`repr(transparent)` for `UnsafeCell`

[wyfo]

I wanted to use loom::cell::UnsafeCell in my project, but i could not, because the loom implementation is not repr(transparent), as it stores an additional state.

Would it be possible to store the state in a global-like map, and use the pointer of the cell as the map key (parking_lot dies this kind of thing if I'm not mistaken).

Actually, I tried to implement a draft of this idea in a branch, and it works almost well, but there is one test failing: unsafe_cell_ok_3. The main difference between the current implementation and my draft is that cell state is initialized lazily when the first read/write access is done, instead of being initialized at creation (that's because moving the cell before using would change the pointer of the cell).

However, in unsafe_cell_ok_3, when the cell state is lazily initialized, causality is not zeroed, which leads to an incorrect write access stored in the state. The issue comes from this line https://github.com/tokio-rs/loom/blob/bcf4e843e2c52f1b41608c2f8f911176f3165db9/src/rt/cell.rs#L88-L100, but I don't really understand it; why read/write accesses should be initialized with causality? I've tested to replace causality by VersionVec::new(), and then all tests pass.

So, I would like to ask why do we need to initialize cell state with causality?