class ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy

DeprecatedInstanceVariableProxy transforms an instance variable into a deprecated one. It takes an instance of a class, a method on that class, an instance variable, and a deprecator as the last argument.

Trying to use the deprecated instance variable will result in a deprecation warning, pointing to the method as a replacement.

class Example
  def initialize
    @request = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new(self, :request, :@request, ActiveSupport::Deprecation.new)
    @_request = :special_request
  end

  def request
    @_request
  end

  def old_request
    @request
  end
end

example = Example.new
# => #<Example:0x007fb9b31090b8 @_request=:special_request, @request=:special_request>

example.old_request.to_s
# => DEPRECATION WARNING: @request is deprecated! Call request.to_s instead of
   @request.to_s
   (Backtrace information…)
   "special_request"

example.request.to_s
# => "special_request"

# File activesupport/lib/active_support/deprecation/proxy_wrappers.rb, line 88
def initialize(instance, method, var = "@#{method}", deprecator:)
  @instance = instance
  @method = method
  @var = var
  @deprecator = deprecator
end