Answer by trojanfoe for Immutable HashMap with mutable object as value

You will need to use interior mutability:

use std::{cell::RefCell, rc::Rc, collections::HashMap};fn main() {    let aa = HashMap::from([(1, Rc::new(RefCell::new(1)))]);    println!("{:?}",aa);    if let Some(value) = aa.get(&1) {        let mut value = value.borrow_mut();        *value = 2;    }    println!("{:?}",aa);}

result:

{1: RefCell { value: 1 }}{1: RefCell { value: 2 }}