Function altstring_set_elt 

#[doc(hidden)]
pub unsafe fn altstring_set_elt<T>(x: SEXP, i: R_xlen_t, v: SEXP)
where
    T: Altrep,

Set_elt for bridge-generated string ALTREPs; R’s default raises an error.

Safety

As for altstring_elt; v is a CHARSXP and 0 <= i < length.