error: Remove the UnmarshalData method and ErrNoData.

Now that the data field is exported from the type, this helper method is no
longer carrying its weight.

error.go

@@ -24,15 +24,6 @@ func (e Error) HasData() bool { return len(e.Data) != 0 }
 // ErrCode trivially satisfies the code.ErrCoder interface for an *Error.
 func (e Error) ErrCode() code.Code { return e.Code }
 
-// UnmarshalData decodes the error data associated with e into v.  It reports
-// ErrNoData without modifying v if there was no data message attached to e.
-func (e Error) UnmarshalData(v interface{}) error {
-	if !e.HasData() {
-		return ErrNoData
-	}
-	return json.Unmarshal(e.Data, v)
-}
-
 // MarshalJSON implements the json.Marshaler interface for Error values.
 func (e Error) MarshalJSON() ([]byte, error) {
 	return json.Marshal(jerror{C: int32(e.Code), M: e.Message, D: e.Data})
@@ -50,9 +41,6 @@ func (e *Error) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
-// ErrNoData indicates that there are no data to unmarshal.
-var ErrNoData = errors.New("no data to unmarshal")
-
 // errServerStopped is returned by Server.Wait when the server was shut down by
 // an explicit call to its Stop method or orderly termination of its channel.
 var errServerStopped = errors.New("the server has been stopped")

jrpc2_test.go

@@ -291,11 +291,6 @@ func TestErrorOnly(t *testing.T) {
 			t.Errorf("ErrorOnly: got %v, want *Error", err)
 		} else if e.Code != 1 || e.Message != errMessage {
 			t.Errorf("ErrorOnly: got (%s, %s), want (1, %s)", e.Code, e.Message, errMessage)
-		} else {
-			var data json.RawMessage
-			if err, want := e.UnmarshalData(&data), jrpc2.ErrNoData; err != want {
-				t.Errorf("UnmarshalData: got %#q, %v, want %v", string(data), err, want)
-			}
 		}
 	})
 	t.Run("CallExpectingOK", func(t *testing.T) {
@@ -467,10 +462,7 @@ func TestErrors(t *testing.T) {
 		if e.Message != errMessage {
 			t.Errorf("Error message: got %q, want %q", e.Message, errMessage)
 		}
-		var data json.RawMessage
-		if err := e.UnmarshalData(&data); err != nil {
-			t.Errorf("Unmarshaling error data: %v", err)
-		} else if s := string(data); s != errData {
+		if s := string(e.Data); s != errData {
 			t.Errorf("Error data: got %q, want %q", s, errData)
 		}
 	} else {