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 does 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

loom/src/rt/cell.rs

Lines 88 to 100 in bcf4e84

	fn new(threads: &thread::Set, location: Location) -> State {
	let version = threads.active().causality;
	State {
	created_location: location,
	is_reading: 0,
	is_writing: false,
	read_access: version,
	read_locations: LocationSet::new(),
	write_access: version,
	write_locations: LocationSet::new(),
	}
	}

, 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?