class Rubocop::Cop::Style::AvoidGlobalVars

It does not report offences for built-in global variables.
This cops looks for uses of global variables.

def check(node) 

def check(node)
  global_var, = *node
  unless BUILT_IN_VARS.include?(global_var)
    add_offence(:convention,
                node.loc.name,
                MSG)
  end
end

def on_gvar(node) 

def on_gvar(node)
  check(node)
end

def on_gvasgn(node) 

def on_gvasgn(node)
  check(node)
end