Add new Style/StringConcatenation cop

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [#8322](https://github.com/rubocop-hq/rubocop/pull/8322): Support autocorrect for `Style/CaseEquality` cop. ([@fatkodima][])
 * [#8376](https://github.com/rubocop-hq/rubocop/pull/8376): Add new `Lint/MissingSuper` cop. ([@fatkodima][])
 * [#8339](https://github.com/rubocop-hq/rubocop/pull/8339): Add `Config#for_badge` as an efficient way to get a cop's config merged with its department's. ([@marcandre][])
+* [#5067](https://github.com/rubocop-hq/rubocop/issues/5067): Add new `Style/StringConcatenation` cop. ([@fatkodima][])
 
 ### Bug fixes
 

config/default.yml

@@ -3940,6 +3940,13 @@ Style/StderrPuts:
   Enabled: true
   VersionAdded: '0.51'
 
+Style/StringConcatenation:
+  Description: 'Checks for places where string concatenation can be replaced with string interpolation.'
+  StyleGuide: '#string-interpolation'
+  Enabled: pending
+  Safe: false
+  VersionAdded: '0.89'
+
 Style/StringHashKeys:
   Description: 'Prefer symbols instead of strings as hash keys.'
   StyleGuide: '#symbols-as-keys'

docs/modules/ROOT/pages/cops.adoc

@@ -471,6 +471,7 @@ In the following section you find all available cops:
 * xref:cops_style.adoc#stylespecialglobalvars[Style/SpecialGlobalVars]
 * xref:cops_style.adoc#stylestabbylambdaparentheses[Style/StabbyLambdaParentheses]
 * xref:cops_style.adoc#stylestderrputs[Style/StderrPuts]
+* xref:cops_style.adoc#stylestringconcatenation[Style/StringConcatenation]
 * xref:cops_style.adoc#stylestringhashkeys[Style/StringHashKeys]
 * xref:cops_style.adoc#stylestringliterals[Style/StringLiterals]
 * xref:cops_style.adoc#stylestringliteralsininterpolation[Style/StringLiteralsInInterpolation]

docs/modules/ROOT/pages/cops_style.adoc

@@ -8908,6 +8908,37 @@ warn('hello')
 
 * https://rubystyle.guide#warn
 
+== Style/StringConcatenation
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Pending
+| No
+| Yes (Unsafe)
+| 0.89
+| -
+|===
+
+This cop checks for places where string concatenation
+can be replaced with string interpolation.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+email_with_name = user.name + ' <' + user.email + '>'
+
+# good
+email_with_name = "#{user.name} <#{user.email}>"
+email_with_name = format('%s <%s>', user.name, user.email)
+----
+
+=== References
+
+* https://rubystyle.guide#string-interpolation
+
 == Style/StringHashKeys
 
 |===

lib/rubocop.rb

@@ -516,6 +516,7 @@
 require_relative 'rubocop/cop/style/special_global_vars'
 require_relative 'rubocop/cop/style/stabby_lambda_parentheses'
 require_relative 'rubocop/cop/style/stderr_puts'
+require_relative 'rubocop/cop/style/string_concatenation'
 require_relative 'rubocop/cop/style/string_hash_keys'
 require_relative 'rubocop/cop/style/string_literals'
 require_relative 'rubocop/cop/style/string_literals_in_interpolation'

lib/rubocop/cli/command/auto_genenerate_config.rb

@@ -113,7 +113,7 @@ def add_inheritance_from_auto_generated_file(config_file)
             return if files.include?(AUTO_GENERATED_FILE)
 
             files.unshift(AUTO_GENERATED_FILE)
-            file_string = "\n  - " + files.join("\n  - ") if files.size > 1
+            file_string = "\n  - #{files.join("\n  - ")}" if files.size > 1
             rubocop_yml_contents = existing_configuration(config_file)
           end
 

lib/rubocop/cli/command/show_cops.rb

@@ -68,7 +68,7 @@ def cops_of_department(cops, department)
 
         def config_lines(cop)
           cnf = @config.for_cop(cop)
-          cnf.to_yaml.lines.to_a.drop(1).map { |line| '  ' + line }
+          cnf.to_yaml.lines.to_a.drop(1).map { |line| "  #{line}" }
         end
       end
     end

lib/rubocop/comment_config.rb

@@ -11,7 +11,7 @@ class CommentConfig
     COPS_PATTERN = "(all|#{COP_NAMES_PATTERN})"
 
