I think this should be:

``` ruby
RSpec.configure do |config|
  config.mock_with :rspec do |mocks|
    mocks.verify_constant_names = true
  end
end
```

This is an rspec-mocks option, not a general rspec-core option.  rspec-core is decoupled from the configuration interface of the mocking libraries it supports; as long as they have a `configure` method, it'll call that and forward the block from `mock_with` on to it so that you can chain the configuration in this fashion.

I'm also not sure about the name `verify_constant_names`; in the context of rspec-fire, where the purpose of the library is much more focused, it's clear what this means.  In the context of rspec-mocks, the name suggests to me that this would verify _all_ constant names, even those that are given to `stub_const` but are not passed to `class_double `or `instance_double`.  I don't think it's useful to verify constants passed to `stub_const`  (I frequently use it define a constant for just one example that doesn't exist otherwise).  Maybe the option should be named `verify_doubled_constant_names` or `verify_doubled_class_names` or something like that?  Thoughts?