Interfaces can require implementing types to also implement other interfaces of the same kind. Interface implementation requirements can be declared by following the interface name with a colon (:) and one or more names of interfaces of the same kind, separated by commas.
// Declare a structure interface named `Shape`.
//
pub struct interface Shape {}
// Declare a structure interface named `Polygon`.
// Require implementing types to also implement the structure interface `Shape`.
//
pub struct interface Polygon: Shape {}
// Declare a structure named `Hexagon` that implements the `Polygon` interface.
// This also is required to implement the `Shape` interface,
// because the `Polygon` interface requires it.
//
pub struct Hexagon: Polygon {}