     COMMENT_DIRECTIVE_REGEXP = Regexp.new(
-      ('# rubocop : ((?:disable|enable|todo))\b ' + COPS_PATTERN)
+      "# rubocop : ((?:disable|enable|todo))\\b #{COPS_PATTERN}"
         .gsub(' ', '\s*')
     )
 

lib/rubocop/config_loader_resolver.rb

@@ -201,7 +201,7 @@ def handle_disabled_by_default(config, new_default_configuration)
         next unless dept_params['Enabled']
 
         new_default_configuration.each do |cop, params|
-          next unless cop.start_with?(dept + '/')
+          next unless cop.start_with?("#{dept}/")
 
           # Retain original default configuration for cops in the department.
           params['Enabled'] = ConfigLoader.default_configuration[cop]['Enabled']

lib/rubocop/cop/correctors/line_break_corrector.rb

@@ -29,8 +29,8 @@ def break_line_before(range:, node:, corrector:, indent_steps: 1,
                               configured_width:)
           corrector.insert_before(
             range,
-            "\n" + ' ' * (node.loc.keyword.column +
-                          indent_steps * configured_width)
+            "\n#{' ' * (node.loc.keyword.column +
+                          indent_steps * configured_width)}"
           )
         end
 
@@ -39,7 +39,7 @@ def move_comment(eol_comment:, node:, corrector:)
 
           text = eol_comment.loc.expression.source
           corrector.insert_before(node,
-                                  text + "\n" + (' ' * node.loc.keyword.column))
+                                  "#{text}\n#{' ' * node.loc.keyword.column}")
           corrector.remove(eol_comment)
         end
 

lib/rubocop/cop/correctors/percent_literal_corrector.rb

@@ -110,7 +110,7 @@ def substitute_escaped_delimiters(content, delimiters)
 
       def end_content(source)
         result = /\A(\s*)\]/.match(source.split("\n").last)
-        ("\n" + result[1]) if result
+        "\n#{result[1]}" if result
       end
     end
   end

lib/rubocop/cop/correctors/punctuation_corrector.rb

@@ -10,7 +10,7 @@ def remove_space(space_before)
         end
 
         def add_space(token)
-          ->(corrector) { corrector.replace(token.pos, token.pos.source + ' ') }
+          ->(corrector) { corrector.replace(token.pos, "#{token.pos.source} ") }
         end
 
         def swap_comma(range)

lib/rubocop/cop/generator/configuration_injector.rb

@@ -25,9 +25,9 @@ def inject
           target_line = find_target_line
           if target_line
             configuration_entries.insert(target_line,
-                                         new_configuration_entry + "\n")
+                                         "#{new_configuration_entry}\n")
           else
-            configuration_entries.push("\n" + new_configuration_entry)
+            configuration_entries.push("\n#{new_configuration_entry}")
           end
 
           File.write(configuration_file_path, configuration_entries.join)

lib/rubocop/cop/layout/block_alignment.rb

@@ -213,7 +213,7 @@ def alt_start_msg(start_loc, source_line_column)
                 start_loc.column == source_line_column[:column]
             ''
           else
-            ' or ' + format_source_line_column(source_line_column)
+            " or #{format_source_line_column(source_line_column)}"
           end
         end
 

lib/rubocop/cop/lint/heredoc_method_call_position.rb

@@ -148,7 +148,7 @@ def call_range_to_safely_reposition(node, heredoc)
         end
 
         def trailing_comma?(call_source, call_line_source)
-          call_source + ',' == call_line_source
+          "#{call_source}," == call_line_source
         end
       end
     end

lib/rubocop/cop/style/hash_syntax.rb

@@ -174,7 +174,7 @@ def autocorrect_ruby19(corrector, pair_node)
 
           corrector.replace(
             range,
-            range.source.sub(/^:(.*\S)\s*=>\s*$/, space.to_s + '\1: ')
+            range.source.sub(/^:(.*\S)\s*=>\s*$/, "#{space}\\1: ")
           )
 
           hash_node = pair_node.parent

lib/rubocop/cop/style/infinite_loop.rb

@@ -99,7 +99,7 @@ def replace_source(range, replacement)
         def modifier_replacement(node)
           body = node.body
           if node.single_line?
-            'loop { ' + body.source + ' }'
+            "loop { #{body.source} }"
           else
             indentation = body.source_range.source_line[LEADING_SPACE]
 

lib/rubocop/cop/style/multiline_memoization.rb

@@ -77,13 +77,13 @@ def keyword_begin_str(node, node_buf)
           if node_buf.source[node.loc.begin.end_pos] == "\n"
             'begin'
           else
-            "begin\n" + (' ' * (node.loc.column + indent))
+            "begin\n#{' ' * (node.loc.column + indent)}"
           end
         end
 
         def keyword_end_str(node, node_buf)
           if /[^\s)]/.match?(node_buf.source_line(node.loc.end.line))
-            "\n" + (' ' * node.loc.column) + 'end'
+            "\n#{' ' * node.loc.column}end"
           else
             'end'
           end

