iterx: Allow forcing scanning as struct

We have a structure type that implements UnmarshalCQL method.
We use it to unmarshal a user defined type. We also want to use the same struct for scanning an entire row.

There is StructScan method available in gocqlx for this purpose when iterating over rows, but no equivalent when doing a Select.
This commit introduces the possibility when doing select/get as well.

Co-authored-by: Michał Matczuk <michal@scylladb.com>

gocqlx.go

@@ -14,22 +14,29 @@ import (
 )
 
 // structOnlyError returns an error appropriate for type when a non-scannable
-// struct is expected but something else is given
+// struct is expected but something else is given.
 func structOnlyError(t reflect.Type) error {
-	isStruct := t.Kind() == reflect.Struct
-	isScanner := reflect.PtrTo(t).Implements(_unmarshallerInterface)
-	if !isStruct {
-		return fmt.Errorf("expected %s but got %s", reflect.Struct, t.Kind())
+	if isStruct := t.Kind() == reflect.Struct; !isStruct {
+		return fmt.Errorf("expected a struct but got %s", t.Kind())
 	}
-	if isScanner {
-		return fmt.Errorf("structscan expects a struct dest but the provided struct type %s implements unmarshaler", t.Name())
+
+	if isUnmarshaller := reflect.PtrTo(t).Implements(unmarshallerInterface); isUnmarshaller {
+		return fmt.Errorf("expected a struct but the provided struct type %s implements gocql.Unmarshaler", t.Name())
 	}
+
+	if isUDTUnmarshaller := reflect.PtrTo(t).Implements(udtUnmarshallerInterface); isUDTUnmarshaller {
+		return fmt.Errorf("expected a struct but the provided struct type %s implements gocql.UDTUnmarshaler", t.Name())
+	}
+
 	return fmt.Errorf("expected a struct, but struct %s has no exported fields", t.Name())
 }
 
 // reflect helpers
 
-var _unmarshallerInterface = reflect.TypeOf((*gocql.Unmarshaler)(nil)).Elem()
+var (
+	unmarshallerInterface    = reflect.TypeOf((*gocql.Unmarshaler)(nil)).Elem()
+	udtUnmarshallerInterface = reflect.TypeOf((*gocql.UDTUnmarshaler)(nil)).Elem()
+)
 
 func baseType(t reflect.Type, expected reflect.Kind) (reflect.Type, error) {
 	t = reflectx.Deref(t)