Code shared from the Rust Playground

playground.rs

use std::collections::HashMap;

trait Iface {
  fn fun(&mut self, c: &mut Container);
}

struct Container {
  map: HashMap<i32, Box<dyn Iface>>,
}
impl Container {
  fn do_it(&mut self) {
    if let Some(i) = self.map.get_mut(&123) {
    //if let Some(mut i) = self.map.remove(&123) {
      i.fun(self);
      //self.map.insert(123, i);
    } else {
      panic!("unexpected");
    }
  }
}

struct Thing {}
impl Iface for Thing {
  fn fun(&mut self, c: &mut Container) {
    c.map.insert(456, Box::new(Thing{}));
  }
}

fn main() {
  let mut c = Container{map:HashMap::new()};
  c.map.insert(123, Box::new(Thing{}));
  c.do_it();
  for key in c.map.keys() {
    println!("{}", key);
  }
}