diff --git a/tool/lib/test/unit/assertions.rb b/tool/lib/test/unit/assertions.rb
index 169a3dcc7e..0c68a93b02 100644
--- a/tool/lib/test/unit/assertions.rb
+++ b/tool/lib/test/unit/assertions.rb
@@ -1,15 +1,12 @@
 # frozen_string_literal: true
 require 'minitest/unit'
+require 'test/unit/core_assertions'
 require 'pp'
 
 module Test
   module Unit
     module Assertions
-      include MiniTest::Assertions
-
-      def mu_pp(obj) #:nodoc:
-        obj.pretty_inspect.chomp
-      end
+      include Test::Unit::CoreAssertions
 
       MINI_DIR = File.join(File.dirname(File.dirname(File.expand_path(__FILE__))), "minitest") #:nodoc:
 
@@ -554,80 +551,6 @@ EOT
         assert !status.signaled?, FailDesc[status, message, out]
       end
 
-      FailDesc = proc do |status, message = "", out = ""|
-        pid = status.pid
-        now = Time.now
-        faildesc = proc do
-          if signo = status.termsig
-            signame = Signal.signame(signo)
-            sigdesc = "signal #{signo}"
-          end
-          log = EnvUtil.diagnostic_reports(signame, pid, now)
-          if signame
-            sigdesc = "SIG#{signame} (#{sigdesc})"
-          end
-          if status.coredump?
-            sigdesc = "#{sigdesc} (core dumped)"
-          end
-          full_message = ''.dup
-          message = message.call if Proc === message
-          if message and !message.empty?
-            full_message << message << "\n"
-          end
-          full_message << "pid #{pid}"
-          full_message << " exit #{status.exitstatus}" if status.exited?
-          full_message << " killed by #{sigdesc}" if sigdesc
-          if out and !out.empty?
-            full_message << "\n" << out.b.gsub(/^/, '| ')
-            full_message.sub!(/(?<!\n)\z/, "\n")
-          end
-          if log
-            full_message << "Diagnostic reports:\n" << log.b.gsub(/^/, '| ')
-          end
-          full_message
-        end
-        faildesc
-      end
-
-      def assert_in_out_err(args, test_stdin = "", test_stdout = [], test_stderr = [], message = nil,
-                            success: nil, **opt)
-        args = Array(args).dup
-        args.insert((Hash === args[0] ? 1 : 0), '--disable=gems')
-        stdout, stderr, status = EnvUtil.invoke_ruby(args, test_stdin, true, true, **opt)
-        if signo = status.termsig
-          EnvUtil.diagnostic_reports(Signal.signame(signo), status.pid, Time.now)
-        end
-        if block_given?
-          raise "test_stdout ignored, use block only or without block" if test_stdout != []
-          raise "test_stderr ignored, use block only or without block" if test_stderr != []
-          yield(stdout.lines.map {|l| l.chomp }, stderr.lines.map {|l| l.chomp }, status)
-        else
-          all_assertions(message) do |a|
-            [["stdout", test_stdout, stdout], ["stderr", test_stderr, stderr]].each do |key, exp, act|
-              a.for(key) do
-                if exp.is_a?(Regexp)
-                  assert_match(exp, act)
-                elsif exp.all? {|e| String === e}
-                  assert_equal(exp, act.lines.map {|l| l.chomp })
-                else
-                  assert_pattern_list(exp, act)
-                end
-              end
-            end
-            unless success.nil?
-              a.for("success?") do
-                if success
-                  assert_predicate(status, :success?)
-                else
-                  assert_not_predicate(status, :success?)
-                end
-              end
-            end
-          end
-          status
-        end
-      end
-
       def assert_ruby_status(args, test_stdin="", message=nil, **opt)
         out, _, status = EnvUtil.invoke_ruby(args, test_stdin, true, :merge_to_stdout, **opt)
         desc = FailDesc[status, message, out]
@@ -636,57 +559,6 @@ EOT
         assert(status.success?, desc)
       end
 
