problem: when I write a herald rule, I have no way to tell if it's correct - if it matches the bad code I want it to catch. As is, I have to wait until a diff actually triggers the rule to see it in action, *or* if it's wrong and matches less than I expect, I have to notice that things I getting by that shouldn't and investigate why; this would be extremely hard for me to find.
It would be really useful, when writing a new herald rule, to be able to see sample results of recent things it would have matched. In my case, I almost always use herald rules to match diffs, so it would be nice if it could run against the most recent ~1000 open diffs or so and show me what sort of diffs it matches. Or maybe instead, run against the current codebase and show me what it would match there, if it were sent in a diff today.