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

1struct Logger {
2    entries: Vec<String>,
3}
4
5impl Logger {
6    fn last_entry(&self) -> Option<&str> {
7        self.entries.last().map(|s| s.as_str())
8    }
9
10    fn entries_since(&self, n: usize) -> &[String] {
11        &self.entries[n..]
12    }
13
14    fn total_entries(&self) -> usize {
15        self.entries.len()
16    }
17}
18
19let logger = Logger { entries: vec!["start".into(), "end".into()] };
20println!("{:?}", logger.last_entry());  // Some("end")
21println!("Total: {}", logger.total_entries());  // 2

1impl Logger {
2    fn log(&mut self, message: String) {
3        self.entries.push(message);
4    }
5
6    fn truncate(&mut self, keep_last: usize) {
7        let len = self.entries.len();
8        if len > keep_last {
9            self.entries.drain(..len - keep_last);
10        }
11    }
12
13    fn clear(&mut self) {
14        self.entries.clear();
15    }
16}
17
18let mut logger = Logger { entries: vec![] };
19logger.log("event1".into());
20logger.truncate(10);