Function generate_altrep_storage 

pub(crate) fn generate_altrep_storage(
    input: &DeriveInput,
    class_name: &str,
) -> Result<TokenStream>

ALTREP stores its owner in an ordinary ExternalPtr, so it needs the same slot visitor and handle accessors. Only storage is generated here: ALTREP has its own IntoR and registration, and isn’t an R class with field methods.