I think it would be worth actually saying the `..` bit, since this error message could be user facing and, also, describing the actual internal state/details isn't particularly useful: as long as the message is vaguely unique, it takes 15 seconds with `git grep` to find the source location, and the failure condition is super simple and obvious from the source (i.e. the fact that we're emitting this implies that `expand` is true, if the condition was something like `some_integer % 2 == 0` then printing the value would be useful).