Undefined behavior because of string_rt passed as const char *restrict being passed as another argument as well.
Extra \n in the end.

Also, your code has undefined behaviour: strings should end with a terminator. But your string did not accomodate for it ;-)

Passing the sample tests does not mean anything, it's only a very tiny subset of the entire test suite ;-)

It's not an issue if you can't pass the tests and you aren't certain why the tests don't pass, but a question ;-)

And nobody could look at it if you don't show what you're doing, so you should post your code marked as spoiler.