rust/src/test/ui/run-pass/issues/issue-20544.rs

// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

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

struct Fun<F>(F);

impl<F, T> FnOnce<(T,)> for Fun<F> where F: Fn(T) -> T {
    type Output = T;

    extern "rust-call" fn call_once(self, (t,): (T,)) -> T {
        (self.0)(t)
    }
}

fn main() {
    let fun = Fun(|i: isize| i * 2);
    println!("{}", fun(3));
}