Function try_vecsxp_from_iter_unchecked 

unsafe fn try_vecsxp_from_iter_unchecked<I, E>(iter: I) -> Result<SEXP, E>
where
    I: ExactSizeIterator<Item = Result<SEXP, E>>,

_unchecked twin of try_vecsxp_from_iter — uses ListBuilder::set_unchecked. The caller’s adaptor must produce children via try_into_sexp_unchecked().

Safety

Must be called from the R main thread, in a context where the checked-FFI assertion is intentionally bypassed.