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

Question

Accepted Answer

**&self** provides immutable access — read but not modify. **&mut self** provides mutable access — read and modify, but exclusively (no other references while active).

**&self — shared, immutable borrow:**
- Multiple &self borrows can exist simultaneously.
- Cannot change any field of the struct.
- Compiler ensures data doesn't change during reads.
- Used for getters, display, and queries.

```rust
struct Config {
    width: u32,
    height: u32,
    title: String,
}

impl Config {
    fn dimensions(&self) -> (u32, u32) {
        (self.width, self.height)
    }

fn area(&self) -> u32 {
        self.width * self.height
    }

fn summary(&self) -> String {
        format!("{}: {}x{}", self.title, self.width, self.height)
    }
}

let config = Config { width: 800, height: 600, title: "MyApp".into() };
let (w, h) = config.dimensions();  // &self
println!("{}", config.summary());   // &self — can coexist
```

**&mut self — exclusive, mutable borrow:**
- Only ONE &mut self can exist at any time.
- Can freely modify any field.
- No other references (&self or &mut self) can exist while active.
- Used for setters, mutators, and state changes.

```rust
impl Config {
    fn resize(&mut self, width: u32, height: u32) {
        self.width = width;
        self.height = height;
    }

fn set_title(&mut self, title: String) {
        self.title = title;
    }

fn scale(&mut self, factor: f64) {
        self.width = (self.width as f64 * factor) as u32;
        self.height = (self.height as f64 * factor) as u32;
    }
}

let mut config = Config { width: 800, height: 600, title: "MyApp".into() };
config.resize(1920, 1080);
config.set_title("FullScreen".into());
```

**When to use &self:**
- Getters, iterators, display methods.
- Any read-only computation.
- When multiple simultaneous borrows are needed.

**When to use &mut self:**
- Setters, mutators, state updates.
- Builder pattern (return &mut Self).
- When exclusive access is required.

**Common mistakes:**
- Forgetting `let mut` when calling &mut self methods.
- Holding &self while calling &mut self (borrow checker error).
- Making methods &mut self when &self works.

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