time: correct time.AppendText's error message

"time.AppendText" returns error messages that start with the prefix
"time.MarshalText: " which seems confusion.

Now correct the message prefix to "time.AppendText: " and add a test
to prevent regression.

Change-Id: I5742c9c3ed802eb79c65d459910deae4f3652ffd
GitHub-Last-Rev: ce96559
GitHub-Pull-Request: #69914
Reviewed-on: https://go-review.googlesource.com/c/go/+/620597
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: apocelipes

src/time/time.go

@@ -1584,24 +1584,28 @@ func (t *Time) UnmarshalJSON(data []byte) error {
 	return err
 }
 
+func (t Time) appendTo(b []byte, errPrefix string) ([]byte, error) {
+	b, err := t.appendStrictRFC3339(b)
+	if err != nil {
+		return nil, errors.New(errPrefix + err.Error())
+	}
+	return b, nil
+}
+
 // AppendText implements the [encoding.TextAppender] interface.
 // The time is formatted in RFC 3339 format with sub-second precision.
 // If the timestamp cannot be represented as valid RFC 3339
 // (e.g., the year is out of range), then an error is returned.
 func (t Time) AppendText(b []byte) ([]byte, error) {
-	b, err := t.appendStrictRFC3339(b)
-	if err != nil {
-		return nil, errors.New("Time.MarshalText: " + err.Error())
-	}
-	return b, nil
+	return t.appendTo(b, "Time.AppendText: ")
 }
 
 // MarshalText implements the [encoding.TextMarshaler] interface. The output
 // matches that of calling the [Time.AppendText] method.
 //
 // See [Time.AppendText] for more information.
 func (t Time) MarshalText() ([]byte, error) {
-	return t.AppendText(make([]byte, 0, len(RFC3339Nano)))
+	return t.appendTo(make([]byte, 0, len(RFC3339Nano)), "Time.MarshalText: ")
 }
 
 // UnmarshalText implements the [encoding.TextUnmarshaler] interface.

src/time/time_test.go

@@ -915,6 +915,16 @@ func TestMarshalInvalidTimes(t *testing.T) {
 		case err == nil || err.Error() != want:
 			t.Errorf("(%v).MarshalText() error = %v, want %v", tt.time, err, want)
 		}
+
+		buf := make([]byte, 0, 64)
+		want = strings.ReplaceAll(tt.want, "MarshalJSON", "AppendText")
+		b, err = tt.time.AppendText(buf)
+		switch {
+		case b != nil:
+			t.Errorf("(%v).AppendText() = %q, want nil", tt.time, b)
+		case err == nil || err.Error() != want:
+			t.Errorf("(%v).AppendText() error = %v, want %v", tt.time, err, want)
+		}
 	}
 }