# `fn_traits`

The tracking issue for this feature is [#29625]

See Also: [`unboxed_closures`](../language-features/unboxed-closures.md)

[#29625]: https://github.com/rust-lang/rust/issues/29625

----

The `fn_traits` feature allows for implementation of the [`Fn*`] traits
for creating custom closure-like types.

[`Fn*`]: https://doc.rust-lang.org/std/ops/trait.Fn.html

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

struct Adder {
    a: u32
}

impl FnOnce<(u32, )> for Adder {
    type Output = u32;
    extern "rust-call" fn call_once(self, b: (u32, )) -> Self::Output {
        self.a + b.0
    }
}

fn main() {
    let adder = Adder { a: 3 };
    assert_eq!(adder(2), 5);
}
```