AcceptToMemoryPoolWorker takes an out-param called coins_to_uncache which it populates with any outpoints-to-be-spent that do not exist in pcoinsTip's in-memory cache.
Confusingly, we then attempt to pcoinsTip->Uncache() those outpoints if the transaction can't be added to the mempool. But we only added those outpoints to coins_to_uncache if they didn't exist in pcoinsTip's cache... so this whole parameter and process seems useless. Maybe I'm missing something?
The point is that these coins were pulled into the cache by ATMPW, but we don't want them there, so they're actively removed afterwards.
The idea is that we uncache the inputs that were added to pcoinsTip during validation of the rejected transaction. Otherwise, someone could send us junk transactions that waste our cache memory, harming performance.
Ah, my bad - thanks for the pointers!
I had trouble finding where the outpoints were actually being added to pcoinsTip; it turns out the addition is buried in a call to view.HaveCoin() -> view.FetchCoin(), both of which are (somewhat confusingly) marked const.
I'll follow up with a doc PR for posterity.