src/variable_bindings/mut.md

# Mutability

Variable bindings are immutable by default, but this can be overridden using
the `mut` modifier.

```rust,editable,ignore,mdbook-runnable
fn main() {
    let _immutable_binding = 1;
    let mut mutable_binding = 1;

    println!("Before mutation: {}", mutable_binding);

    // Ok
    mutable_binding += 1;

    println!("After mutation: {}", mutable_binding);

    // Error!
    _immutable_binding += 1;
    // FIXME ^ Comment out this line
}
```

The compiler will throw a detailed diagnostic about mutability errors.