Function altstring_dataptr 

#[doc(hidden)]
pub unsafe fn altstring_dataptr<T>(x: SEXP, writable: bool) -> *mut c_void
where
    T: Altrep + AltrepExtract + AltStringData + 'static,

Dataptr for bridge-generated string ALTREPs: fill every cache slot and return the cache’s pointer. A writable request marks the cache written first (see altstring_cache_written), since R writes through it.

A panic part-way leaves the filled slots cached; a retry fills the rest.

Safety

As for altstring_elt.