Fix Rails/FilePath to detect offenses from complex string interpolation

Author: r7kamura

changelog/fix_rails_file_path_to_detect_offenses_from_complex_string_interpolation.md

@@ -0,0 +1 @@
+* [#989](https://github.com/rubocop/rubocop-rails/pull/989): Fix `Rails/FilePath` to detect offenses from complex string interpolation. ([@r7kamura][])

lib/rubocop/cop/rails/file_path.rb

@@ -56,11 +56,8 @@ class FilePath < Base
 
         def on_dstr(node)
           return unless rails_root_nodes?(node)
-          return unless node.children.last.str_type?
-
-          last_child_source = node.children.last.source
-          return unless last_child_source.start_with?('.') || last_child_source.include?(File::SEPARATOR)
-          return if last_child_source.start_with?(':')
+          return if dstr_separated_by_colon?(node)
+          return unless dstr_ending_with_file_extension?(node) || dstr_including_file_separator?(node)
 
           register_offense(node, require_to_s: true)
         end
@@ -119,6 +116,22 @@ def build_message(require_to_s)
 
           format(message_template, to_s: to_s)
         end
+
+        def dstr_ending_with_file_extension?(node)
+          node.children.last.str_type? && node.children.last.source.start_with?('.')
+        end
+
+        def dstr_including_file_separator?(node)
+          node.children.any? do |child|
+            child.str_type? && child.source.include?(File::SEPARATOR)
+          end
+        end
+
+        def dstr_separated_by_colon?(node)
+          node.children[1..].any? do |child|
+            child.str_type? && child.source.start_with?(':')
+          end
+        end
       end
     end
   end

spec/rubocop/cop/rails/file_path_spec.rb

@@ -88,6 +88,15 @@
       end
     end
 
+    context 'when using Rails.root called by double quoted string that ends with string interpolation' do
+      it 'registers an offense' do
+        expect_offense(<<~'RUBY')
+          "#{Rails.root}/a/#{b}"
+          ^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to')`.
+        RUBY
+      end
+    end
+
     context 'when concat Rails.root and file separator using string interpolation' do
       it 'registers an offense' do
         expect_offense(<<~'RUBY')