Function map_vecsxp_with_unchecked 

pub(crate) unsafe fn map_vecsxp_with_unchecked<U>(
    sexp: SEXP,
    map: impl FnMut(usize, SEXP) -> Result<U, SexpError>,
) -> Result<Vec<U>, SexpError>

Unchecked-FFI variant of map_vecsxp_with — uses len_unchecked / vector_elt_unchecked for the type-check and walk.

Safety

Must be called from R’s main thread (same contract as TryFromSexp::try_from_sexp_unchecked).