Check for useless square bracket assignment

[agrimm]

The following feature request is like #383 , but rather than concerning foo.bar = baz, which is covered by UselessSetterCall, it's about foo[bar] = baz.

Using []= on the last line of a method on a local variable is usually a mistake.

# Bad - returns 'o', and the string referred to by the variable foo is lost without doing anything
def create_string_bad
  foo = 'String'
  foo[2..2] = 'o'
end

# Good - returns the full string
def create_string_good
  foo = 'String'
  foo.tap do |foo|
    foo[2..2] = 'o'
  end
end

# Good - no objects created and lost
def modify_string
  @foo[2..2] = 'o'
end

[bbatsov]

Sounds reasonable. I'll look into that.

[fuadsaud]

Just a comment: rubocop also raises an offence for something like this:

class Doge
  attr_writer :wow

  def initialize(*)
    super

    # whatever
    wow = 'foo'
  end
end

It identifies that method call as a local variable assignment. I didn't find any closed issues about this topic, should a open a new one?

[agrimm]

@fuadsaud that is a local variable assignment, just as rubocop says. You should have done self.wow = 'foo'.

If you have

# Incorrect
class Doge
  attr_writer :wow

  def initialize(*)
    super

    # whatever
    wow = 'foo'
  end
end

doge = Doge.new
puts doge.inspect

class Correct
  attr_writer :wow

  def initialize(*)
    super

    # whatever
    self.wow = 'foo'
  end
end

correct = Correct.new
puts correct.inspect

Then you'll get

#<Doge:0x007f7bfc5b8720>
#<Correct:0x007f7bfc5b8540 @wow="foo">

[fuadsaud]

@agrimm very correct. It was my bad, I didn't know self was required in a case like this. Thanks.

[bbatsov]

@yujinakayama @jonas054 What do you think about this? Are we doing it or closing it?

[yujinakayama]

Though I haven't looked into this, maybe this can be done by extending UselessSetterCall?

[bbatsov]

@yujinakayama That's what I thought as well. Would you, please, look into this?