'Useless assignment' reported when setting atttribute on method parameter

[dirkbolte]

The following code results in a warning: Useless assignment to local variable test_class. for test_class.member = value:

# encoding: UTF-8
# TestClass
class TestClass
  attr_accessor :member
end

def add_to_hash(test_class, value)
  test_class.member = value
end
tc = TestClass.new
add_to_hash(tc, 'a')
puts tc.member

[bbatsov]

Parameters are treated as local vars by the parser and assignments to local vars as the result of a functions generally make no sense. Assignment to parameters is considered extremely bad style anyways...

[dirkbolte]

I completely agree on style. Nevertheless the code actually does something and has an effect outside of that method call.

[bbatsov]

The problem is that we cannot simply allow assignments to params, because of code like this:

def add_to_hash(test_class, value)
  test_class = NewClass.new
  # there is a param test_class, but this is no longer it
  test_class.member = value
end

Checking for assignments to all the params might be one solution (not sure how robust, though) - if there are no, we assume this is still a param and it's OK to assign to it.

@yujinakayama Would you volunteer to fix this issue? :-)

[yujinakayama]

OK 👍

[jonas054]

Then there are a number of special cases that make the assignment "not useless". I don't know if they're too far-fetched or if we should worry about them.

• Aliasing such as @tc = test_class or store_reference(test_class)
• If the TestClass#member= method is not just assigning an attribute, but has side effects.