Fix a false positive for Style/FetchEnvVar in the body with assignm…

…ent method

changelog/fix_false_positive_for_fetch_env_var.md

@@ -0,0 +1 @@
+* [#10670](https://github.com/rubocop/rubocop/pull/10670): Fix a false positive for `Style/FetchEnvVar` in the body with assignment method. ([@ydah][])

lib/rubocop/cop/style/fetch_env_var.rb

@@ -112,13 +112,20 @@ def used_if_condition_in_body(node)
         end
 
         def used_in_condition?(node, condition)
-          if condition.send_type? && (!condition.comparison_method? && !condition.predicate_method?)
-            return false
+          if condition.send_type?
+            return true if condition.assignment_method? && partial_matched?(node, condition)
+            return false if !condition.comparison_method? && !condition.predicate_method?
           end
 
           condition.child_nodes.any?(node)
         end
 
+        # Avoid offending in the following cases:
+        # `ENV['key'] if ENV['key'] = x`
+        def partial_matched?(node, condition)
+          node.child_nodes == node.child_nodes & condition.child_nodes
+        end
+
         def offensive?(node)
           !(allowed_var?(node) || allowable_use?(node))
         end

spec/rubocop/cop/style/fetch_env_var_spec.rb

@@ -364,6 +364,29 @@
         end
       RUBY
     end
+
+    it 'registers no offenses when using the same `ENV` var as `if` condition in the body with assignment method' do
+      expect_no_offenses(<<~RUBY)
+        if ENV['X'] = x
+          puts ENV['X']
+        end
+      RUBY
+    end
+
+    it 'registers an offense with using an `ENV` var as `if` condition in the body with assignment method' do
+      expect_offense(<<~RUBY)
+        if ENV['X'] = x
+          puts ENV['Y']
+               ^^^^^^^^ Use `ENV.fetch('Y')` or `ENV.fetch('Y', nil)` instead of `ENV['Y']`.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        if ENV['X'] = x
+          puts ENV.fetch('Y', nil)
+        end
+      RUBY
+    end
   end
 
   it 'registers an offense when using `ENV && x` that is different from `if` condition in the body' do