Manual implementation of a Fn* trait.

Erroneous code example:

struct MyClosure {
    foo: i32
}

impl FnOnce<()> for MyClosure {  // error
    type Output = ();
    extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
        println!("{}", self.foo);
    }
}

Manually implementing Fn, FnMut or FnOnce is unstable and requires #![feature(fn_traits, unboxed_closures)].

#![feature(fn_traits, unboxed_closures)]

struct MyClosure {
    foo: i32
}

impl FnOnce<()> for MyClosure {  // ok!
    type Output = ();
    extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
        println!("{}", self.foo);
    }
}

The arguments must be a tuple representing the argument list. For more info, see the tracking issue: