chore: various embedding fixes

This PR does several things:
- Nil pointer embedded structs are now handled correctly.
- Embedded structs no longer need to have `protobuf:<n>` tags since they didn't use them to begin with.
- Embedding primitive types now reports an error instead of silently breaking.

Signed-off-by: Dmitriy Matrenichev <dmitry.matrenichev@siderolabs.com>

Author: Dmitriy Matrenichev

field_test.go

@@ -55,8 +55,8 @@ func TestEncodeNested(t *testing.T) {
 
 //nolint:govet
 type StructWithEmbed struct {
-	A            int32 `protobuf:"1"`
-	*EmbedStruct `protobuf:"2"`
+	A int32 `protobuf:"1"`
+	*EmbedStruct
 }
 
 type EmbedStruct struct {

marshal.go

@@ -115,11 +115,23 @@ func (m *marshaller) encodeFields(val reflect.Value, fieldsData []FieldData) {
 // fieldByIndex returns the field of the struct by its index if the field is exported.
 // Otherwise, it returns empty reflect.Value.
 func fieldByIndex(structVal reflect.Value, data FieldData) reflect.Value {
-	if data.Field.IsExported() && structVal.IsValid() {
-		return structVal.FieldByIndex(data.FieldIndex)
+	if !structVal.IsValid() || !data.Field.IsExported() || len(data.FieldIndex) == 0 {
+		return reflect.Value{}
 	}
 
-	return reflect.Value{}
+	var result reflect.Value
+
+	for i := 0; i < len(data.FieldIndex); i++ {
+		index := data.FieldIndex[:i+1]
+
+		result = structVal.FieldByIndex(index)
+		if len(data.FieldIndex) > 1 && result.Kind() == reflect.Ptr && result.IsNil() {
+			// Embedded field is nil, return empty reflect.Value. Avo
+			return reflect.Value{}
+		}
+	}
+
+	return result
 }
 
 //nolint:cyclop

marshal_test.go

@@ -269,7 +269,7 @@ func TestStringKey(t *testing.T) {
 
 func TestInternalStructMarshal(t *testing.T) {
 	encoded := hasInternalCanMarshal[string]{
-		Field:  canMarshal[string]{private: "test for tests"},
+		Field:  Sequence[string]{field: "test for tests"},
 		Field2: 150,
 	}
 
@@ -284,22 +284,22 @@ func TestInternalStructMarshal(t *testing.T) {
 }
 
 type hasInternalCanMarshal[T string | []byte] struct {
-	Field  canMarshal[T] `protobuf:"1"`
-	Field2 int           `protobuf:"2"`
+	Field  Sequence[T] `protobuf:"1"`
+	Field2 int         `protobuf:"2"`
 }
 
-type canMarshal[T string | []byte] struct {
-	private T
+type Sequence[T string | []byte] struct {
+	field T
 }
 
 //nolint:revive
-func (cm *canMarshal[T]) MarshalBinary() ([]byte, error) {
-	return []byte(cm.private), nil
+func (cm *Sequence[T]) MarshalBinary() ([]byte, error) {
+	return []byte(cm.field), nil
 }
 
 //nolint:revive
-func (cm *canMarshal[T]) UnmarshalBinary(data []byte) error {
-	cm.private = T(data)
+func (cm *Sequence[T]) UnmarshalBinary(data []byte) error {
+	cm.field = T(data)
 
