Auto-correct dstr nodes in StringConcatenation

The cop documentation says that complex cases are skipped in auto-correction,
but the concatenation of strings where at least one has interpolation isn't
really that complex. I think we can safely add auto-correction in those cases.

lib/rubocop/cop/mixin/percent_literal.rb

@@ -27,12 +27,6 @@ def begin_source(node)
       def type(node)
         node.loc.begin.source[0..-2]
       end
-
-      # A range containing only the contents of the percent literal (e.g. in
-      # %i{1 2 3} this will be the range covering '1 2 3' only)
-      def contents_range(node)
-        range_between(node.loc.begin.end_pos, node.loc.end.begin_pos)
-      end
     end
   end
 end

lib/rubocop/cop/mixin/range_help.rb

@@ -27,6 +27,12 @@ def source_range(source_buffer, line_number, column, length = 1)
         Parser::Source::Range.new(source_buffer, begin_pos, end_pos)
       end
 
+      # A range containing only the contents of a literal with delimiters (e.g. in
+      # `%i{1 2 3}` this will be the range covering `1 2 3` only).
+      def contents_range(node)
+        range_between(node.loc.begin.end_pos, node.loc.end.begin_pos)
+      end
+
       def range_between(start_pos, end_pos)
         Parser::Source::Range.new(processed_source.buffer, start_pos, end_pos)
       end

lib/rubocop/cop/style/string_concatenation.rb

@@ -29,6 +29,7 @@ module Style
       #
       class StringConcatenation < Base
         include Util
+        include RangeHelp
         extend AutoCorrector
 
         MSG = 'Prefer string interpolation to string concatenation.'
@@ -103,7 +104,6 @@ def plus_node?(node)
 
         def uncorrectable?(part)
           part.multiline? ||
-            part.dstr_type? ||
             (part.str_type? && part.heredoc?) ||
             part.each_descendant(:block).any?
         end
@@ -115,12 +115,12 @@ def corrected_ancestor?(node)
         def replacement(parts)
           interpolated_parts =
             parts.map do |part|
-              if part.str_type?
-                if single_quoted?(part)
-                  part.value.gsub(/(\\|")/, '\\\\\&')
-                else
-                  part.value.inspect[1..-2]
-                end
+              case part.type
+              when :str
+                value = part.value
+                single_quoted?(part) ? value.gsub(/(\\|")/, '\\\\\&') : value.inspect[1..-2]
+              when :dstr
+                contents_range(part).source
               else
                 "\#{#{part.source}}"
               end

spec/rubocop/cop/style/string_concatenation_spec.rb

@@ -109,13 +109,15 @@
   end
 
   context 'nested interpolation' do
-    it 'registers an offense but does not correct' do
-      expect_offense(<<~RUBY)
-        "foo" + "bar: \#{baz}"
+    it 'registers an offense and corrects' do
+      expect_offense(<<~'RUBY')
+        "foo" + "bar: #{baz}"
         ^^^^^^^^^^^^^^^^^^^^^ Prefer string interpolation to string concatenation.
       RUBY
 
-      expect_no_corrections
+      expect_correction(<<~'RUBY')
+        "foobar: #{baz}"
+      RUBY
     end
   end