Cop Feature Request: Catch assert expression.nil? and suggest assert_nil instead

[Ryan1729]

Is your feature request related to a problem? Please describe.

The AssertNil and RefuteNil cops do catch cases like the following, and suggests assert_nil instead:

assert_equal nil, expression

But they don't catch cases that use .nil? like this:

assert expression.nil?

The minitest style guide currently even uses a .nil? example for Refute Nil.

Describe the solution you'd like

It would be nice if the AssertNil and RefuteNil cops could catch these .nil? cases too.

Describe alternatives you've considered

It's not hard to write a regex for this myself, but having this be part of rubocop-minitest would be more convenient.

Additional context

N/A

[abhaynikam]

IMO, it is not a bad practice to use assert expression.nil? or refute expression.nil?. I think the documentation should be updated.

The cop was introduced because assert_nil and refute_nil is deprecated in minitest. 😊

[andyw8]

Do you know when they were deprecated? I can't see a mention in https://github.com/seattlerb/minitest/blob/master/History.rdoc

[abhaynikam]

The cop was introduced because assert_nil and refute_nil is deprecated in minitest.

Sorry. My bad. I meant assert_equal nil, expression was deprecated. Here is the deprecation warning: https://github.com/seattlerb/minitest/blob/master/lib/minitest/assertions.rb#L227 😊

[Ryan1729]

I think a good reason to prefer assert_nil expression over assert expression.nil? is because with assert_nil you get more informative error messages.

For example, this assert:

assert 1.nil?

results in this error message:

Minitest::Assertion: Expected false to be truthy.

But if you use assert_nil instead like this:

assert_nil 1

you get this more informative error message:

Minitest::Assertion: Expected 1 to be nil.

Yes, you can go look at the test to figure out what happened, but it's slightly nicer to see the expression that caused the issue right in the error message.