code:

use std::collections::HashMap;

struct Test<'a> {
    map: HashMap<&'a str, u32>
}

impl<'a> Test<'a> {
    fn new() -> Test<'a> {
        return Test {
            map: HashMap::new()
        }
    }

    fn size(&self) -> usize {
        return self.map.keys().count();
    }

    fn add(&self, key: &'a str, count: u32) {  <------ I don't want to have to pass &mut self
        self.map.insert(key, count);
    }

}

fn main() {
    let t = Test::new();

    t.add("test", 1);
    println!("{}", t.size());
}