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.

Cornstalks · Accepted Answer · 2017-02-26 17:03:58Z

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.

I looked up the implementation of UnsafeCell<T> and wanted to experiment with raw pointers. The important part I overlooked is the attribute #[lang = "unsafe_cell"]. Thanks.

mnemotronic · Accepted Answer · 2023-03-13 07:24:22Z

let x = 42; let rx = &x;

Here you create a new variable on stack and write the value 42 that comes from the code of your program. When creating a reference, it is referencing this value on stack. When you change the value by *px = 0; you gain access to the value on stack and can change it.

In the second case

let rx = &42;

you create a reference directly to the value stored in the part of memory that stores the code (instructions and other data). And this part of memory is immutable.

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