Function num_interior_rings 

pub fn num_interior_rings<G: NumInteriorRings>(g: &G) -> usize

Number of interior rings (holes) in g.

• Polygon → interiors().count()
• MultiPolygon → sum over members
• everything else → 0

Mirrors boost::geometry::num_interior_rings(g) from boost/geometry/algorithms/num_interior_rings.hpp.