It sounds like our instance types will end up being a bit different. Right now we don't need to send the hit instance back with the differential geometry since it doesn't tell us anything extra but later we'll attach different materials and lights to instances of the same geometry and will then need access to the hit instance so we can compute the material properties.

Sending the instance to the geometry to fill it out in the struct is a good idea, if Rust had default parameter support I'd do this and have the instance's intersect just default to None and pass itself to the geometry's intersect. Then the functions would have the same signature and we could mark the case of a None instance in geometry as unreachable.