() StartElement
 pkg encoding/xml, method (UnmarshalError) Error() string

src/pkg/encoding/xml/marshal_test.go

@@ -289,6 +289,31 @@ type ChardataEmptyTest struct {
 	Contents *string `xml:",chardata"`
 }
 
+type MyMarshalerTest struct {
+}
+
+var _ Marshaler = (*MyMarshalerTest)(nil)
+
+func (m *MyMarshalerTest) MarshalXML(e *Encoder, start StartElement) error {
+	e.EncodeToken(start)
+	e.EncodeToken(CharData([]byte("hello world")))
+	e.EncodeToken(EndElement{start.Name})
+	return nil
+}
+
+type MyMarshalerAttrTest struct {
+}
+
+var _ MarshalerAttr = (*MyMarshalerAttrTest)(nil)
+
+func (m *MyMarshalerAttrTest) MarshalXMLAttr(name Name) (Attr, error) {
+	return Attr{name, "hello world"}, nil
+}
+
+type MarshalerStruct struct {
+	Foo MyMarshalerAttrTest `xml:",attr"`
+}
+
 var (
 	nameAttr     = "Sarah"
 	ageAttr      = uint(12)
@@ -844,6 +869,15 @@ var marshalTests = []struct {
 		ExpectXML: `<Strings><A></A></Strings>`,
 		Value:     &Strings{},
 	},
+	// Custom marshalers.
+	{
+		ExpectXML: `<MyMarshalerTest>hello world</MyMarshalerTest>`,
+		Value:     &MyMarshalerTest{},
+	},
+	{
+		ExpectXML: `<MarshalerStruct Foo="hello world"></MarshalerStruct>`,
+		Value:     &MarshalerStruct{},
+	},
 }
 
 func TestMarshal(t *testing.T) {