trait Foo {
    type Output;

    fn foo<F: FnOnce() -> i32>(self, f: F) -> Self::Output;
}

fn bar<F: FnOnce() -> i32>(_: F) -> &'static str {"blah"}

struct Bar;

impl Foo for Bar {
    type Output = &'static str;

    fn foo<F: FnOnce() -> i32>(self, f: F) -> Self::Output {
        bar(f)
    }
}

fn never_gonna_give_you_up<F: Foo>(f: F) -> F::Output {
    f.foo(|| 42)
}

fn main() {
    println!("{}", never_gonna_give_you_up(Bar));
}