Do not mutate method parameter names when `super` (zsuper) is used

[dgollahon]

When I have code like this:

def initialize(a, b = 5)
  super(a, b)
end

mutant will correctly point out that I could write:

def initialize(a, b = 5)
  super
end

Unfortunately, it will then also mutate the names to

def initialize(_a, b = 5)
  super
end

and

def initialize(a, _b = 5)
  super
end

I find this weird/confusing because the variables are actually not unused (which is the general convention for _variable_name). I'd rather keep my non-underscored, descriptive names in this case and I think mutant should not emit the rename mutations when local variables can be statically determined as referenced like in this case with the implicit reference via zsuper.

[mbj]

Detecting the presence of zsuper in the body is easy. I support this change.

[dgollahon]

@mbj I would be happy to implement this but I am not sure what the best way to check for the presence zsuper would be since the Argument mutator does not have access to the method body. Is there a recommended pattern for making that kind of context available? Should the Define mutator try to reach into the arguments node and handle the _ mutation since it is able to inspect its body?

[mbj]

@dgollahon You can use the parent_node method to peek up and into a sibling. But this is discouraged and a method of last resort.

For now this may be the best solution, till I push the new mutation generation engine here that is more flexible, and is more declarative.

[dgollahon]

Aha! I didn't realize there was a parent_node method. I think that makes the most sense for this case until we have something more expressive.