Function altrep_values_written 

#[doc(hidden)]
pub unsafe fn altrep_values_written(x: SEXP) -> bool

Whether R may have changed the vector’s values since it was built.

Until then the Rust data is current, so a class may answer from it: serialize its state, subset it, report its sortedness, NAs, sum or range. A numeric cache exists only once Dataptr handed R a pointer it may write through, so for those families that is until data2 exists. A string cache also fills on reads, so a string ALTREP counts as written only once Set_elt or a writable Dataptr has marked its cache.

Safety

As for materialized_altrep_data2.