compiler/rustc_error_codes/src/error_codes/E0183.md

   1 Manual implementation of a `Fn*` trait.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0183
   6 struct MyClosure {
   7     foo: i32
   8 }
   9
  10 impl FnOnce<()> for MyClosure {  // error
  11     type Output = ();
  12     extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
  13         println!("{}", self.foo);
  14     }
  15 }
  16 ```
  17
  18 Manually implementing `Fn`, `FnMut` or `FnOnce` is unstable
  19 and requires `#![feature(fn_traits, unboxed_closures)]`.
  20
  21 ```
  22 #![feature(fn_traits, unboxed_closures)]
  23
  24 struct MyClosure {
  25     foo: i32
  26 }
  27
  28 impl FnOnce<()> for MyClosure {  // ok!
  29     type Output = ();
  30     extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
  31         println!("{}", self.foo);
  32     }
  33 }
  34 ```
  35
  36 The arguments must be a tuple representing the argument list.
  37 For more info, see the [tracking issue][iss29625]:
  38
  39 [iss29625]: https://github.com/rust-lang/rust/issues/29625