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
Ord are examples of such traits from the std library.

  1. struct Cardinal;
  2. struct BlueJay;
  3. struct Turkey;
  5. trait Red {}
  6. trait Blue {}
  8. impl Red for Cardinal {}
  9. impl Blue for BlueJay {}
  11. // These functions are only valid for types which implement these
  12. // traits. The fact that the traits are empty is irrelevant.
  13. fn red<T: Red>(_: &T)   -> &'static str { "red" }
  14. fn blue<T: Blue>(_: &T) -> &'static str { "blue" }
  16. fn main() {
  17.     let cardinal = Cardinal;
  18.     let blue_jay = BlueJay;
  19.     let _turkey   = Turkey;
  21.     // `red()` won't work on a blue jay nor vice versa
  22.     // because of the bounds.
  23.     println!("A cardinal is {}", red(&cardinal));
  24.     println!("A blue jay is {}", blue(&blue_jay));
  25.     //println!("A turkey is {}", red(&_turkey));
  26.     // ^ TODO: Try uncommenting this line.
  27. }

See also:

std::cmp::Eq, std::cmp::Ords, and traits