the macro creates a _layout struct which is a named struct with fields _0, _1, ... that is then used in the ReprC trait impl where it uses the layout in its is_valid. but when using the fields it still treats the _layout struct as a tuple struct which then causes a compile error.

im using 0.2.0-alpha.0 but im pretty sure it happened on the lastest stable too. if needed i can create a pr to fix this issue.