Function try_vecsxp_from_iter 

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

Fallible twin of vecsxp_from_iter: stops at the first row whose conversion fails. The list stays protected by the local scope until it returns, so the partially filled list is released on the error path.

Safety

Must be called from the R main thread.