 	return nil
 }
@@ -342,3 +342,110 @@ func TestMarshal(t *testing.T) {
 	assert.Equal(t, a, testA)
 	assert.Equal(t, b, testB)
 }
+
+type EmbeddedStruct struct {
+	Value  int    `protobuf:"1"`
+	Value2 uint32 `protobuf:"2"`
+}
+
+type AnotherEmbeddedStruct struct {
+	Value1 int    `protobuf:"3"`
+	Value2 uint32 `protobuf:"4"`
+}
+
+func TestEmbedding(t *testing.T) {
+	structs := map[string]struct {
+		fn func(t *testing.T)
+	}{
+		"should embed struct": {
+			fn: makeEmbedTest(struct {
+				EmbeddedStruct
+			}{
+				EmbeddedStruct: EmbeddedStruct{
+					Value:  0x11,
+					Value2: 0x12,
+				},
+			}),
+		},
+		"should embed struct pointer": {
+			fn: makeEmbedTest(struct {
+				*EmbeddedStruct
+			}{
+				EmbeddedStruct: &EmbeddedStruct{
+					Value:  0x15,
+					Value2: 0x16,
+				},
+			}),
+		},
+		"should embed nil pointer struct and not nil pointer struct": {
+			fn: makeEmbedTest(struct {
+				*EmbeddedStruct
+				*AnotherEmbeddedStruct
+			}{
+				EmbeddedStruct: nil,
+				AnotherEmbeddedStruct: &AnotherEmbeddedStruct{
+					Value1: 0x21,
+					Value2: 0x22,
+				},
+			}),
+		},
+		"should embed struct with marshaller": {
+			fn: makeEmbedTest(struct {
+				Sequence[string]
+			}{
+				Sequence: Sequence[string]{
+					"test",
+				},
+			}),
+		},
+		"should not embed nil struct pointer": {
+			fn: makeIncorrectEmbedTest(struct {
+				*EmbeddedStruct
+			}{
+				EmbeddedStruct: nil,
+			}),
+		},
+		"should not embed simple type": {
+			fn: makeIncorrectEmbedTest(struct {
+				int
+			}{
+				0x11,
+			}),
+		},
+		"should not embed pointer to simple type": {
+			fn: makeIncorrectEmbedTest(struct {
+				*int
+			}{
+				int: new(int),
+			}),
+		},
+	}
+
+	for name, test := range structs {
+		t.Run(name, test.fn)
+	}
+}
+
+func makeEmbedTest[V any](v V) func(t *testing.T) {
+	return func(t *testing.T) {
+		t.Helper()
+		encoded := must(protoenc.Marshal(&v))(t)
+
+		t.Logf("\n%s", hex.Dump(encoded))
+
+		var result V
+
+		require.NoError(t, protoenc.Unmarshal(encoded, &result))
+		require.Equal(t, v, result)
+	}
+}
+
+func makeIncorrectEmbedTest[V any](v V) func(t *testing.T) {
+	return func(t *testing.T) {
+		t.Helper()
+
+		_, err := protoenc.Marshal(&v)
+
+		require.Error(t, err)
+	}
+}

type_cache.go

@@ -63,6 +63,24 @@ func structFields(typ reflect.Type) ([]FieldData, error) {
 			continue
 		}
 
+		if typField.Anonymous {
+			if deref(typField.Type).Kind() != reflect.Struct {
+				return nil, fmt.Errorf("%s.%s.%s is not a struct type", typ.PkgPath(), typ.Name(), typField.Name)
+			}
+
+			fields, err := structFields(typField.Type)
+			if err != nil {
+				return nil, err
+			}
+
+			for _, innerField := range fields {
+				innerField.FieldIndex = append([]int{i}, innerField.FieldIndex...)
+				result = append(result, innerField)
+			}
+
+			continue
+		}
+
 		num := ParseTag(typField)
 		switch num {
 		case 0:
@@ -74,23 +92,15 @@ func structFields(typ reflect.Type) ([]FieldData, error) {
 			return nil, fmt.Errorf("%s.%s.%s has invalid protobuf tag", typ.PkgPath(), typ.Name(), typField.Name)
 		}
 
-		if typField.Anonymous {
-			fields, err := structFields(typField.Type)
-			if err != nil {
-				return nil, err
-			}
+		result = append(result, FieldData{
+			Num:        protowire.Number(num),
+			FieldIndex: []int{i},
+			Field:      typField,
+		})
+	}
 
-			for _, innerField := range fields {
-				innerField.FieldIndex = append([]int{i}, innerField.FieldIndex...)
-				result = append(result, innerField)
-			}
-		} else {
-			result = append(result, FieldData{
-				Num:        protowire.Number(num),
-				FieldIndex: []int{i},
-				Field:      typField,
-			})
-		}
+	if len(result) == 0 {
+		return nil, fmt.Errorf("%s.%s has no exported fields", typ.PkgPath(), typ.Name())
 	}
 
 	return result, nil