tests/ui/rfc-2632-const-trait-impl/specialization/non-const-default-const-specialized.rs

   1 // Tests that a non-const default impl can be specialized by a const trait impl,
   2 // but that the default impl cannot be used in a const context.
   3
   4 // run-pass
   5
   6 #![feature(const_trait_impl)]
   7 #![feature(min_specialization)]
   8
   9 #[const_trait]
  10 trait Value {
  11     fn value() -> u32;
  12 }
  13
  14 const fn get_value<T: ~const Value>() -> u32 {
  15     T::value()
  16 }
  17
  18 impl<T> Value for T {
  19     default fn value() -> u32 {
  20         println!("You can't do that (constly)");
  21         0
  22     }
  23 }
  24
  25 struct FortyTwo;
  26
  27 impl const Value for FortyTwo {
  28     fn value() -> u32 {
  29         42
  30     }
  31 }
  32
  33 fn main() {
  34     let zero = get_value::<()>();
  35     assert_eq!(zero, 0);
  36
  37     const FORTY_TWO: u32 = get_value::<FortyTwo>();
  38     assert_eq!(FORTY_TWO, 42);
  39 }