Mutability

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

  1. fn main() {
  2.     let _immutable_binding = 1;
  3.     let mut mutable_binding = 1;
  5.     println!("Before mutation: {}", mutable_binding);
  7.     // Ok
  8.     mutable_binding += 1;
  10.     println!("After mutation: {}", mutable_binding);
  12.     // Error!
  13.     _immutable_binding += 1;
  14.     // FIXME ^ Comment out this line
  15. }

The compiler will throw a detailed diagnostic about mutability errors.