Rust: What is the difference between &mut self and &self?

1struct Cache {
2    entries: std::collections::HashMap<String, String>,
3}
4
5impl Cache {
6    fn get(&self, key: &str) -> Option<&str> {
7        self.entries.get(key).map(|s| s.as_str())
8    }
9
10    fn contains(&self, key: &str) -> bool {
11        self.entries.contains_key(key)
12    }
13
14    fn len(&self) -> usize {
15        self.entries.len()
16    }
17}
18
19let cache = Cache { entries: vec![("a".into(), "1".into())].into_iter().collect() };
20println!("a: {:?}", cache.get("a"));  // Some("1")
21println!("len: {}", cache.len());      // 1

1impl Cache {
2    fn insert(&mut self, key: String, value: String) {
3        self.entries.insert(key, value);
4    }
5
6    fn remove(&mut self, key: &str) -> Option<String> {
7        self.entries.remove(key)
8    }
9
10    fn clear(&mut self) {
11        self.entries.clear();
12    }
13}
14
15let mut cache = Cache { entries: HashMap::new() };
16cache.insert("x".into(), "10".into());
17cache.remove("x");