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This DeprecatedInstanceVariableProxy transforms instance variable to deprecated instance variable.

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

  def request
    @_request
  end

  def old_request
    @request
  end
end

When someone execute any method on @request variable this will trigger warn method on deprecator_instance and will fetch @_request variable via request method and execute the same method on non-proxy instance variable.

Default deprecator is ActiveSupport::Deprecation.

Methods
N
Class Public methods
new(instance, method, var = "@#{method}", deprecator = ActiveSupport::Deprecation.instance)

Source: show | on GitHub 

# File activesupport/lib/active_support/deprecation/proxy_wrappers.rb, line 79
def initialize(instance, method, var = "@#{method}", deprecator = ActiveSupport::Deprecation.instance)
  @instance = instance
  @method = method
  @var = var
  @deprecator = deprecator
end