Shared via Rust Playground

playground.rs

fn get_c() -> Box<FnMut(i32)> {
    Box::new(|val: i32| {
        println!("value --> {}", val);
    })
}

pub fn set_c<F>(mut callback: F)
    where F: FnMut(i32)
{
    callback(100);
}

fn main() {
    let mut cb = get_c();

    set_c(|x| cb(x));
    set_c(|val: i32| println!("value --> {}", val));
}