marshal: Rcpp::sourceCpp()

From https://future.futureverse.org/articles/future-4-non-exportable-objects.html#package-rcpp:

```r
Rcpp::sourceCpp(code = "
#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
int my_length(NumericVector x) {
    return x.size();
}
")
```

This produces:

```r
my_length()
```

which holds an external reference pointer.

Can such an object be marshalled?