dubious `&raw const|mut value` compile error

[Paladynee]

Code:

fn minimal() {
    let mut target = 10;
    let mutref = &mut target;
    let ptr = &raw mut target; // can also be &raw const target;
    drop(mutref)
}

According to the reference, the &raw mut|const val syntax should be used "whenever creating a reference would introduce incorrect aliasing assumptions". We also know that pointers acting as funny usize's is safe as long as we don't read from/write to them(or invoke other unsafe functions or unsafe superpowers with them) [citation needed]. The compiler error we get from this contradicts this:

error[E0499]: cannot borrow `target` as mutable more than once at a time
 --> src/lib.rs:4:15
  |
3 |     let mutref = &mut target;
  |                  ----------- first mutable borrow occurs here
4 |     let ptr = &raw mut target;
  |               ^^^^^^^^^^^^^^^ second mutable borrow occurs here
5 |     drop(mutref)
  |          ------ first borrow later used here

For more information about this error, try `rustc --explain E0499`.
warning: `sdfvkj` (lib) generated 1 warning
error: could not compile `sdfvkj` (lib) due to 1 previous error; 1 warning emitted

The whole premise of the addr_of! and addr_of_mut! macros and the &raw mut|const value syntax is to not go through the intermediary &T or &mut T where we can't guarantee the soundness of the existence of such references.

The actual real life scenario that caused this problem stems from using the pointer as an indicator of which value is being pointed to:

let mut val1 = ...;
let mut val2 = ...;

let ref1 = &mut val1;
let ref2 = &mut val2;

loop {
    (ref1, ref2) = (ref2, ref1); // just a normal swap

    let ptr = &raw const val1; // error!
    if std::ptr::eq(ptr, ref1) {
        // ref1 points to val1
    } else {
        // ref1 points to val2
    }
}

Which doesn't use the pointer to read from/write to the value at all, just an indicator to which stack value is being pointed.

[kpreid]

As one of the people involved in the discussion where this came up: I think that this restriction might be

• a “lint” behavior intended to catch not certain but highly likely occurrences of prohibited aliasing, or
• actually necessary because even retrieving the place expression’s address while it is exclusively borrowed is UB

But if either of those is true, it should be documented in the Reference.

[saethlin]

We also know that pointers acting as funny usize's is safe as long as we don't read from/write to them [citation needed].

This is true for plain old reads and writes of pointers themselves, not necessarily all the other operations on pointers. offset is an easy example of a raw pointer being more than a usize even without doing a read or write through the pointer.

[Paladynee]

As one of the people involved in the discussion where this came up: I think that this restriction might be

* a “lint” behavior intended to catch _not certain but highly likely_ occurrences of prohibited aliasing, or

* actually necessary because even retrieving the place expression’s address while it is exclusively borrowed is UB

But if either of those is true, it should be documented in the Reference.

The first bullet was my first guess, if this is an intended change. Although, I don't understand why retrieving the address of the place expression would be UB? Does a unique reference allow the owning value's address to change? Wouldn't that invalidate the unique reference?

Also, if the discussion was made in zulip, can I get a link?

[Paladynee]

We also know that pointers acting as funny usize's is safe as long as we don't read from/write to them [citation needed].

This is true for plain old reads and writes of pointers themselves, not necessarily all the other operations on pointers. offset is an easy example of a raw pointer being more than a usize even without doing a read or write through the pointer.

I see, let's add offset along with every other unsafe method on raw pointers as well. Would this be a sound operation?

[kpreid]

Also, if the discussion was made in zulip, can I get a link?

Sorry, I meant the discussion on Discord today.

[moxian]

@rustbot label: +T-opsem +A-raw-pointers