Shared via Rust Playground

playground.rs

macro_rules! defer {
    ($x:expr) => {
        let _x = {
            struct Deferred<F: Fn()>(F);
            
            impl<F: Fn()> Drop for Deferred<F> {
                fn drop(&mut self) {
                    self.0();
                }
            }
            Deferred(||$x)
        };
    }
}

use std::ops::{Deref, DerefMut};

pub struct Guard<T, F> where
    F: FnMut(&mut T)
{
    __dropfn: F,
    __value: T,
}

pub fn guard<T, F>(v: T, dropfn: F) -> Guard<T, F> where
    F: FnMut(&mut T)
{
    Guard{__value: v, __dropfn: dropfn}
}

impl<T, F> Deref for Guard<T, F> where
    F: FnMut(&mut T)
{
    type Target = T;
    fn deref(&self) -> &T
    {
        &self.__value
    }

}

impl<T, F> DerefMut for Guard<T, F> where
    F: FnMut(&mut T)
{
    fn deref_mut(&mut self) -> &mut T
    {
        &mut self.__value
    }
}

impl<T, F> Drop for Guard<T, F> where
    F: FnMut(&mut T)
{
    fn drop(&mut self) {
        (self.__dropfn)(&mut self.__value)
    }
}

fn incr(i : &mut i32) {*i += 1;}


fn main() {
    let x = Box::new(5);
    {
    defer!(println!("Defer 1: {:?}",x));
    //*x += 1;
    defer!(println!("Defer 2"));
    println!("x: {:?}", x);
    }
    let mut y = guard(x, |x|println!("y is now: {:?}",x));
    **y += 4;
    println!("y = {:?}", *y);
    incr(&mut y);
    
    
    let mut z = guard(1, |x|println!("z is now: {:?}",x));
    *z += 3;
    //let b : Box<str> = box *"x";
    //let a : _ = &*box b;
}

/*
    let _x = {
            #[derive(Debug)]
            struct Deferred<F: FnOnce()>(F);
            
            impl<F: FnOnce()> Drop for Deferred<F> {
                fn drop(&mut self) {
                    //println!("Dropping {:?}", $x);
                    
                    replace(&mut self.0, ||{} as F)();
                }
            }
            Deferred(|| println!("Drop {:?}",x))
    };
*/