The compiler can't evaluate the strcmp tree generated by MP_ERROR_TEXT(x) at compile time if x is determined at runtime (for example a call to an old style xxx_strerror function):

mp_raise_msg(&mp_type_OSError, MP_ERROR_TEXT(ffs_strerror(res)));

This results in a copy of the tree for every invocation of MP_ERROR_TEXT, so the return result can't be compressed and must be cast to mp_rom_error_text_t instead. This should be safe for now, but will it change in the future ? And if so, shouldn't there be a function or macro to hide this conversion since all exceptions formally accept mp_rom_error_text_t arg ? Something like:

#define MP_ERROR_TEXT_FROM_STR(x) (mp_rom_error_text_t) x