fn never_gonna_give_you_up<F: FnOnce(fn() -> i32) -> R, R>(f: F) -> R {
    f(|| 42)
}

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

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