Error code E0183

Manual implementation of a Fn* trait.

Erroneous code example:

#![allow(unused)] fn main() { 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)].

#![allow(unused)] #![feature(fn_traits, unboxed_closures)] fn main() { 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: