#![feature(fn_traits, unboxed_closures)] use std::{fmt, ops}; struct Debuger<T> { x: T } impl<T: fmt::Debug> ops::FnOnce<(),> for Debuger<T> { type Output = (); fn call_once(self, _args: ()) { //~^ ERROR `call_once` has an incompatible type for trait //~| expected signature `extern "rust-call" fn //~| found signature `fn println!("{:?}", self.x); } } fn make_shower<T>(x: T) -> Debuger<T> { Debuger { x: x } } pub fn main() { let show3 = make_shower(3); show3(); }