Support custom unmarshaler (#394)

Co-authored-by: Thomas Pelletier <pelletier.thomas@gmail.com>

marshal.go

@@ -70,6 +70,7 @@ const (
 
 var timeType = reflect.TypeOf(time.Time{})
 var marshalerType = reflect.TypeOf(new(Marshaler)).Elem()
+var unmarshalerType = reflect.TypeOf(new(Unmarshaler)).Elem()
 var textMarshalerType = reflect.TypeOf(new(encoding.TextMarshaler)).Elem()
 var textUnmarshalerType = reflect.TypeOf(new(encoding.TextUnmarshaler)).Elem()
 var localDateType = reflect.TypeOf(LocalDate{})
@@ -156,6 +157,14 @@ func callTextMarshaler(mval reflect.Value) ([]byte, error) {
 	return mval.Interface().(encoding.TextMarshaler).MarshalText()
 }
 
+func isCustomUnmarshaler(mtype reflect.Type) bool {
+	return mtype.Implements(unmarshalerType)
+}
+
+func callCustomUnmarshaler(mval reflect.Value, tval interface{}) error {
+	return mval.Interface().(Unmarshaler).UnmarshalTOML(tval)
+}
+
 func isTextUnmarshaler(mtype reflect.Type) bool {
 	return mtype.Implements(textUnmarshalerType)
 }
@@ -170,6 +179,12 @@ type Marshaler interface {
 	MarshalTOML() ([]byte, error)
 }
 
+// Unmarshaler is the interface implemented by types that
+// can unmarshal a TOML description of themselves.
+type Unmarshaler interface {
+	UnmarshalTOML(interface{}) error
+}
+
 /*
 Marshal returns the TOML encoding of v.  Behavior is similar to the Go json
 encoder, except that there is no concept of a Marshaler interface or MarshalTOML
@@ -676,6 +691,17 @@ func (d *Decoder) valueFromTree(mtype reflect.Type, tval *Tree, mval1 *reflect.V
 	if mtype.Kind() == reflect.Ptr {
 		return d.unwrapPointer(mtype, tval, mval1)
 	}
+
+	// Check if pointer to value implements the Unmarshaler interface.
+	if mvalPtr := reflect.New(mtype); isCustomUnmarshaler(mvalPtr.Type()) {
+		d.visitor.visitAll()
+
+		if err := callCustomUnmarshaler(mvalPtr, tval.ToMap()); err != nil {
+			return reflect.ValueOf(nil), fmt.Errorf("unmarshal toml: %v", err)
+		}
+		return mvalPtr.Elem(), nil
+	}
+
 	var mval reflect.Value
 	switch mtype.Kind() {
 	case reflect.Struct:
@@ -1151,6 +1177,16 @@ func (s *visitorState) visit() {
 	}
 }
 
+func (s *visitorState) visitAll() {
+	if s.active {
+		for k := range s.keys {
+			if strings.HasPrefix(k, strings.Join(s.path, ".")) {
+				delete(s.keys, k)
+			}
+		}
+	}
+}
+
 func (s *visitorState) validate() error {
 	if !s.active {
 		return nil