Code shared from the Rust Playground

playground.rs

use core::ops::*;

fn main() {
    let mut times_vec_updated = 0;
    let mut vec = OnUpdate(vec![], |v| {
        eprintln!("Vec updated: {:?}", v);
        times_vec_updated += 1;
    });
    
    vec.borrow_mut().push(5);
    vec.borrow_mut().push(6);
    vec.borrow_mut().push(7);
    
    dbg!(times_vec_updated);
    
    let mut int = OnUpdate(10, |v| {
        eprintln!("Int updated: {:?}", v);
    });
    *int.borrow_mut() += 5;
}

#[derive(Debug, Clone, Copy)]
struct OnUpdate<T, F: FnMut(&T) -> ()>(T, F);

impl<T, F: FnMut(&T) -> ()> OnUpdate<T, F> {
    fn borrow_mut(&mut self) -> MutBorrow<T, F> {
        MutBorrow(&mut self.0, &mut self.1)
    }
}

#[derive(Debug)]
struct MutBorrow<'a, T, F: FnMut(&T) -> ()>(&'a mut T, &'a mut F);

impl<'a, T, F: FnMut(&T) -> ()> Drop for MutBorrow<'a, T, F> {
    fn drop(&mut self) {
        (self.1)(&self.0)
    }
}

impl<'a, T, F: FnMut(&T) -> ()> Deref for MutBorrow<'a, T, F> {
    type Target = T;
    
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<'a, T, F: FnMut(&T) -> ()> DerefMut for MutBorrow<'a, T, F> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}