encoding/json: disable anonymous fields

We should, after Go 1, make them work the same as
package xml, that is, make them appear in the outer
struct.  For now turn them off so that people do not
depend on the old behavior.

Fixing them is issue 3069.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5656102

src/pkg/encoding/json/decode.go

@@ -496,6 +496,12 @@ func (d *decodeState) object(v reflect.Value) {
 					// Pretend this field doesn't exist.
 					continue
 				}
+				if sf.Anonymous {
+					// Pretend this field doesn't exist,
+					// so that we can do a good job with
+					// these in a later version.
+					continue
+				}
 				// First, tag match
 				tagName, _ := parseTag(tag)
 				if tagName == key {
@@ -963,3 +969,11 @@ func unquoteBytes(s []byte) (t []byte, ok bool) {
 	}
 	return b[0:w], true
 }
+
+// The following is issue 3069.
+
+// BUG(rsc): This package ignores anonymous (embedded) struct fields
+// during encoding and decoding.  A future version may assign meaning
+// to them.  To force an anonymous field to be ignored in all future
+// versions of this package, use an explicit `json:"-"` tag in the struct
+// definition.

src/pkg/encoding/json/decode_test.go

@@ -619,3 +619,32 @@ func TestRefUnmarshal(t *testing.T) {
 		t.Errorf("got %+v, want %+v", got, want)
 	}
 }
+
+// Test that anonymous fields are ignored.
+// We may assign meaning to them later.
+func TestAnonymous(t *testing.T) {
+	type S struct {
+		T
+		N int
+	}
+
+	data, err := Marshal(new(S))
+	if err != nil {
+		t.Fatalf("Marshal: %v", err)
+	}
+	want := `{"N":0}`
+	if string(data) != want {
+		t.Fatalf("Marshal = %#q, want %#q", string(data), want)
+	}
+
+	var s S
+	if err := Unmarshal([]byte(`{"T": 1, "T": {"Y": 1}, "N": 2}`), &s); err != nil {
+		t.Fatalf("Unmarshal: %v", err)
+	}
+	if s.N != 2 {
+		t.Fatal("Unmarshal: did not set N")
+	}
+	if s.T.Y != 0 {
+		t.Fatal("Unmarshal: did set T.Y")
+	}
+}

src/pkg/encoding/json/encode.go

@@ -538,6 +538,11 @@ func encodeFields(t reflect.Type) []encodeField {
 		if f.PkgPath != "" {
 			continue
 		}
+		if f.Anonymous {
+			// We want to do a better job with these later,
+			// so for now pretend they don't exist.
+			continue
+		}
 		var ef encodeField
 		ef.i = i
 		ef.tag = f.Name