[Fix #9905] Fix single line concatenation false positive

By checking that the inspected node is multiline, we should
get rid of this false positive. Happens for code like

puts 'a'"#{b}"

We're also checking that none of the children is multiline.
This is done to skip weird corner cases that are
impossible to have a consistent alignment rule for.

It turns out that the new multiline checks make it
possible to remove a couple of old conditions when checking
if it's a concatenation with backslashes we're inspecting.
It is perhaps possible to search for backslash and newline
in the code, but I'd like to avoid that, because I'm
afraid of other weird corner cases if we take the
regexp search approach.

changelog/fix_line_end_string_concatenation_indentation.md

@@ -0,0 +1 @@
+* [#9905](https://github.com/rubocop/rubocop/issues/9905): Fix false positive for single line concatenation in `Layout/LineEndStringConcatenationIndentation`. ([@jonas054][])

lib/rubocop/cop/layout/line_end_string_concatenation_indentation.rb

@@ -85,14 +85,9 @@ def autocorrect(corrector, node)
         private
 
         def strings_concatenated_with_backslash?(dstr_node)
-          !dstr_node.heredoc? &&
-            !single_string_literal?(dstr_node) &&
-            dstr_node.children.length > 1 &&
-            dstr_node.children.all? { |c| c.str_type? || c.dstr_type? }
-        end
-
-        def single_string_literal?(dstr_node)
-          dstr_node.loc.respond_to?(:begin) && dstr_node.loc.begin
+          dstr_node.multiline? &&
+            dstr_node.children.all? { |c| c.str_type? || c.dstr_type? } &&
+            dstr_node.children.none?(&:multiline?)
         end
 
         def always_indented?(dstr_node)

spec/rubocop/cop/layout/line_end_string_concatenation_indentation_spec.rb

@@ -14,6 +14,21 @@
   let(:cop_indent) { nil } # use indentation width from Layout/IndentationWidth
 
   shared_examples 'common' do
+    it 'accepts single line string literal concatenation' do
+      expect_no_offenses(<<~'RUBY')
+        text = 'offense'
+        puts 'This probably should not be '"an #{text}"
+      RUBY
+    end
+
+    it 'accepts string literal with line break concatenated with other string' do
+      expect_no_offenses(<<~'RUBY')
+        text = 'offense'
+        puts 'This probably
+              should not be '"an #{text}"
+      RUBY
+    end
+
     it 'accepts a multiline string literal' do
       expect_no_offenses(<<~'RUBY')
         puts %(