I’d like to pass a closure to a methods which mutates a captured variable.

I can do it, but then the compiler suggests me to give the static lifetime to the captured variables, or they may not live long enough. I have a hunch that this is not what I want, but I’m a bit stuck.

Here is an example:

fn main() {
    struct NeedleFinder {
        on_needle_found: Box<dyn FnMut(usize)>,
    }
    
    impl NeedleFinder {
        fn new(on_needle_found: Box<dyn FnMut(usize)>) -> Self {
            Self { on_needle_found }
        }

        fn find_needles(& mut self, haystack: &Vec<&str>) {
            for (i, element) in haystack.iter().enumerate() {
                if *element == "needle" {
                    (self.on_needle_found)(i);
                }
            }
        }
    }

    // This poses no problem 
    let mut needle_finder = NeedleFinder::new(Box::new(|index| {
        println!("Found needle at index {}", index);
    }));
    needle_finder.find_needles(&vec!["haystack", "needle", "haystack", "needle"]);
    
    
    /*
    // This on the other hand does not work
    
    let mut needle_positions: Vec<usize> = vec![];

    let mut needle_finder = NeedleFinder::new(Box::new(|index| {
        println!("Found needle at index {}", index);
        needle_positions.push(index);
    }));

    needle_finder.find_needles(&vec!["haystack", "needle", "haystack", "needle"]);
    */
}

and the playground link.

I imagine this is a trivial problem for someone versed in rust, but I’m struggling a bit here, I’ll take any suggestion!