Right now, the query_plan_cache library builds both the classic and SBE plan caches under the same rule. Ideally, they would be split.

• classic_plan_cache would build just the stuff necessary for the classic plan cache.
• sbe_plan_cache would build just the stuff necessary for the SBE plan cache.
• query_plan_cache can use them as dependencies.

The biggest advantage is isolating the plan cache implementations and making it possible to test them in isolation (if only our tests were structured as such). It's also generally a good idea to separate distinct build units from each other with distinct rules.