tests/ui/generic-associated-types/projection-bound-cycle.rs

   1 // Test case from Chalk.
   2 // Make sure that we make sure that we don't allow arbitrary bounds to be
   3 // proven when a bound and a where clause of an associated type are the same.
   4
   5 #![feature(trivial_bounds)]
   6
   7 trait Print {
   8     fn print();
   9 }
  10
  11 trait Foo {
  12     type Item: Sized where <Self as Foo>::Item: Sized;
  13 }
  14
  15 struct Number { }
  16
  17 impl Foo for Number {
  18     // Well-formedness checks require that the following
  19     // goal is true:
  20     // ```
  21     // if (str: Sized) { # if the where clauses hold
  22     //     str: Sized # then the bound on the associated type hold
  23     // }
  24     // ```
  25     // which it is :)
  26     type Item = str where str: Sized;
  27     //~^ ERROR overflow evaluating the requirement `<Number as Foo>::Item == _`
  28 }
  29
  30 struct OnlySized<T> where T: Sized { f: T }
  31 impl<T> Print for OnlySized<T> {
  32     fn print() {
  33         println!("{}", std::mem::size_of::<T>());
  34     }
  35 }
  36
  37 trait Bar {
  38     type Assoc: Print;
  39 }
  40
  41 impl<T> Bar for T where T: Foo {
  42     // This is not ok, we need to prove `wf(<T as Foo>::Item)`, which requires
  43     // knowing that `<T as Foo>::Item: Sized` to satisfy the where clause. We
  44     // can use the bound on `Foo::Item` for this, but that requires
  45     // `wf(<T as Foo>::Item)`, which is an invalid cycle.
  46     type Assoc = OnlySized<<T as Foo>::Item>;
  47 }
  48
  49 fn foo<T: Print>() {
  50     T::print() // oops, in fact `T = OnlySized<str>` which is ill-formed
  51 }
  52
  53 fn bar<T: Bar>() {
  54     // we have `FromEnv(T: Bar)` hence
  55     // `<T as Bar>::Assoc` is well-formed and
  56     // `Implemented(<T as Bar>::Assoc: Print)` hold
  57     foo::<<T as Bar>::Assoc>()
  58 }
  59
  60 fn main() {
  61     bar::<Number>()
  62 }