The majority of 83e85c058ab7b63adcd9ead3ac80c70d64cf31eb “coinselection: option to exclude change cost from waste score” feels like a layer violation. If we don’t create change, that should be obvious from the excess being smaller than min_viable_change + change_fee. Let’s please not use change_cost as a dual-intent variable. change_cost/cost_of_change is the amount that it would cost to create and spend a change if there is one. If you want to communicate the intent of not creating change explicitly, please use a separate designated variable that only communicates that and only that.
If we can identify the change output, we should remove it before coin selection, redo coin selection with the existing inputs as preset inputs and then create a new change output. The change output should absolutely be part of the waste calculation, since the replacement transaction at a higher feerate may have multiple possible input sets, some of which might be changeless, which then should be preferred by the scoring accordingly.
Generally, if a custom change output address is provided, I would read that as “if there will be change, please send it to this”. If the user wants a specific output, they should add it as a recipient. If the user wants a specific output of at least some amount without creating change, they should send a higher amount and subtract the fee from that output, or use a sendall with preset inputs. So I don’t see how there should ever be a case of “the user explicitly wanted a specific change”—that would be the description of a recipient output.
