Fix decoding of interface type

decode_interface.go

@@ -1,6 +1,7 @@
 package json
 
 import (
+	"encoding"
 	"reflect"
 	"unsafe"
 )
@@ -39,11 +40,56 @@ var (
 	)
 )
 
+func decodeWithUnmarshaler(s *stream, unmarshaler Unmarshaler) error {
+	start := s.cursor
+	if err := s.skipValue(); err != nil {
+		return err
+	}
+	src := s.buf[start:s.cursor]
+	dst := make([]byte, len(src))
+	copy(dst, src)
+
+	if err := unmarshaler.UnmarshalJSON(dst); err != nil {
+		return err
+	}
+	return nil
+}
+
+func decodeWithTextUnmarshaler(s *stream, unmarshaler encoding.TextUnmarshaler) error {
+	start := s.cursor
+	if err := s.skipValue(); err != nil {
+		return err
+	}
+	src := s.buf[start:s.cursor]
+	dst := make([]byte, len(src))
+	copy(dst, src)
+
+	if err := unmarshaler.UnmarshalText(dst); err != nil {
+		return err
+	}
+	return nil
+}
+
 func (d *interfaceDecoder) decodeStream(s *stream, p unsafe.Pointer) error {
 	s.skipWhiteSpace()
 	for {
 		switch s.char() {
 		case '{':
+			runtimeInterfaceValue := *(*interface{})(unsafe.Pointer(&interfaceHeader{
+				typ: d.typ,
+				ptr: p,
+			}))
+			rv := reflect.ValueOf(runtimeInterfaceValue)
+			if rv.NumMethod() > 0 && rv.CanInterface() {
+				if u, ok := rv.Interface().(Unmarshaler); ok {
+					return decodeWithUnmarshaler(s, u)
+				}
+				if u, ok := rv.Interface().(encoding.TextUnmarshaler); ok {
+					return decodeWithTextUnmarshaler(s, u)
+				}
+				return nil
+			}
+			// empty interface
 			var v map[string]interface{}
 			ptr := unsafe.Pointer(&v)
 			if err := newMapDecoder(