Monkey patching String with #to_ivar

Question

I would like to monkey patch String with #to_ivar.

class String
  def to_ivar
    instance_variable_get("@#{self}")
  end
end

So that when I have an environment with instance variables like this:

instance variables:
  @__cucumber_runtime  @browser       @browser_type  @debug     @sauce_config  @user_A

I can do this in a step definition:

Given(/^user (.*) does something$/) do |user|
  @user = user.to_ivar
...
end

So I did, but, it seems that String#to_ivar does not know about the instance variables:

    13: def to_ivar
 => 14:   binding.pry
    15:   instance_variable_get("@#{self}")
    16: end

    [2] pry("user_A")> instance_variables
     => []

Is there a way for an instance of String to know about the Cucumber world and its instance variables?