Fix a false positive for Style/FetchEnvVar in the body with assignment method

[ydah]

Follow up: #10653 (comment)

if ENV['X'] = x
  puts ENV['X']
end

# The above is corrected into,

if ENV['X'] = x
  puts ENV.fetch('X', nil)
end

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
  * [ ] Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[koic]

Is this condition necessary? If the condition can be eliminated, the following looks simple.

diff --git a/lib/rubocop/cop/style/fetch_env_var.rb b/lib/rubocop/cop/style/fetch_env_var.rb
index 06b3fdfc4..5e8bb5247 100644
--- a/lib/rubocop/cop/style/fetch_env_var.rb
+++ b/lib/rubocop/cop/style/fetch_env_var.rb
@@ -112,22 +112,12 @@ module RuboCop
         end

         def used_in_condition?(node, condition)
-          if condition.send_type? &&
-             (!condition.comparison_method? &&
-              !condition.predicate_method? &&
-              !condition.assignment_method?)
-            return false
+          if condition.send_type?
+            return true if condition.assignment_method?
+            return false if !condition.comparison_method? && !condition.predicate_method?
           end

-          used_in_condition_with_assignment?(node, condition) || condition.child_nodes.any?(node)
-        end
-
-        def used_in_condition_with_assignment?(node, condition)
-          if condition.send_type? && condition.assignment_method?
-            node.child_nodes == node.child_nodes & condition.child_nodes
-          else
-            false
-          end
+          condition.child_nodes.any?(node)
         end

         def offensive?(node)

So just update used_in_condition? method.

         def used_in_condition?(node, condition)
           if condition.send_type?
             return true if condition.assignment_method?
             return false if !condition.comparison_method? && !condition.predicate_method?
           end

           condition.child_nodes.any?(node)
         end

[ydah]

Thank you very much.
I've modified the conditional statement. However, the following conditional statement is still needed:

node.child_nodes == node.child_nodes & condition.child_nodes

The following code cannot be considered offensive:

if ENV['X'] = x
  puts ENV['Y']
end

[ydah]

I updated this PR. Thank you so much.

[bbatsov]

It's funny how many edge cases something that seemed so simply ended up having. :) Thanks!