Handle empty input and pass it to debugger

Since `rdbg` accepts empty input to repeat the previous command, IRB
should take empty input in `irb:rdbg` sessions and pass them to the
debugger.

Currently, IRB simply ignores empty input and does nothing. This commit
creates `EmptyInput` to represent empty input so it can fit into the
current IRB's input processing flow in `Irb#eval_input`.

lib/irb.rb

@@ -1074,16 +1074,17 @@ def each_top_level_statement
       loop do
         code = readmultiline
         break unless code
-
-        if code != "\n"
-          yield build_statement(code), @line_no
-        end
+        yield build_statement(code), @line_no
         @line_no += code.count("\n")
       rescue RubyLex::TerminateLineInput
       end
     end
 
     def build_statement(code)
+      if code.match?(/\A\n*\z/)
+        return Statement::EmptyInput.new
+      end
+
       code.force_encoding(@context.io.encoding)
       command_or_alias, arg = code.split(/\s/, 2)
       # Transform a non-identifier alias (@, $) or keywords (next, break)

lib/irb/statement.rb

@@ -20,6 +20,29 @@ def evaluable_code
       raise NotImplementedError
     end
 
+    class EmptyInput < Statement
+      def is_assignment?
+        false
+      end
+
+      def suppresses_echo?
+        true
+      end
+
+      # Debugger takes empty input to repeat the last command
+      def should_be_handled_by_debugger?
+        true
+      end
+
+      def code
+        ""
+      end
+
+      def evaluable_code
+        code
+      end
+    end
+
     class Expression < Statement
       def initialize(code, is_assignment)
         @code = code

test/irb/test_debug_cmd.rb → test/irb/test_debugger_integration.rb

@@ -6,7 +6,7 @@
 require_relative "helper"
 
 module TestIRB
-  class DebugCommandTest < IntegrationTestCase
+  class DebuggerIntegrationTest < IntegrationTestCase
     def setup
       super
 
@@ -434,5 +434,25 @@ def test_multi_irb_commands_are_not_available_after_activating_the_debugger
       assert_match(/irb\(main\):001> next/, output)
       assert_include(output, "Multi-IRB commands are not available when the debugger is enabled.")
     end
+
+    def test_irb_passes_empty_input_to_debugger_to_repeat_the_last_command
+      write_ruby <<~'ruby'
+        binding.irb
+        puts "foo"
+        puts "bar"
+        puts "baz"
+      ruby
+
+      output = run_ruby_file do
+        type "next"
+        type ""
+        type "\n\r"
+        type "\n"
+      end
+
+      assert_include(output, "=>   2\| puts \"foo\"")
+      assert_include(output, "=>   3\| puts \"bar\"")
+      assert_include(output, "=>   4\| puts \"baz\"")
+    end
   end
 end