Rust mutating borrowed struct pattern - memory

Question

Here is an example of some structs:
enum VehicleType {
Car,
Motorcycle,
}
struct Vehicle {
name: String,
horsepowers: i32,
vehicle_type: VehicleType,
}
struct Person<'a> {
vehicle: &'a Vehicle,
name: &'a str,
}
and in main function:
let mut car = Vehicle {
name: "Nissan GTR".to_string(),
horsepowers: 300,
vehicle_type: VehicleType::Car,
};
let alice = Person {
name: "Alice",
vehicle: &car, // Share a reference to the same car
};
let bob = Person {
name: "Bob",
vehicle: &car, // Share a reference to the same car
};
println!("{} drives {}", alice.name, alice.vehicle.name);
println!("{} drives {}", bob.name, bob.vehicle.name);
Now let's say we want to update the name of the car while preserving that Alice and Bob drive the same car
car.name = "Lamborghini".to_string();
car.horsepowers = 684;
println!("{} drives {}", alice.name, alice.vehicle.name);
println!("{} drives {}", bob.name, bob.vehicle.name);
This of course fails because car is borrowed by both Alice and Bob.
Why wouldn't rust compiler allow this? How does this introduce memory safety issues? How to go about a pattern like this?

Answer 1

You cannot mutate an object when it is immutably borrowed.
If you want to reference the same Vehicle object in the Person structs and be able to mutate it, you may want to use a reference to a RefCell<T>:
use std::cell::RefCell;
enum VehicleType {
Car,
Motorcycle,
}
struct Vehicle {
name: String,
horsepowers: i32,
vehicle_type: VehicleType,
}
struct Person<'a> {
vehicle: &'a RefCell<Vehicle>,
name: &'a str,
}
fn main() {
let mut car = RefCell::new(Vehicle {
name: "Nissan GTR".to_string(),
horsepowers: 300,
vehicle_type: VehicleType::Car,
});
let alice = Person {
name: "Alice",
vehicle: &car,
};
let bob = Person {
name: "Bob",
vehicle: &car,
};
println!("{} drives {}", alice.name, alice.vehicle.borrow().name);
println!("{} drives {}", bob.name, bob.vehicle.borrow().name);
car.borrow_mut().name = "Lamborghini".to_string();
car.borrow_mut().horsepowers = 684;
println!("{} drives {}", alice.name, alice.vehicle.borrow().name);
println!("{} drives {}", bob.name, bob.vehicle.borrow().name);
}
NB: i32 is a poor choice for the horsepower of the car, since a negative horsepower does not make any sense. Consider using u32 instead.