lib/rubocop/cop/style/string_concatenation.rb

@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # This cop checks for places where string concatenation
+      # can be replaced with string interpolation.
+      #
+      # @example
+      #   # bad
+      #   email_with_name = user.name + ' <' + user.email + '>'
+      #
+      #   # good
+      #   email_with_name = "#{user.name} <#{user.email}>"
+      #   email_with_name = format('%s <%s>', user.name, user.email)
+      #
+      class StringConcatenation < Base
+        include Util
+        extend AutoCorrector
+
+        MSG = 'Prefer string interpolation instead of string concatenation.'
+
+        def_node_matcher :string_concatenation?, <<~PATTERN
+          {
+            (send str_type? :+ _)
+            (send _ :+ str_type?)
+          }
+        PATTERN
+
+        def on_send(node)
+          return unless node.method?(:+)
+          return unless string_concatenation?(node)
+
+          topmost_plus_node = find_topmost_plus_node(node)
+
+          parts = []
+          collect_parts(topmost_plus_node, parts)
+
+          add_offense(topmost_plus_node) do |corrector|
+            corrector.replace(topmost_plus_node, replacement(parts))
+          end
+        end
+
+        private
+
+        def find_topmost_plus_node(node)
+          current = node
+          while (parent = current.parent) && plus_node?(parent)
+            current = parent
+          end
+          current
+        end
+
+        def collect_parts(node, parts)
+          return unless node
+
+          if plus_node?(node)
+            collect_parts(node.receiver, parts)
+            collect_parts(node.first_argument, parts)
+          else
+            parts << node
+          end
+        end
+
+        def plus_node?(node)
+          node.send_type? && node.method?(:+)
+        end
+
+        def replacement(parts)
+          interpolated_parts =
+            parts.map do |part|
+              if part.str_type?
+                if single_quoted?(part)
+                  part.value.gsub('\\') { '\\\\' }
+                else
+                  escape_string(part.value)
+                end
+              else
+                "\#{#{part.source}}"
+              end
+            end
+
+          "\"#{interpolated_parts.join}\""
+        end
+
+        def single_quoted?(str_node)
+          str_node.source.start_with?("'")
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/style/symbol_array.rb

@@ -74,7 +74,7 @@ def correct_bracketed(node)
             if c.dsym_type?
               string_literal = to_string_literal(c.source)
 
-              ':' + trim_string_interporation_escape_character(string_literal)
+              ":#{trim_string_interporation_escape_character(string_literal)}"
             else
               to_symbol_literal(c.value.to_s)
             end

lib/rubocop/cop/style/symbol_proc.rb

@@ -84,7 +84,7 @@ def autocorrect_with_args(corrector, node, args, method_name)
           arg_range = args.last.source_range
           arg_range = range_with_surrounding_comma(arg_range, :right)
           replacement = " &:#{method_name}"
-          replacement = ',' + replacement unless arg_range.source.end_with?(',')
+          replacement = ",#{replacement}" unless arg_range.source.end_with?(',')
           corrector.insert_after(arg_range, replacement)
           corrector.remove(block_range_with_space(node))
         end

lib/rubocop/cop/style/trailing_method_end_statement.rb

@@ -45,7 +45,7 @@ def on_def(node)
           add_offense(node.loc.end) do |corrector|
             corrector.insert_before(
               node.loc.end,
-              "\n" + ' ' * node.loc.keyword.column
+              "\n#{' ' * node.loc.keyword.column}"
             )
           end
         end

spec/rubocop/cli/cli_options_spec.rb

@@ -144,7 +144,7 @@
     it 'exits cleanly' do
       expect(cli.run(['-v'])).to eq(0)
       expect(cli.run(['--version'])).to eq(0)
-      expect($stdout.string).to eq((RuboCop::Version::STRING + "\n") * 2)
+      expect($stdout.string).to eq("#{RuboCop::Version::STRING}\n" * 2)
     end
   end
 
@@ -537,7 +537,7 @@ class SomeCop < Cop
     context 'when a namespace is given' do
       it 'runs all enabled cops in that namespace' do
         create_file('example.rb', ['if x== 100000000000000 ',
-                                   '  ' + '#' * 130,
+                                   "  #{'#' * 130}",
                                    "\ty",
                                    'end'])
         expect(cli.run(%w[-f offenses --only Layout example.rb])).to eq(1)
@@ -559,7 +559,7 @@ class SomeCop < Cop
     context 'when three namespaces are given' do
       it 'runs all enabled cops in those namespaces' do
         create_file('example.rb', ['if x== 100000000000000 ',
-                                   '  # ' + '-' * 130,
+                                   "  # #{'-' * 130}",
                                    "\ty",
                                    'end'])
         create_file('.rubocop.yml', <<~YAML)