Avoid crash in Rails/EnumUniqueness cop

[pocke]

Rails/EnumUniqueness cop crashes when enum method receives a variable.

For example

class Foo < ApplicationRecord
  var = {status: {open: 1, closed: 2}}
  enum var
end

$ rubocop --rails
An error occurred while Rails/EnumUniqueness cop was inspecting /tmp/tmp.sIFH7vguiW/app/models/foo.rb.

1 error occurred:
An error occurred while Rails/EnumUniqueness cop was inspecting /tmp/tmp.sIFH7vguiW/app/models/foo.rb.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
Mention the following information in the issue report:
0.46.0 (using Parser 2.3.2.0, running on ruby 2.3.1 x86_64-linux)
For /tmp/tmp.sIFH7vguiW: configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/default.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/enabled.yml
Inheriting configuration from /home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/config/disabled.yml
Inspecting 1 file
Scanning /tmp/tmp.sIFH7vguiW/app/models/foo.rb
undefined method `child_nodes' for nil:NilClass
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/rails/enum_uniqueness.rb:61:in `parse_args'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/rails/enum_uniqueness.rb:28:in `on_send'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:41:in `block (2 levels) in on_send'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:96:in `with_cop_error_handling'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:40:in `block in on_send'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:39:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:39:in `on_send'
(eval):2:in `block in on_begin'
(eval):2:in `each'
(eval):2:in `on_begin'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:45:in `on_begin'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/ast_node/traversal.rb:88:in `on_class'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:45:in `on_class'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/ast_node/traversal.rb:12:in `walk'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/commissioner.rb:58:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:120:in `investigate'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:108:in `offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cop/team.rb:51:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:243:in `inspect_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:190:in `block in do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:222:in `block in iterate_until_no_changes'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:215:in `loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:215:in `iterate_until_no_changes'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:186:in `do_inspection_loop'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:101:in `block in file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:111:in `file_offense_cache'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:99:in `file_offenses'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:90:in `process_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:68:in `block in each_inspected_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `each'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `reduce'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:65:in `each_inspected_file'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:57:in `inspect_files'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/runner.rb:36:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cli.rb:72:in `execute_runner'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/lib/rubocop/cli.rb:27:in `run'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/bin/rubocop:13:in `block in <top (required)>'
/usr/lib/ruby/2.3.0/benchmark.rb:308:in `realtime'
/home/pocke/.gem/ruby/2.3.0/gems/rubocop-0.46.0/bin/rubocop:12:in `<top (required)>'
/home/pocke/.gem/ruby/2.3.0/bin/rubocop:23:in `load'
/home/pocke/.gem/ruby/2.3.0/bin/rubocop:23:in `<main>'
C

Offenses:

app/models/foo.rb:3:9: C: Style/SpaceInsideHashLiteralBraces: Space inside { missing.
  var = {status: {open: 1, closed: 2}}
        ^
app/models/foo.rb:3:18: C: Style/SpaceInsideHashLiteralBraces: Space inside { missing.
  var = {status: {open: 1, closed: 2}}
                 ^
app/models/foo.rb:3:37: C: Style/SpaceInsideHashLiteralBraces: Space inside } missing.
  var = {status: {open: 1, closed: 2}}
                                    ^
app/models/foo.rb:3:38: C: Style/SpaceInsideHashLiteralBraces: Space inside } missing.
  var = {status: {open: 1, closed: 2}}
                                     ^

1 file inspected, 4 offenses detected
Finished in 0.10733168199658394 seconds

I fixed the problem.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[olliebennett]

Nice one - thanks for this, @pocke! I didn't consider that syntax when implementing the cop, sorry.