Last week I upgraded Rubocop to 0.53. I ran the test suite and noticed a new warning called Naming/MemoizedInstanceVariableName, which targets methods that use an instance variable for memoization. Here is an example that would trigger the warning:

def greeting
  @hello ||= user_greeting

Rubocop raises an error because the method name is different from the name of the instance variable, used for memoization. This seems sensible so I started working through the failing tests and fixing them. As I fixed tests I began to find bugs. I realised that the Naming/MemoizedInstanceVariableName warning is super useful. Here’s an example of a bug that I uncovered:

def user
  @user ||= User.create!

def admin
  @user ||= Admin.create!

This bug occurred in my test code, probably due to careless copy/pasting. It could occur in application code just as easily. The bug is that calling the user method will create a User and memoize it. When admin is called, instead of creating an Admin, the User object is returned.

Upgrading ensures that this type of bug will never occur again. Thanks Rubocop!