This sounds like a regression, but it was masking a "race condition".
DynamicGuard runs the change callbacks on drop in a background thread.
Validations was using the guard to not have to lock twice.
This led to an issue where the invalid count might be non-zero due to
the callbacks not being invoked, preventing the closure from being
invoked even though there are no validation errors.
Introducing compare_swap gives a higher-level API for Validations to
use, and it also ensures the callbacks are able to be run in the current
thread.
