Remove redundant to_s suggestion from offense message for Rails/FilePath

Follow up #968.

`to_s` should be called implicitly when string interpolated.

Author: koic

lib/rubocop/cop/rails/file_path.rb

@@ -106,6 +106,8 @@ def string_with_slash?(node)
         def register_offense(node, require_to_s:)
           line_range = node.loc.column...node.loc.last_column
           source_range = source_range(processed_source.buffer, node.first_line, line_range)
+          require_to_s = false if node.dstr_type?
+
           message = build_message(require_to_s)
 
           add_offense(source_range, message: message)

spec/rubocop/cop/rails/file_path_spec.rb

@@ -83,7 +83,7 @@
       it 'registers an offense' do
         expect_offense(<<~'RUBY')
           "#{Rails.root}/app/models/goober"
-          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to').to_s`.
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to')`.
         RUBY
       end
     end
@@ -92,7 +92,7 @@
       it 'registers an offense' do
         expect_offense(<<~'RUBY')
           system "rm -rf #{Rails.root}/foo/bar"
-                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to').to_s`.
+                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to')`.
         RUBY
       end
     end
@@ -101,7 +101,7 @@
       it 'registers an offense' do
         expect_offense(<<~'RUBY')
           "#{Rails.root.join('tmp', user.id, 'icon')}.png"
-          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to').to_s`.
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path/to')`.
         RUBY
       end
     end
@@ -211,7 +211,7 @@
       it 'registers an offense' do
         expect_offense(<<~'RUBY')
           "#{Rails.root}/app/models/goober"
-          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path', 'to').to_s`.
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path', 'to')`.
         RUBY
       end
     end
@@ -220,7 +220,7 @@
       it 'registers an offense' do
         expect_offense(<<~'RUBY')
           system "rm -rf #{Rails.root}/foo/bar"
-                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path', 'to').to_s`.
+                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path', 'to')`.
         RUBY
       end
     end
@@ -229,7 +229,7 @@
       it 'registers an offense' do
         expect_offense(<<~'RUBY')
           "#{Rails.root.join('tmp', user.id, 'icon')}.png"
-          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path', 'to').to_s`.
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Rails.root.join('path', 'to')`.
         RUBY
       end
     end