TB: add Cell state to support more fine-grained tracking of interior mutable data

[yoctocell]

This adds a new Cell state to TB to represent shared references to interior mutable data. This is still a WIP for making the tracking of interior mutable data per-byte, like in Stacked Borrows.

cc @RalfJung @JoJoDeveloping

[RalfJung]

Testing if this works:
r? @RalfJung

[RalfJung]

Looks good, thanks. :) My comments are all minor.

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[yoctocell]

@rustbot ready

[RalfJung]

@rustbot author
Please post @rustbot ready when the PR is ready for the next round of review.

Also, why is this marked as "draft"? Since you were asking for a review, I assume this is ready to land, so please update the PR status accordingly. Usually "draft" means "not yet ready for review".

[yoctocell]

@rustbot ready

[RalfJung]

This looks great, thanks! Please squash the commits, then we can land this. Please use the --keep-base flag when squashing so that the force-push diff is easier to review; do not rebase on the latest master branch. Then write @rustbot ready after you pushed the squashed PR.

@rustbot author

[RalfJung]

It seems like something went wrong with the rebase. Make sure you have the latest upstream commit fetched into your local clone.

[yoctocell]

So you mean that I should run git pull on my master branch?

[RalfJung]

It depends on your setup. For me, it's git fetch origin, but you have to replace "origin" with whatever the name of the Miri upstream remote (https://github.com/rust-lang/miri, not your clone) is. Then I do git rebase origin/master --keep-base.

[yoctocell]

Ok, now I have fetched the latest commit from the upstream remote. What would be the next step?

[RalfJung]

I have linked the tutorial for squashing above: https://rustc-dev-guide.rust-lang.org/git.html#squash-your-commits

[yoctocell]

Running git rebase origin/master --keep-base outputs Current branch new-cell-state is up to date.. Is this expected?

[RalfJung]

Then your origin is out-of-date. Which URL is origin? What does git remote get-url origin say?

[yoctocell]

I get https://github.com/rust-lang/miri. Running git fetch origin doesn't do anything now, so seems to be the latest version.

[RalfJung]

Hm that doesn't check out, your branch is not based on origin/master. Maybe try without --keep-base?

Or you can check git reflog for the commit ID before the broken rebase, to reset to a good state, and then try again.

[RalfJung]

The commit ID before the broken rebase seems to be 64b11de, so the best fix is probably git reset --hard 64b11de958c114f6de837a25daec9ba8eda6d971, and then trying again.

[yoctocell]

Doing git reset seemed to fix things :)

[RalfJung]

Great. :)