`cell-project` used incorrect variance when projecting through `&Cell<T>`
Advisory: GHSA-p75v-367r-2v23
Published: 2022-09-16T21:08:19Z

## Overview

The issue lies in the implementation of the `cell_project` macro which used `field as *const _` instead of `field as *mut _`.
The problem being that `*const T` is covariant in `T` while `*mut T` is invariant in `T`. Keep in mind that `&Cell<T>` is invariant in `T`,
so casting to `*const T` relaxed the variance, and lead to unsoundness, as shown in the example below.

```rs
use std::cell::Cell;
use cell_project::cell_project as cp;

struct Foo<'a> {
    x: Option<&'a Cell<Foo<'a>>>,
}

impl<'a> Drop for Foo<'a> {
    fn drop(&mut self) {
        // `ourselves` is an &Cell<Self>.
        // NB: `Drop` is unsound.
        if let Some(ourselves) = self.x.as_ref() {
            // replace `self` (but this doesn't actually replace `self`)
            let is_x_none = ourselves.replace(Foo {
                x: None,
            }).x.as_ref().is_none();
            // if we just moved out of `self`, and we had a `Some` originally,
            // how come this is a `None`?
            if is_x_none {
                println!("how did we get a None?");
            }
        }
    }
}

fn main() {
    let foo = Cell::new(Foo {
        x: None,
    });
    let x = cp!(Foo<'_>, foo.x);
    x.set(Some(&foo));
}
```

### MIRI error

```rs
$ cargo +nightly miri run
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `<snip>`
error: Undefined Behavior: not granting access to tag <untagged> because incompatible item is protected: [Unique for <2472> (call 796)]
   --> $RUST_STD_PATH/src/rust/library/core/src/cell.rs:404:31
    |
404 |         mem::replace(unsafe { &mut *self.value.get() }, val)
    |                               ^^^^^^^^^^^^^^^^^^^^^^ not granting access to tag <untagged> because incompatible item is protected: [Unique for <2472> (call 796)]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

    = note: inside `std::cell::Cell::<Foo>::replace` at $RUST_STD_PATH/src/rust/library/core/src/cell.rs:404:31
note: inside `<Foo as std::ops::Drop>::drop` at src/main.rs:14:29
   --> src/main.rs:14:29
    |
14  |               let is_x_none = ourselves.replace(Foo {
    |  _____________________________^
15  | |                 x: None,
16  | |             }).x.as_ref().is_none();
    | |______________^
    = note: inside `std::ptr::drop_in_place::<Foo> - shim(Some(Foo))` at $RUST_STD_PATH/src/rust/library/core/src/ptr/mod.rs:486:1
    = note: inside `std::ptr::drop_in_place::<std::cell::UnsafeCell<Foo>> - shim(Some(std::cell::UnsafeCell<Foo>))` at $RUST_STD_PATH/src/rust/library/core/src/ptr/mod.rs:486:1
    = note: inside `std::ptr::drop_in_place::<std::cell::Cell<Foo>> - shim(Some(std::cell::Cell<Foo>))` at $RUST_STD_PATH/src/rust/library/core/src/ptr/mod.rs:486:1
note: inside `main` at src/main.rs:32:1
   --> src/main.rs:32:1
    |
32  | }
    | ^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error
```

## Affected Versions

All versions of the cell-project crate before `0.1.4` are affected.

## Mitigation

This was fixed in [Issues/4], and released as version `0.1.4`.
So just updating to the latest version will include the fix (which may result in a compile error on unsound usage).

## Acknowledgements

This was discovered and fixed by @SoniEx2 in cell-project: [Issues/3] and [Issues/4]

[issues/3]: https://github.com/RustyYato/cell-project/issues/3
[issues/4]: https://github.com/RustyYato/cell-project/issues/4