class RuboCop::Cop::RSpec::InstanceVariable

end
it { expect(foo).to be_empty }
let(:foo) { [] }
describe MyClass do
# good
end
it { expect(@foo).to be_empty }
before { @foo = [] }
describe MyClass do
# bad
@example
variable, use let.
When you have to assign a variable instead of using an instance

def on_block(node) 

def on_block(node)
  method, _args, _body = *node
  _receiver, method_name, _object = *method
  @in_spec = true if EXAMPLE_GROUP_METHODS.include?(method_name)
end

def on_ivar(node) 

def on_ivar(node)
  add_offense(node, :expression, MESSAGE) if @in_spec
end