Fix wrong autocorrect for Rails/FilePath when passing an array to `…

…File.join`

`File.join` actually accepts nested arrays and will unflatten them as necessary.

Don't bother implementing autocorrect for these cases, seems complicated
and not worth the effort.

The current correction for these testcases look like this:
```rb
Rails.root.join().to_s
```

changelog/fix_wrong_autocorrect_for_rails_file_path.md

@@ -0,0 +1 @@
+* [#1326](https://github.com/rubocop/rubocop-rails/pull/1326): Fix wrong autocorrect for `Rails/FilePath` when passing an array to `File.join`. ([@earlopain][])

lib/rubocop/cop/rails/file_path.rb

@@ -97,7 +97,7 @@ def check_for_file_join_with_rails_root(node)
           return unless node.arguments.any? { |e| rails_root_nodes?(e) }
 
           register_offense(node, require_to_s: true) do |corrector|
-            autocorrect_file_join(corrector, node)
+            autocorrect_file_join(corrector, node) unless node.first_argument.array_type?
           end
         end
 

spec/rubocop/cop/rails/file_path_spec.rb

@@ -114,6 +114,26 @@
       end
     end
 
+    context 'when using File.join with an array' do
+      it 'registers an offense' do
+        expect_offense(<<~RUBY)
+          File.join([Rails.root, 'foo'])
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to').to_s`.
+        RUBY
+
+        expect_no_corrections
+      end
+
+      it 'registers an offense for nested arrays' do
+        expect_offense(<<~RUBY)
+          File.join([Rails.root, 'foo', ['bar']])
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to').to_s`.
+        RUBY
+
+        expect_no_corrections
+      end
+    end
+
     context 'when using Rails.root.join with slash separated path string' do
       it 'does not register an offense' do
         expect_no_offenses("Rails.root.join('app/models/goober')")