skip scanning struct fields for ItemMarshaler structs (#229)

encode_test.go

@@ -133,3 +133,57 @@ func TestMarshalItemAsymmetric(t *testing.T) {
 		})
 	}
 }
+
+type isValue_Kind interface {
+	isValue_Kind()
+}
+
+type myStruct struct {
+	OK    bool
+	Value isValue_Kind
+}
+
+func (ms *myStruct) MarshalDynamoItem() (map[string]*dynamodb.AttributeValue, error) {
+	world := "world"
+	return map[string]*dynamodb.AttributeValue{
+		"hello": {S: &world},
+	}, nil
+}
+
+func (ms *myStruct) UnmarshalDynamoItem(item map[string]*dynamodb.AttributeValue) error {
+	hello := item["hello"]
+	if hello == nil || hello.S == nil || *hello.S != "world" {
+		ms.OK = false
+	} else {
+		ms.OK = true
+	}
+	return nil
+}
+
+var _ ItemMarshaler = &myStruct{}
+var _ ItemUnmarshaler = &myStruct{}
+
+func TestMarshalItemBypass(t *testing.T) {
+	something := &myStruct{}
+	got, err := MarshalItem(something)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	world := "world"
+	expect := map[string]*dynamodb.AttributeValue{
+		"hello": {S: &world},
+	}
+	if !reflect.DeepEqual(got, expect) {
+		t.Error("bad marshal. want:", expect, "got:", got)
+	}
+
+	var dec myStruct
+	err = UnmarshalItem(got, &dec)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !dec.OK {
+		t.Error("bad unmarshal")
+	}
+}

encoding.go

@@ -14,8 +14,9 @@ import (
 var typeCache sync.Map // unmarshalKey → *typedef
 
 type typedef struct {
-	decoders map[unmarshalKey]decodeFunc
-	fields   []structField
+	decoders  map[unmarshalKey]decodeFunc
+	fields    []structField
+	marshaler bool
 }
 
 func newTypedef(rt reflect.Type) (*typedef, error) {
@@ -27,6 +28,7 @@ func newTypedef(rt reflect.Type) (*typedef, error) {
 }
 
 func (def *typedef) init(rt reflect.Type) error {
+	rt0 := rt
 	for rt.Kind() == reflect.Pointer {
 		rt = rt.Elem()
 	}
@@ -37,6 +39,11 @@ func (def *typedef) init(rt reflect.Type) error {
 		return nil
 	}
 
+	// skip visiting struct fields if encoding will be bypassed by a custom marshaler
+	if shouldBypassEncodeItem(rt0) || shouldBypassEncodeItem(rt) {
+		return nil
+	}
+
 	var err error
 	def.fields, err = structFields(rt)
 	return err