rspec-mocks VS rspec-core

It's nowhere near as equivalent, where's the behavior testing? Where stuff like allow_any_instance_of(...).to receive(...).and_return(...)? What about shared examples? How can you patch globally the current time?

Have a look on the examples here https://github.com/rspec/rspec-mocks

That being said, the code and documentation for rspec is generally clear and descriptive, making it a fantastic repository for learning about metaprogramming techniques. In rspec-core/lib/rspec/core/dsl.rb, for example, the implementation is spelled out in code comments.

Something like that. You almost certainly want to use bundler because it's pretty much the easiest way to add 3rd party libs to your app. (Read more about bundler). Tests aren't an absolute requirement but are a generally considered to be an important part of ruby (and rails) culture. They can be difficult to learn in the beginning, but get easier with practice -- start soon and practice often! (Good rule of thumb: test any public methods of your objects that you define, at a minimum -- a guide. There's also relishapp, which seems to be down at the moment?)

Take this small excerpt[9] from rspec-core of the describe public api:

From what I understand RSpec creates a class per spec (e.g. #) and it stores the instance variables (defined in the before :context DSL) on the RSpec::Core::ExampleGroup class on class level. The RSpec::Core::ExampleGroup is the parent class of the spec classes.

From the RSpec docs