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

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**&self** = immutable borrow, multiple allowed, read-only. **&mut self** = mutable borrow, exclusive, read-write. The borrow checker enforces these rules at compile time. **&self — shared read access:** - Multiple &self borrows can coexist. - Cannot modify struct fields. - Guarantees data stability during reads. - Used for getters, display, queries. ```rust struct Database { records: Vec, } impl Database { fn count(&self) -> usize { self.records.len() } fn find(&self, query: &str) -> Vec<&str> { self.records.iter() .filter(|r| r.contains(query)) .map(|s| s.as_str()) .collect() } } let db = Database { records: vec!["Alice".into(), "Bob".into()] }; println!("Count: {}", db.count()); // 2 println!("A: {:?}", db.find("A")); // ["Alice"] ``` **&mut self — exclusive write access:** - Only ONE &mut self at a time. - Can modify struct fields. - No other references while active. - Used for setters and state changes. ```rust impl Database { fn add(&mut self, record: String) { self.records.push(record); } fn remove(&mut self, index: usize) -> Option { if index < self.records.len() { Some(self.records.remove(index)) } else { None } } } let mut db = Database { records: vec![] }; db.add("New record".into()); ``` **When to use &self:** - Read-only methods. - Multiple simultaneous borrows. - Display, comparison, computation. **When to use &mut self:** - State-changing methods. - Builder pattern. - Exclusive access required. **Common mistakes:** - Missing `let mut` for &mut self methods. - Holding &self while calling &mut self. - Overusing &mut self.

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