Function classify_null_address 

pub(crate) unsafe fn classify_null_address<T: TypedExternal>(
    sexp: SEXP,
) -> NullAddress

Reads prot[0] of a pointer without an address and says what it is for T. Symbol names live for the session, so the borrowed parts are 'static.

Safety

Must be called from R’s main thread with a valid EXTPTRSXP.