tracked-structs: access to field whilst the value is being initialized

[MichaReiser]

We see more spurious panics in the read_lock function of TrackedStructs:

panicked at /home/runner/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/d758691/src/tracked_struct.rs:824:21:
access to field whilst the value is being initialized

I haven't been able to create a minimal reproduction yet. We see it both in our tests (where we heavily relly on incrementality for faster tests) and in the playground (IDE like environment)

This might be solved by #741

I'm opening this issue to document the problem. I still hope to find some time to push #741 over the line but I'm not sure yet when this will be.