unmarshal: support encoding.TextUnmarshaler (#375)

* unmarshal: support encoding.TextUnmarshaler

This PR adds support for decoding fields of primitive types that implement
encoding.TextUnmarshaler by calling the custom method.

Fields in anonymous structs are not supported at this point.

Co-authored-by: Lorenz Bauer <lmb@users.noreply.github.com>

marshal.go

@@ -71,6 +71,7 @@ const (
 var timeType = reflect.TypeOf(time.Time{})
 var marshalerType = reflect.TypeOf(new(Marshaler)).Elem()
 var textMarshalerType = reflect.TypeOf(new(encoding.TextMarshaler)).Elem()
+var textUnmarshalerType = reflect.TypeOf(new(encoding.TextUnmarshaler)).Elem()
 var localDateType = reflect.TypeOf(LocalDate{})
 var localTimeType = reflect.TypeOf(LocalTime{})
 var localDateTimeType = reflect.TypeOf(LocalDateTime{})
@@ -155,6 +156,14 @@ func callTextMarshaler(mval reflect.Value) ([]byte, error) {
 	return mval.Interface().(encoding.TextMarshaler).MarshalText()
 }
 
+func isTextUnmarshaler(mtype reflect.Type) bool {
+	return mtype.Implements(textUnmarshalerType)
+}
+
+func callTextUnmarshaler(mval reflect.Value, text []byte) error {
+	return mval.Interface().(encoding.TextUnmarshaler).UnmarshalText(text)
+}
+
 // Marshaler is the interface implemented by types that
 // can marshal themselves into valid TOML.
 type Marshaler interface {
@@ -866,6 +875,14 @@ func (d *Decoder) valueFromToml(mtype reflect.Type, tval interface{}, mval1 *ref
 		return reflect.ValueOf(nil), fmt.Errorf("Can't convert %v(%T) to a slice", tval, tval)
 	default:
 		d.visitor.visit()
+		// Check if pointer to value implements the encoding.TextUnmarshaler.
+		if mvalPtr := reflect.New(mtype); isTextUnmarshaler(mvalPtr.Type()) && !isTimeType(mtype) {
+			if err := d.unmarshalText(tval, mvalPtr); err != nil {
+				return reflect.ValueOf(nil), fmt.Errorf("unmarshal text: %v", err)
+			}
+			return mvalPtr.Elem(), nil
+		}
+
 		switch mtype.Kind() {
 		case reflect.Bool, reflect.Struct:
 			val := reflect.ValueOf(tval)
@@ -983,6 +1000,12 @@ func (d *Decoder) unwrapPointer(mtype reflect.Type, tval interface{}, mval1 *ref
 	return mval, nil
 }
 
+func (d *Decoder) unmarshalText(tval interface{}, mval reflect.Value) error {
+	var buf bytes.Buffer
+	fmt.Fprint(&buf, tval)
+	return callTextUnmarshaler(mval, buf.Bytes())
+}
+
 func tomlOptions(vf reflect.StructField, an annotation) tomlOpts {
 	tag := vf.Tag.Get(an.tag)
 	parse := strings.Split(tag, ",")