Function altstring_elt 

#[doc(hidden)]
pub unsafe fn altstring_elt<T>(x: SEXP, i: R_xlen_t) -> SEXP
where
    T: Altrep + AltrepExtract + AltStringData + 'static,

Elt for bridge-generated string ALTREPs: each element is computed once.

Safety

x must be a live ALTSTRING using the bridge’s data2 convention, on R’s main thread.