Function __mx_external_arg 

#[doc(hidden)]
pub fn __mx_external_arg<T: TypedExternal>(
    sexp: SEXP,
) -> Result<ExternalPtr<T>, SexpError>

The handle a generated wrapper borrows for a &T / &mut T / ExternalPtrRef / ExternalPtrMut argument: converted like an ExternalPtr<T> argument, without its T: Send bound, since a borrowed argument keeps the call on R’s main thread.