Unsoundness in `OwningRef::map_with_owner` and more

[noamtashma]

I found more unsoundness problems. This extends #61 .

In particular, OwningRef::map_with_owner allows creating an OwningRef that points at the owner, which can be moved.

fn unstable_address() {
    let ow_ref = OwningRef::new(Box::new(5));
    let new_ow_ref = ow_ref.map_with_owner(|owner, _my_ref| owner);
    println!("Reading memory that was moved from: {}", *new_ow_ref);
}

This by itself can be fixed by replacing map_with_owner with a method that only gives out a reference to the referent of the owner, like this:

pub fn map_with_owner<F, U: ?Sized>(self, f: F) -> OwningRef<'t, O, U>
    where O: StableAddress + Deref,
        F: for<'a> FnOnce(&'a O::Target, &'a T) -> &'a U

In addition, There's unsoundness combining a conversion from OwningRefMut to OwningRef together with methods that can read the owner of an OwningRef, like so:

fn ref_mut_to_ref() {
    use core::cell::RefCell; // `Cell` and any other kind of cell also works
    let ow_ref = OwningRefMut::new(Box::new(RefCell::new(Box::new(5))));
    let new_ow_ref = ow_ref.map(|cell| &**cell.get_mut());
    // Have to convert to `OwningRef` In order to use `OwningRef::as_owner`
    // instead of `OwningRefMut::as_owner`.
    *new_ow_ref.as_owner().borrow_mut() = Box::new(9);
    println!("Reading deallocated memory: {}", *new_ow_ref);
}

OwningRefMut::{as_owner, as_owner_mut} can also be used, as #61 shows.

There are two ways to fix this, and each choice corresponds to a small difference in the meaning and invariants of OwningRef:

• Disallow conversions from OwningRefMut to OwningRef, and allow shared access to the owner of an OwningRef. Invariant: the reference of the OwningRef may only borrow immutably from the owner.
• Allow conversions from OwningRefMut to OwningRef, but disallow shared access to the owner of an OwningRef. Invariant: the reference of the OwningRef may borrow immutably or mutably from the owner.

Essentially, these are two distinct types, which are both sound by themselves, and a third option is to have both types.

[slonik-az]

This repo has not received any commits for two years (since Feb 27, 2020) and seems unmaintained. Did you try to contact the owners directly?

[noamtashma]

Hey everyone, since so long has passed and still the maintainer hasn't showed up and no one made a replacement crate, I decided to make my pull request into a crate.

It's available as safer_owning_ref.