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

// Test a case where you have an impl of `Foo<X>` for all `X` that
// is being applied to `for<'a> Foo<&'a mut X>`. Issue #19730.

trait Foo<X> {
    fn foo(&mut self, x: X) { }
}

trait Bar<X> {
    fn bar(&mut self, x: X) { }
}

impl<'a,X,F> Foo<X> for &'a mut F
    where F : Foo<X> + Bar<X>
{
}

impl<'a,X,F> Bar<X> for &'a mut F
    where F : Bar<X>
{
}

fn no_hrtb<'b,T>(mut t: T)
    where T : Bar<&'b isize>
{
    // OK -- `T : Bar<&'b isize>`, and thus the impl above ensures that
    // `&mut T : Bar<&'b isize>`.
    no_hrtb(&mut t);
}

fn bar_hrtb<T>(mut t: T)
    where T : for<'b> Bar<&'b isize>
{
    // OK -- `T : for<'b> Bar<&'b isize>`, and thus the impl above
    // ensures that `&mut T : for<'b> Bar<&'b isize>`.  This is an
    // example of a "perfect forwarding" impl.
    bar_hrtb(&mut t);
}

fn foo_hrtb_bar_not<'b,T>(mut t: T)
    where T : for<'a> Foo<&'a isize> + Bar<&'b isize>
{
    // Not OK -- The forwarding impl for `Foo` requires that `Bar` also
    // be implemented. Thus to satisfy `&mut T : for<'a> Foo<&'a
    // isize>`, we require `T : for<'a> Bar<&'a isize>`, but the where
    // clause only specifies `T : Bar<&'b isize>`.
    foo_hrtb_bar_not(&mut t); //~ ERROR mismatched types
                              //~| ERROR mismatched types
}

fn foo_hrtb_bar_hrtb<T>(mut t: T)
    where T : for<'a> Foo<&'a isize> + for<'b> Bar<&'b isize>
{
    // OK -- now we have `T : for<'b> Bar&'b isize>`.
    foo_hrtb_bar_hrtb(&mut t);
}

fn main() { }