Change "check syntax" message when it isn't a ruby file

lib/rdoc/rdoc.rb

@@ -357,13 +357,25 @@ def parse_file(filename)
 this is not your library please report a bug to the author.
     EOF
   rescue => e
-    $stderr.puts <<-EOF
+    syntax_check_command = syntax_check_command_for filename, parser&.class
+    syntax_check_message = if syntax_check_command
+      <<~MESSAGE
 Before reporting this, could you check that the file you're documenting
 has proper syntax:
 
-  #{Gem.ruby} -c #{filename}
+  #{syntax_check_command}
+      MESSAGE
+    else
+      <<~MESSAGE
+Before reporting this, could you check that the file you're documenting
+has proper syntax for its language?
+      MESSAGE
+    end
 
-RDoc is not a full Ruby parser and will fail when fed invalid ruby programs.
+    $stderr.puts <<-EOF
+#{syntax_check_message}
+RDoc's parsers are not full language parsers and may fail when fed invalid
+source files.
 
 The internal error was:
 
@@ -376,6 +388,16 @@ def parse_file(filename)
     raise e
   end
 
+  def syntax_check_command_for(filename, parser_class = RDoc::Parser.can_parse_by_name(filename))
+    if parser_class == RDoc::Parser::Ruby
+      "#{Gem.ruby} -c #{filename}"
+    elsif parser_class == RDoc::Parser::C
+      cc = ENV['CC']
+      cc = 'cc' if cc.nil? || cc.empty?
+      "#{cc} -fsyntax-only #{filename}"
+    end
+  end
+
   ##
   # Returns the relative path for +filename+ against +options.root+ (and
   # +options.page_dir+ when set).  This is the key used by RDoc::Store to

test/rdoc/rdoc_rdoc_test.rb

@@ -493,6 +493,23 @@ def test_parse_file_forbidden
     tf.close!
   end
 
+  def test_syntax_check_command_for_c_file
+    command = @rdoc.syntax_check_command_for 'extension.c'
+
+    assert_includes command, ' -fsyntax-only extension.c'
+    refute_includes command, "#{Gem.ruby} -c"
+  end
+
+  def test_syntax_check_command_for_ruby_file
+    command = @rdoc.syntax_check_command_for 'library.rb'
+
+    assert_equal "#{Gem.ruby} -c library.rb", command
+  end
+
+  def test_syntax_check_command_for_unknown_file_type
+    assert_nil @rdoc.syntax_check_command_for('README')
+  end
+
   def test_remove_unparseable
     file_list = %w[
       blah.class