Function altstring_cached_elt 

unsafe fn altstring_cached_elt<T>(x: SEXP, cache: SEXP, i: R_xlen_t) -> SEXP
where
    T: AltrepExtract + AltStringData + 'static,

Read cache slot i, computing it from the Rust data on first use.

Safety

As for altstring_cache; cache is x’s cache and 0 <= i < length.