Rust by Example

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 Copy 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)   -> &'static str { "red" }

fn 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.

}

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