[Fix #14502] Fix wrong autocorrect for Style/StringConcatenation when a single-quoted string contains interpolation

Earlopain · bbatsov · commit ca1b8136837a · 2025-09-03T18:11:29.000+03:00
This only escapes `#` when it looks like interpolation, not all the time.
If it turns out to not be interpolation after all, `Style/RedundantStringEscape` will take care of it.
diff --git a/changelog/fix_string_concat_interpolation.md b/changelog/fix_string_concat_interpolation.md
@@ -0,0 +1 @@
+* [#14502](https://github.com/rubocop/rubocop/issues/14502): Fix wrong autocorrect for `Style/StringConcatenation` when a single-quoted string contains interpolation like `'#{foo}'`. ([@earlopain][])
diff --git a/lib/rubocop/cop/style/string_concatenation.rb b/lib/rubocop/cop/style/string_concatenation.rb
@@ -149,7 +149,11 @@ def replacement(parts)
         def adjust_str(part)
           case part.type
           when :str
-            single_quoted?(part) ? part.value.gsub(/(\\|")/, '\\\\\&') : part.value.inspect[1..-2]
+            if single_quoted?(part)
+              part.value.gsub(/(\\|"|#\{|#@|#\$)/, '\\\\\&')
+            else
+              part.value.inspect[1..-2]
+            end
           when :dstr, :begin
             part.children.map do |child|
               adjust_str(child)
diff --git a/spec/rubocop/cop/style/string_concatenation_spec.rb b/spec/rubocop/cop/style/string_concatenation_spec.rb
@@ -293,6 +293,52 @@
     end
   end
 
+  context 'characters for interpolation inside single quotes' do
+    it 'registers an offense for `#{}`' do
+      expect_offense(<<~'RUBY')
+        "foo" + '#{bar}'
+        ^^^^^^^^^^^^^^^^ Prefer string interpolation to string concatenation.
+      RUBY
+
+      expect_correction(<<~'RUBY')
+        "foo\#{bar}"
+      RUBY
+    end
+
+    it 'registers an offense for `#@`' do
+      expect_offense(<<~'RUBY')
+        "foo" + '#@bar'
+        ^^^^^^^^^^^^^^^ Prefer string interpolation to string concatenation.
+      RUBY
+
+      expect_correction(<<~'RUBY')
+        "foo\#@bar"
+      RUBY
+    end
+
+    it 'registers an offense for `#@@`' do
+      expect_offense(<<~'RUBY')
+        "foo" + '#@@bar'
+        ^^^^^^^^^^^^^^^^ Prefer string interpolation to string concatenation.
+      RUBY
+
+      expect_correction(<<~'RUBY')
+        "foo\#@@bar"
+      RUBY
+    end
+
+    it 'registers an offense for `#$`' do
+      expect_offense(<<~'RUBY')
+        "foo" + '#$bar'
+        ^^^^^^^^^^^^^^^ Prefer string interpolation to string concatenation.
+      RUBY
+
+      expect_correction(<<~'RUBY')
+        "foo\#$bar"
+      RUBY
+    end
+  end
+
   context 'Mode = conservative' do
     let(:cop_config) { { 'Mode' => 'conservative' } }
 
