Function saved_slot 

pub(crate) fn saved_slot(
    prot: SEXP,
    index: usize,
    name: &str,
) -> Option<R_xlen_t>

The position in the saved protection list prot of Sidecar field index, named name: found by name when the save carries slot names, else by position (a save by this version made before slots were named). None when the save has no slot for the field.