Pointer dereferencing in Rust

Question

I am rather confused about the following problem. If I understand it correctly

let x = &42;

is expanded internally to

let x_value = 42;
let x = &x;

I think I have seen this in the Rust book somewhere but I cannot find the reference to it.

My problem is concerning the following code:

let x = 42;
let rx = &x;
let px = rx as *const i32 as *mut i32;
unsafe {
  *px = 0;
}
println!("{}", x);

As expected this prints 0. However, if I write

let rx = &42;
let px = rx as *const i32 as *mut i32;
unsafe {
  println!("Deref");
  *px = 0;
}
println!("{}", x);

the program terminates after printing out Deref. Aparently something goes wrong when px gets dereferenced. I guess my first assessment about let x = &42 getting expanded internally is wrong.

Answer 1

You're invoking undefined behavior. From the Rust Reference:

12.3 Behavior considered undefined

The following is a list of behavior which is forbidden in all Rust code, including within unsafe blocks and unsafe functions. [...]

• Mutating non-mutable data (that is, data reached through a shared reference or data owned by a let binding), unless that data is contained within an UnsafeCell<U>.

Since you're mutating non-mutable data, you're invoking undefined behavior. The fact that it works at all in the first version is just (bad) luck.