uiop:native-namestring

(uiop:native-namestring pathname)

The pathname's namestring in the host operating system's own spelling. A rontolisp namestring already IS the host spelling -- no backend translates between a Lisp and a native syntax -- so this is namestring. Libraries call it where a path leaves Lisp (jzon stringifies a pathname value through it, trivial-mimes hands one to an external probe).

Backend support

All four backends: the interpreter as a built-in, the compile paths lowered onto namestring.