Function vecsxp_from_iter_unchecked 

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

_unchecked twin of vecsxp_from_iter — uses ListBuilder::set_unchecked. For ALTREP / with_r_thread / unwind contexts. The caller’s adaptor must produce children via into_sexp_unchecked().

Safety

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