Testcase: empty bounds

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.

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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.
}
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See also:

std::cmp::Eq, std::marker::Copy, and traits