[Fix #9945] Fix heredoc indentation in trailing space corrections

Whitespace-only lines within heredocs that are shorter than the
indentation level, or equal to it, should be removed when correcting
trailing whitespace. Inserting string interpolation tokens on these
lines changes the indentation level for squiggly heredocs.

changelog/fix_heredoc_trailing_spaces.md

@@ -0,0 +1 @@
+* [#9945](https://github.com/rubocop/rubocop/issues/9945): Fix auto-correction of lines in heredocs with only spaces in `Layout/TrailingWhitespace`. ([@jonas054][])

lib/rubocop/cop/layout/heredoc_indentation.rb

@@ -143,13 +143,6 @@ def base_indent_level(node)
           indent_level(base_line)
         end
 
-        def indent_level(str)
-          indentations = str.lines
-                            .map { |line| line[/^\s*/] }
-                            .reject { |line| line.end_with?("\n") }
-          indentations.empty? ? 0 : indentations.min_by(&:size).size
-        end
-
         # Returns '~', '-' or nil
         def heredoc_indent_type(node)
           node.source[/^<<([~-])/, 1]

lib/rubocop/cop/layout/trailing_whitespace.rb

@@ -41,6 +41,7 @@ module Layout
       #
       class TrailingWhitespace < Base
         include RangeHelp
+        include Heredoc
         extend AutoCorrector
 
         MSG = 'Trailing whitespace detected.'
@@ -54,6 +55,8 @@ def on_new_investigation
           end
         end
 
+        def on_heredoc(_node); end
+
         private
 
         def process_line(line, lineno)
@@ -63,13 +66,33 @@ def process_line(line, lineno)
           range = offense_range(lineno, line)
           add_offense(range) do |corrector|
             if heredoc
-              corrector.wrap(range, "\#{'", "'}") unless static?(heredoc)
+              process_line_in_heredoc(corrector, range, heredoc)
             else
               corrector.remove(range)
             end
           end
         end
 
+        def process_line_in_heredoc(corrector, range, heredoc)
+          indent_level = indent_level(find_heredoc(range.line).loc.heredoc_body.source)
+          whitespace_only = whitespace_only?(range)
+          if whitespace_only && whitespace_is_indentation?(range, indent_level)
+            corrector.remove(range)
+          elsif !static?(heredoc)
+            range = range_between(range.begin_pos + indent_level, range.end_pos) if whitespace_only
+            corrector.wrap(range, "\#{'", "'}")
+          end
+        end
+
+        def whitespace_is_indentation?(range, level)
+          range.source[/ +/].length <= level
+        end
+
+        def whitespace_only?(range)
+          source = range_with_surrounding_space(range: range).source
+          source.start_with?("\n") && source.end_with?("\n")
+        end
+
         def static?(heredoc)
           heredoc.loc.expression.source.end_with? "'"
         end

lib/rubocop/cop/mixin/heredoc.rb

@@ -20,6 +20,13 @@ def on_heredoc(_node)
 
       private
 
+      def indent_level(str)
+        indentations = str.lines
+                          .map { |line| line[/^\s*/] }
+                          .reject { |line| line.end_with?("\n") }
+        indentations.empty? ? 0 : indentations.min_by(&:size).size
+      end
+
       def delimiter_string(node)
         node.source.match(OPENING_DELIMITER).captures[1]
       end

spec/rubocop/cop/layout/trailing_whitespace_spec.rb

@@ -144,6 +144,49 @@
       RUBY
     end
 
+    it 'corrects by removing trailing whitespace used for indentation in a heredoc string' do
+      expect_offense(<<~RUBY)
+        x = <<~EXAMPLE
+          no trailing
+         #{trailing_whitespace}
+        ^^ Trailing whitespace detected.
+          no trailing
+        #{trailing_whitespace}
+        ^ Trailing whitespace detected.
+          no trailing
+        EXAMPLE
+      RUBY
+
+      expect_correction(<<~RUBY)
+        x = <<~EXAMPLE
+          no trailing
+
+          no trailing
+
+          no trailing
+        EXAMPLE
+      RUBY
+    end
+
+    it 'corrects a whitespace line in a heredoc string that is longer than the indentation' do
+      expect_offense(<<~RUBY)
+        x = <<~EXAMPLE
+          no trailing
+          #{trailing_whitespace}
+        ^^^ Trailing whitespace detected.
+          no trailing
+        EXAMPLE
+      RUBY
+
+      expect_correction(<<~RUBY)
+        x = <<~EXAMPLE
+          no trailing
+          \#{' '}
+          no trailing
+        EXAMPLE
+      RUBY
+    end
+
     it 'does not correct trailing whitespace in a static heredoc string' do
       expect_offense(<<~RUBY)
         x = <<~'EXAMPLE'