Macro impl_vec_option_into_r 

macro_rules! impl_vec_option_into_r {
    ($t:ty, $na_value:expr) => { ... };
}

Macro for NA-aware Vec<Option<T>> → R vector conversions.

Uses alloc_r_vector to get a mutable slice, then fills it.

Unlike the sibling Vec<Option<T>> macros (impl_vec_option_smart_i64_into_r! / impl_vec_option_coerce_into_r!, which route through into_r_infallible! and inherit the default-unchecked policy), this one is written out by hand because it carries a real into_sexp_unchecked path (alloc_r_vector_unchecked) that the others have no analogue for. The divergence is deliberate.