code:

struct Container {
    cache: std::collections::HashMap<u32, String>,
}

impl Container {
    pub fn get(&mut self) -> &str {
        let id = 42;
        let cache = &mut self.cache;
        if let Some(value) = cache.get(&id) {
            return value;
        }
        cache.insert(id, "foo".to_owned());
        return cache.get(&id).unwrap();
    }
}