src/test/compile-fail/typeck-auto-trait-no-supertraits.rs

   1 // Copyright 2016 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 // This test is for #29859, we need to ensure auto traits,
  12 // (also known previously as default traits), do not have
  13 // supertraits. Since the compiler synthesizes these
  14 // instances on demand, we are essentially enabling
  15 // users to write axioms if we view trait selection,
  16 // as a proof system.
  17 //
  18 // For example the below test allows us to add the rule:
  19 //  forall (T : Type), T : Copy
  20 //
  21 // Providing a copy instance for *any* type, which
  22 // is most definitely unsound. Imagine copying a
  23 // type that contains a mutable reference, enabling
  24 // mutable aliasing.
  25 //
  26 // You can imagine an even more dangerous test,
  27 // which currently compiles on nightly.
  28 //
  29 // fn main() {
  30 //    let mut i = 10;
  31 //    let (a, b) = copy(&mut i);
  32 //    println!("{:?} {:?}", a, b);
  33 // }
  34
  35 #![feature(optin_builtin_traits)]
  36
  37 trait Magic: Copy {} //~ ERROR E0568
  38 #[allow(auto_impl)]
  39 impl Magic for .. {}
  40 impl<T:Magic> Magic for T {}
  41
  42 fn copy<T: Magic>(x: T) -> (T, T) { (x, x) }
  43
  44 #[derive(Debug)]
  45 struct NoClone;
  46
  47 fn main() {
  48     let (a, b) = copy(NoClone);
  49     println!("{:?} {:?}", a, b);
  50 }