A consequence of how bounds work is that even if a trait doesn't
include any functionality, you can still use it as a bound. Eq and
Ord are examples of such traits from the std library.
struct Cardinal;
struct BlueJay;
struct Turkey;
trait Red {}
trait Blue {}
impl Red for Cardinal {}
impl Blue for BlueJay {}
// These functions are only valid for types which implement these
// traits. The fact that the traits are empty is irrelevant.
fn red<T: Red>(_: &T) -> &'static str { "red" }
fn blue<T: Blue>(_: &T) -> &'static str { "blue" }
fn main() {
let cardinal = Cardinal;
let blue_jay = BlueJay;
let _turkey = Turkey;
// `red()` won't work on a blue jay nor vice versa
// because of the bounds.
println!("A cardinal is {}", red(&cardinal));
println!("A blue jay is {}", blue(&blue_jay));
//println!("A turkey is {}", red(&_turkey));
// ^ TODO: Try uncommenting this line.
}