expansion-based box exacerbates pretty-printing to unstableness

[pnkfelix]

Spawned off of #23616

The new expansion-based box protocol (#22181) will internally expanded into calls to API methods that are unstable.

This is going to mean that tests that use box that today can be tagged with pretty-expanded FIXME #23616 are going to have to switch to not opting-into the pretty-expanded test, I think due to the same issue as #23616.

(Thinking about the problem now, we could work around this problem by having the pretty-printer inject attributes to turn off the stability checking. This would only work on nightly so it wouldn't resolve #23616 as written, but it would allow us to continue testing pretty-expanded on things that use box, I think...)

[steveklabnik]

Triage: no changes that I'm aware of.

[Mark-Simulacrum]

Closing in favor of #23616. I don't see this as being a separate problem, really.