enumunsafety.rs

struct Spaceship {
  subsystem: Subsystem
}
enum Subsystem {
  Engine { fuel: i64 },
  Navigation { antenna: Box<Antenna> },
}
struct Antenna {
  frequency: i64
}
fn main() -> () {
  let mut ship =
      Spaceship{
          subsystem: Subsystem::Navigation{
              antenna: Box::new(
                  Antenna { frequency: 100 }
              )
          }
      };
  match &ship.subsystem {
    Subsystem::Navigation{antenna} => {
      ship.subsystem = Subsystem::Engine{fuel: 10};

      antenna.frequency = 200;
      println!("{:?}", antenna.frequency);
    }
    Subsystem::Engine{fuel} => {
      println!("{:?}", fuel);
    }
  }
}

// error[E0506]: cannot assign to `ship.subsystem` because it is borrowed
//   --> enumunsafety.rs:22:7
//    |
// 20 |   match &ship.subsystem {
//    |         --------------- borrow of `ship.subsystem` occurs here
// 21 |     Subsystem::Navigation{antenna} => {
// 22 |       ship.subsystem = Subsystem::Engine{fuel: 10};
//    |       ^^^^^^^^^^^^^^ assignment to borrowed `ship.subsystem` occurs here
// 23 | 
// 24 |       antenna.frequency = 200;
//    |       ----------------------- borrow later used here
// 
// error[E0594]: cannot assign to `antenna.frequency` which is behind a `&` reference
//   --> enumunsafety.rs:24:7
//    |
// 24 |       antenna.frequency = 200;
//    |       ^^^^^^^^^^^^^^^^^^^^^^^ `antenna` is a `&` reference, so the data it refers to cannot be written
// 
// error: aborting due to 2 previous errors