src/test/compile-fail/variance-trait-matching.rs

   1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 #![allow(dead_code)]
  12
  13 // Get<T> is covariant in T
  14 trait Get<T> {
  15     fn get(&self) -> T;
  16 }
  17
  18 struct Cloner<T:Clone> {
  19     t: T
  20 }
  21
  22 impl<T:Clone> Get<T> for Cloner<T> {
  23     fn get(&self) -> T {
  24         self.t.clone()
  25     }
  26 }
  27
  28 fn get<'a, G>(get: &G) -> i32
  29     where G : Get<&'a i32>
  30 {
  31     // This fails to type-check because, without variance, we can't
  32     // use `G : Get<&'a i32>` as evidence that `G : Get<&'b i32>`,
  33     // even if `'a : 'b`.
  34     pick(get, &22) //~ ERROR cannot infer
  35 }
  36
  37 fn pick<'b, G>(get: &'b G, if_odd: &'b i32) -> i32
  38     where G : Get<&'b i32>
  39 {
  40     let v = *get.get();
  41     if v % 2 != 0 { v } else { *if_odd }
  42 }
  43
  44 fn main() {
  45     let x = Cloner { t: &23 };
  46     let y = get(&x);
  47     assert_eq!(y, 23);
  48 }