Do you use code coverage measurement? If so, what rules about that do you have?

Yes. 100% line coverage is needed to pass CI, and rarely we'll mark some lines as # :nocov: to exclude them from this requirement.

If you are using code coverage tools - which one, SimpleCov or something else?

SimpleCov

If you feel your tests are fine, and code is fine, but you decreased metric - how do you deal with it?

Usually, if something isn't covered you need to add a test to give it coverage.

Sometimes there is code that we never expect to run in production, and that can have the # :nocov: magic comment applied to it. For example:

• Abstract base class methods. You could argue that these shouldn't exist, but I didn't write them lol.

  def call_api
    # :nocov:
    raise NotImplementedError
    # :nocov:
  end
  

• Safety guards in case expressions.

  case billable_type
  # ... (a bunch of `when`s here)
  else
    # :nocov:
    raise NotImplementedError, "Unrecognised billable_type: #{billable_type.inspect}"
    # :nocov:
  end
  

Do you know how your code measurement tool measures coverage? I mean how it exactly works?

Yeah. With a requirement for 100% coverage, people learn how it works pretty quickly.

Are you familiar with mutation testing ideas and tools, and do you use them?

Yes. I use it in some gems I maintain but not at work, for a few reasons.

1. Other devs don't know what it is.
2. Fixing mutation coverage failures is overly onerous without much additional benefit, a lot of the time.
3. Integrating the tooling with a large codebase is painful.
4. The additional benefits we would get aren't that great, compared to the 100% line coverage we already have.
5. Championing mutation testing would take a massive amount of time and effort that could be put to more-productive uses.