once.rs

#![feature(fn_traits, unboxed_closures, tuple_trait)]
use std::marker::Tuple;

pub struct Once<F> {
    f: Option<F>,
}

impl<Args: Tuple, F: FnOnce<Args, Output = ()>> FnMut<Args> for Once<F> {
    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
        if let Some(f) = self.f.take() {
            f.call_once(args)
        }
    }
}

impl<Args: Tuple, F: FnOnce<Args, Output = ()>> FnOnce<Args> for Once<F> {
    type Output = ();
    extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
        if let Some(f) = self.f {
            f.call_once(args)
        }
    }
}

pub fn once<Args: Tuple, F: FnOnce<Args>>(f: F) -> Once<F> {
    Once { f: Some(f) }
}

fn foo(a: i32, b: i32) {
    println!("{a} {b}");
}

fn main() {
    let mut f = once(foo);

    f(1, 2);
    f(3, 4);
}