Warn about re-assigned but not re-used variables

[agrimm]

The following code

def reassigned_unused
  a = 42
  b = a.to_s
  a = 56
  b
end

Should be regarded as problematic, because of the second assignment to a which isn't used. It'd be nice if rubocop could generate a warning about it.

I proposed the following as an enhancement for MRI, but it hasn't attracted any attention yet: https://bugs.ruby-lang.org/issues/8691 . The Rubinius team isn't planning on adding this kind of thing to Rubinius itself, based on rubinius/rubinius#2054. The JRuby team isn't enthusiastic about this kind of thing, based on https://jira.codehaus.org/browse/JRUBY-6905

[bbatsov]

I think that such a check makes sense indeed. @yujinakayama Would you look into it?

[yujinakayama]

👍

Actually I wondered why MRI does not warn about it when I was implementing UnusedLocalVariable. However I was not sure if warning of it is valid at that time, so I've made the cop mimic MRI. But yes, I'll try.

[agrimm]

Thanks for all the work you put into doing this!

It's detected several useless assignments in my application, so it's proved to be useful!