src/test/ui/generic-associated-types/projection-bound-cycle.rs

// Test case from Chalk.
// Make sure that we make sure that we don't allow arbitrary bounds to be
// proven when a bound and a where clause of an associated type are the same.

#![feature(generic_associated_types)]
//~^ WARNING the feature `generic_associated_types` is incomplete
#![feature(trivial_bounds)]

trait Print {
    fn print();
}

trait Foo {
    type Item: Sized where <Self as Foo>::Item: Sized;
}

struct Number { }

impl Foo for Number {
    // Well-formedness checks require that the following
    // goal is true:
    // ```
    // if (str: Sized) { # if the where clauses hold
    //     str: Sized # then the bound on the associated type hold
    // }
    // ```
    // which it is :)
    type Item where str: Sized = str;
}

struct OnlySized<T> where T: Sized { f: T }
impl<T> Print for OnlySized<T> {
    fn print() {
        println!("{}", std::mem::size_of::<T>());
    }
}

trait Bar {
    type Assoc: Print;
}

impl<T> Bar for T where T: Foo {
    // This is not ok, we need to prove `wf(<T as Foo>::Item)`, which requires
    // knowing that `<T as Foo>::Item: Sized` to satisfy the where clause. We
    // can use the bound on `Foo::Item` for this, but that requires
    // `wf(<T as Foo>::Item)`, which is an invalid cycle.
    type Assoc = OnlySized<<T as Foo>::Item>;
    //~^ ERROR overflow evaluating the requirement `<T as Foo>::Item: Sized`
}

fn foo<T: Print>() {
    T::print() // oops, in fact `T = OnlySized<str>` which is ill-formed
}

fn bar<T: Bar>() {
    // we have `FromEnv(T: Bar)` hence
    // `<T as Bar>::Assoc` is well-formed and
    // `Implemented(<T as Bar>::Assoc: Print)` hold
    foo::<<T as Bar>::Assoc>()
}

fn main() {
    bar::<Number>()
}