> * `errors_on` has been moved to `rspec-collection_matchers` so the user can't use it w/o loading that gem at which point they might as well keep the expectation the same and not re-write it, right? I'm not actually sure what the replacement is since I don't know the activemodel API well.

The idea of the rewrite @cupakromer suggests here would negate be to the need for the collection matchers gem.

> * Doesn't errors_on require an argument (e.g. errors_on(:name))? It's confusing to me to see it used without the argument

I think it does, @cupakromer?