src/test/compile-fail/hrtb-perfect-forwarding.rs

   1 // Copyright 2014 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 // Test a case where you have an impl of `Foo<X>` for all `X` that
  12 // is being applied to `for<'a> Foo<&'a mut X>`. Issue #19730.
  13
  14 trait Foo<X> {
  15     fn foo(&mut self, x: X) { }
  16 }
  17
  18 trait Bar<X> {
  19     fn bar(&mut self, x: X) { }
  20 }
  21
  22 impl<'a,X,F> Foo<X> for &'a mut F
  23     where F : Foo<X> + Bar<X>
  24 {
  25 }
  26
  27 impl<'a,X,F> Bar<X> for &'a mut F
  28     where F : Bar<X>
  29 {
  30 }
  31
  32 fn no_hrtb<'b,T>(mut t: T)
  33     where T : Bar<&'b isize>
  34 {
  35     // OK -- `T : Bar<&'b isize>`, and thus the impl above ensures that
  36     // `&mut T : Bar<&'b isize>`.
  37     no_hrtb(&mut t);
  38 }
  39
  40 fn bar_hrtb<T>(mut t: T)
  41     where T : for<'b> Bar<&'b isize>
  42 {
  43     // OK -- `T : for<'b> Bar<&'b isize>`, and thus the impl above
  44     // ensures that `&mut T : for<'b> Bar<&'b isize>`.  This is an
  45     // example of a "perfect forwarding" impl.
  46     bar_hrtb(&mut t);
  47 }
  48
  49 fn foo_hrtb_bar_not<'b,T>(mut t: T)
  50     where T : for<'a> Foo<&'a isize> + Bar<&'b isize>
  51 {
  52     // Not OK -- The forwarding impl for `Foo` requires that `Bar` also
  53     // be implemented. Thus to satisfy `&mut T : for<'a> Foo<&'a
  54     // isize>`, we require `T : for<'a> Bar<&'a isize>`, but the where
  55     // clause only specifies `T : Bar<&'b isize>`.
  56     foo_hrtb_bar_not(&mut t); //~ ERROR `for<'a> T: Bar<&'a isize>` is not satisfied
  57 }
  58
  59 fn foo_hrtb_bar_hrtb<T>(mut t: T)
  60     where T : for<'a> Foo<&'a isize> + for<'b> Bar<&'b isize>
  61 {
  62     // OK -- now we have `T : for<'b> Bar&'b isize>`.
  63     foo_hrtb_bar_hrtb(&mut t);
  64 }
  65
  66 fn main() { }