add support of dst structs with pointers (#22)

README.md

@@ -70,8 +70,8 @@ fieldmask_utils.StructToStruct(mask, request.User, userDst)
 
 2.  Masks inside a protobuf `Map` are not supported.
 3.  When copying from a struct to struct the destination struct must have the same fields (or a subset)
-    as the source struct. Pointers must also be coherent: if a field is a pointer in the source struct, then
-    it also must be a pointer (not a value field) in the destination struct.
+    as the source struct. Either of source or destination fields can be a pointer as long as it is a pointer to
+    the type of the corresponding field.
 4. `oneof` fields are represented differently in `fieldmaskpb.FieldMask` compared to `fieldmask_util.Mask`. In 
     [FieldMask](https://pkg.go.dev/google.golang.org/protobuf/types/known/fieldmaskpb#:~:text=%23%20Field%20Masks%20and%20Oneof%20Fields)
     the fields are represented using their property name, in this library they are prefixed with the `oneof` name

copy.go

@@ -34,9 +34,24 @@ func StructToStruct(filter FieldFilter, src, dst interface{}, userOpts ...Option
 	return structToStruct(filter, &srcVal, &dstVal, opts)
 }
 
+func ensureCompatible(src, dst *reflect.Value) error {
+	srcKind := src.Kind()
+	if srcKind == reflect.Ptr {
+		srcKind = src.Type().Elem().Kind()
+	}
+	dstKind := dst.Kind()
+	if dstKind == reflect.Ptr {
+		dstKind = dst.Type().Elem().Kind()
+	}
+	if srcKind != dstKind {
+		return errors.Errorf("src kind %s differs from dst kind %s", srcKind, dstKind)
+	}
+	return nil
+}
+
 func structToStruct(filter FieldFilter, src, dst *reflect.Value, userOptions *options) error {
-	if src.Kind() != dst.Kind() {
-		return errors.Errorf("src kind %s differs from dst kind %s", src.Kind(), dst.Kind())
+	if err := ensureCompatible(src, dst); err != nil {
+		return err
 	}
 
 	switch src.Kind() {
@@ -46,6 +61,14 @@ func structToStruct(filter FieldFilter, src, dst *reflect.Value, userOptions *op
 			return nil
 		}
 
+		if dst.Kind() == reflect.Ptr {
+			if dst.IsNil() {
+				dst.Set(reflect.New(dst.Type().Elem()))
+			}
+			v := dst.Elem()
+			dst = &v
+		}
+
 		for i := 0; i < src.NumField(); i++ {
 			srcType := src.Type()
 			fieldName := srcType.Field(i).Name
@@ -78,7 +101,7 @@ func structToStruct(filter FieldFilter, src, dst *reflect.Value, userOptions *op
 			dst.Set(reflect.Zero(dst.Type()))
 			break
 		}
-		if dst.IsNil() {
+		if dst.Kind() == reflect.Ptr && dst.IsNil() {
 			// If dst is nil create a new instance of the underlying type and set dst to the pointer of that instance.
 			dst.Set(reflect.New(dst.Type().Elem()))
 		}
@@ -117,7 +140,11 @@ func structToStruct(filter FieldFilter, src, dst *reflect.Value, userOptions *op
 			break
 		}
 
-		srcElem, dstElem := src.Elem(), dst.Elem()
+		srcElem, dstElem := src.Elem(), *dst
+		if dst.Kind() == reflect.Ptr {
+			dstElem = dst.Elem()
+		}
+
 		if err := structToStruct(filter, &srcElem, &dstElem, userOptions); err != nil {
 			return err
 		}
@@ -185,7 +212,14 @@ func structToStruct(filter FieldFilter, src, dst *reflect.Value, userOptions *op
 		if !dst.CanSet() {
 			return errors.Errorf("dst %s, %s is not settable", dst, dst.Type())
 		}
-		dst.Set(*src)
+		if dst.Kind() == reflect.Ptr {
+			if !src.CanAddr() {
+				return errors.Errorf("src %s, %s is not addressable", src, src.Type())
+			}
+			dst.Set(src.Addr())
+		} else {
+			dst.Set(*src)
+		}
 	}
 
 	return nil

copy_test.go

@@ -47,6 +47,104 @@ func TestStructToStruct_PtrToInt(t *testing.T) {
 	}, dst)
 }
 
+func TestStructToStruct_StructToPointer(t *testing.T) {
+	v15 := 15
+	v42 := 42
+
+	type N struct {
+		Field1 int
+	}
+	type S struct {
+		Field1 N
+		Field2 int
+	}
+	src := &S{
+		Field1: N{
+			Field1: v15,
+		},
+		Field2: v42,
+	}
+	type SN struct {
+		Field1 *int
+	}
+	type D struct {
+		Field1 *SN
+		Field2 *int
+	}
+	dst := new(D)
+
+	mask := fieldmask_utils.MaskFromString("Field1,Field2")
+	err := fieldmask_utils.StructToStruct(mask, src, dst)
+	require.NoError(t, err)
+	assert.Equal(t, &D{
+		Field1: &SN{
+			Field1: &v15,
+		},
+		Field2: &v42,
+	}, dst)
+}
+
+func TestStructToStruct_IntToPointer(t *testing.T) {
+	v := 42
+
+	type S struct {
+		Field2 int
+	}
+	src := &S{
+		Field2: v,
+	}
+	type D struct {
+		Field2 *int
+	}
+	dst := new(D)
+
+	mask := fieldmask_utils.MaskFromString("Field2")
+	err := fieldmask_utils.StructToStruct(mask, src, dst)
+	require.NoError(t, err)
+	assert.Equal(t, &D{
+		Field2: &v,
+	}, dst)
+}
+
+func TestStructToStruct_PointerToInt(t *testing.T) {
+	v := 42
+
+	type S struct {
+		Field2 *int
+	}
+	src := &S{
+		Field2: &v,
+	}
+	type D struct {
+		Field2 int
+	}
+	dst := new(D)
+
+	mask := fieldmask_utils.MaskFromString("Field2")
+	err := fieldmask_utils.StructToStruct(mask, src, dst)
+	require.NoError(t, err)
+	assert.Equal(t, &D{
+		Field2: 42,
+	}, dst)
+}
+
+func TestStructToStruct_Incompatible(t *testing.T) {
+	type S struct {
+		Field2 int
+	}
+	src := &S{
+		Field2: 42,
+	}
+	type D struct {
+		Field2 string
+	}
+	dst := new(D)
+
+	mask := fieldmask_utils.MaskFromString("Field2")
+	err := fieldmask_utils.StructToStruct(mask, src, dst)
+	require.EqualError(t, err, "src kind int differs from dst kind string")
+}
+
 func TestStructToStruct_PtrToStruct_EmptyDst(t *testing.T) {
 	type A struct {
 		Field1 string