I just noticed this a recurring message format, in other matchers:
``` 
expected "a string" not to match /str/
expected 27.5 not to be within 0.5 of 27
expected <Planet: Earth> not to exist
```

which can be easily solved in most cases by adding a default:
``` ruby
class BaseMatcher
   def description_when_negated
          "not #{description}"
        end
````
and patching ```failure_message_when_negated```:
https://github.com/rspec/rspec-expectations/pull/505/files#r11254470