dispatch_5.rs

let cat = Cat {
  meow_factor: 7
  purr_factor: 8
};

let dog = Dog { ... };

let clone_mammal: &CloneMammal;
if get_random_bool() == true {
  clone_mammal = &cat;
} else {
  clone_mammal = &dog;
}

// Error! How can the compiler know what vtable to
// point to?
let clone: &Clone = &clone_mammal;