Code shared from the Rust Playground

playground.rs

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

use std::marker::PhantomData;
use std::ops::{Add, FnOnce};

struct WhateverFn<F, T>(F, PhantomData<T>);

impl<F, T> FnOnce<(T,)> for WhateverFn<F, T> where F: FnOnce<(T,)> {
    type Output = F::Output;

    extern "rust-call" fn call_once(self, arg: (T,)) -> Self::Output {
        self.0(arg.0)
    }
}

impl<F, T, U> Add<U> for WhateverFn<F, T> where F: FnOnce<(T,)>, F::Output: Add<U> {
    type Output = WhateverFn<impl FnOnce<T, Output=Self::Output>, T>;

    fn add(self, rhs: U) -> Self::Output {
        WhateverFn(|x| self(x) + rhs, PhantomData)
    }
}

fn main() {
    let what = WhateverFn(|n: usize| n, PhantomData);
    let erm = what + 3;
    println!("{}", erm(7));
}