#![feature(fn_traits)]
#![feature(unboxed_closures)]

use std::ops;

struct Bar;
struct Foo;

impl ops::FnOnce<usize> for Foo {
    type Output = Bar;

    extern "rust-call" fn call_once(self, idx: usize) -> Self::Output {
        unimplemented!()
    }
}

fn main() {}