src/test/ui/hrtb/hrtb-perfect-forwarding.rs

   1 // Test a case where you have an impl of `Foo<X>` for all `X` that
   2 // is being applied to `for<'a> Foo<&'a mut X>`. Issue #19730.
   3
   4 trait Foo<X> {
   5     fn foo(&mut self, x: X) { }
   6 }
   7
   8 trait Bar<X> {
   9     fn bar(&mut self, x: X) { }
  10 }
  11
  12 impl<'a,X,F> Foo<X> for &'a mut F
  13     where F : Foo<X> + Bar<X>
  14 {
  15 }
  16
  17 impl<'a,X,F> Bar<X> for &'a mut F
  18     where F : Bar<X>
  19 {
  20 }
  21
  22 fn no_hrtb<'b,T>(mut t: T)
  23     where T : Bar<&'b isize>
  24 {
  25     // OK -- `T : Bar<&'b isize>`, and thus the impl above ensures that
  26     // `&mut T : Bar<&'b isize>`.
  27     no_hrtb(&mut t);
  28 }
  29
  30 fn bar_hrtb<T>(mut t: T)
  31     where T : for<'b> Bar<&'b isize>
  32 {
  33     // OK -- `T : for<'b> Bar<&'b isize>`, and thus the impl above
  34     // ensures that `&mut T : for<'b> Bar<&'b isize>`.  This is an
  35     // example of a "perfect forwarding" impl.
  36     bar_hrtb(&mut t);
  37 }
  38
  39 fn foo_hrtb_bar_not<'b,T>(mut t: T)
  40     where T : for<'a> Foo<&'a isize> + Bar<&'b isize>
  41 {
  42     // Not OK -- The forwarding impl for `Foo` requires that `Bar` also
  43     // be implemented. Thus to satisfy `&mut T : for<'a> Foo<&'a
  44     // isize>`, we require `T : for<'a> Bar<&'a isize>`, but the where
  45     // clause only specifies `T : Bar<&'b isize>`.
  46     foo_hrtb_bar_not(&mut t); //~ ERROR mismatched types
  47                               //~| ERROR mismatched types
  48 }
  49
  50 fn foo_hrtb_bar_hrtb<T>(mut t: T)
  51     where T : for<'a> Foo<&'a isize> + for<'b> Bar<&'b isize>
  52 {
  53     // OK -- now we have `T : for<'b> Bar&'b isize>`.
  54     foo_hrtb_bar_hrtb(&mut t);
  55 }
  56
  57 fn main() { }