src/doc/unstable-book/src/library-features/fn-traits.md

   1 # `fn_traits`
   2
   3 The tracking issue for this feature is [#29625]
   4
   5 See Also: [`unboxed_closures`](language-features/unboxed-closures.html)
   6
   7 [#29625]: https://github.com/rust-lang/rust/issues/29625
   8
   9 ----
  10
  11 The `fn_traits` feature allows for implementation of the [`Fn*`] traits
  12 for creating custom closure-like types.
  13
  14 [`Fn*`]: https://doc.rust-lang.org/std/ops/trait.Fn.html
  15
  16 ```rust
  17 #![feature(unboxed_closures)]
  18 #![feature(fn_traits)]
  19
  20 struct Adder {
  21     a: u32
  22 }
  23
  24 impl FnOnce<(u32, )> for Adder {
  25     type Output = u32;
  26     extern "rust-call" fn call_once(self, b: (u32, )) -> Self::Output {
  27         self.a + b.0
  28     }
  29 }
  30
  31 fn main() {
  32     let adder = Adder { a: 3 };
  33     assert_eq!(adder(2), 5);
  34 }
  35 ```