-      ABORT_SIGNALS = Signal.list.values_at(*%w"ILL ABRT BUS SEGV TERM")
-
-      def assert_separately(args, file = nil, line = nil, src, ignore_stderr: nil, **opt)
-        unless file and line
-          loc, = caller_locations(1,1)
-          file ||= loc.path
-          line ||= loc.lineno
-        end
-        src = <<eom
-# -*- coding: #{line += __LINE__; src.encoding}; -*-
-  require #{__dir__.dump};include Test::Unit::Assertions
-  END {
-    puts [Marshal.dump($!)].pack('m'), "assertions=\#{self._assertions}"
-  }
-#{line -= __LINE__; src}
-  class Test::Unit::Runner
-    @@stop_auto_run = true
-  end
-eom
-        args = args.dup
-        args.insert((Hash === args.first ? 1 : 0), "-w", "--disable=gems", *$:.map {|l| "-I#{l}"})
-        stdout, stderr, status = EnvUtil.invoke_ruby(args, src, true, true, **opt)
-        abort = status.coredump? || (status.signaled? && ABORT_SIGNALS.include?(status.termsig))
-        assert(!abort, FailDesc[status, nil, stderr])
-        self._assertions += stdout[/^assertions=(\d+)/, 1].to_i
-        begin
-          res = Marshal.load(stdout.unpack("m")[0])
-        rescue => marshal_error
-          ignore_stderr = nil
-        end
-        if res
-          if bt = res.backtrace
-            bt.each do |l|
-              l.sub!(/\A-:(\d+)/){"#{file}:#{line + $1.to_i}"}
-            end
-            bt.concat(caller)
-          else
-            res.set_backtrace(caller)
-          end
-          raise res unless SystemExit === res
-        end
-
-        # really is it succeed?
-        unless ignore_stderr
-          # the body of assert_separately must not output anything to detect error
-          assert(stderr.empty?, FailDesc[status, "assert_separately failed with error message", stderr])
-        end
-        assert(status.success?, FailDesc[status, "assert_separately failed", stderr])
-        raise marshal_error if marshal_error
-      end
-
       def assert_warning(pat, msg = nil)
         result = nil
         stderr = EnvUtil.with_default_internal(pat.encoding) {
@@ -799,10 +671,6 @@ eom
         assert(1.0/f == -Float::INFINITY, "#{f} is not -0.0")
       end
 
-      def assert_file
-        AssertFile
-      end
-
       # pattern_list is an array which contains regexp and :*.
       # :* means any sequence.
       #
@@ -878,77 +746,6 @@ eom
         end
       end
 
-      class << (AssertFile = Struct.new(:failure_message).new)
-        include Assertions
-        def assert_file_predicate(predicate, *args)
-          if /\Anot_/ =~ predicate
-            predicate = $'
-            neg = " not"
-          end
-          result = File.__send__(predicate, *args)
-          result = !result if neg
-          mesg = "Expected file ".dup << args.shift.inspect
-          mesg << "#{neg} to be #{predicate}"
-          mesg << mu_pp(args).sub(/\A\[(.*)\]\z/m, '(\1)') unless args.empty?
-          mesg << " #{failure_message}" if failure_message
-          assert(result, mesg)
-        end
-        alias method_missing assert_file_predicate
-
-        def for(message)
-          clone.tap {|a| a.failure_message = message}
-        end
-      end
-
-      class AllFailures
-        attr_reader :failures
-
-        def initialize
-          @count = 0
-          @failures = {}
-        end
-
-        def for(key)
-          @count += 1
-          yield
-        rescue Exception => e
-          @failures[key] = [@count, e]
-        end
-
-        def foreach(*keys)
-          keys.each do |key|
-            @count += 1
-            begin
-              yield key
-            rescue Exception => e
-              @failures[key] = [@count, e]
-            end
-          end
-        end
-
-        def message
-          i = 0
-          total = @count.to_s
-          fmt = "%#{total.size}d"
-          @failures.map {|k, (n, v)|
-            v = v.message
-            "\n#{i+=1}. [#{fmt%n}/#{total}] Assertion for #{k.inspect}\n#{v.b.gsub(/^/, '   | ').force_encoding(v.encoding)}"
-          }.join("\n")
-        end
-
-        def pass?
-          @failures.empty?
-        end
-      end
-
-      def assert_all_assertions(msg = nil)
-        all = AllFailures.new
-        yield all
-      ensure
-        assert(all.pass?, message(msg) {all.message.chomp(".")})
-      end
-      alias all_assertions assert_all_assertions
-
       def assert_all_assertions_foreach(msg = nil, *keys, &block)
         all = AllFailures.new
         all.foreach(*keys, &block)
diff --git a/tool/lib/test/unit/core_assertions.rb b/tool/lib/test/unit/core_assertions.rb
new file mode 100644
index 0000000000..21be860176
--- /dev/null
+++ b/tool/lib/test/unit/core_assertions.rb
@@ -0,0 +1,216 @@
+# frozen_string_literal: true
+
+require_relative '../../envutil'
+
+module Test
+  module Unit
+    module CoreAssertions
+      include MiniTest::Assertions
+
+      def mu_pp(obj) #:nodoc:
+        obj.pretty_inspect.chomp
+      end
+
+      def assert_file
+        AssertFile
+      end
+
+      FailDesc = proc do |status, message = "", out = ""|
+        pid = status.pid
+        now = Time.now
+        faildesc = proc do
+          if signo = status.termsig
+            signame = Signal.signame(signo)
+            sigdesc = "signal #{signo}"
+          end
+          log = EnvUtil.diagnostic_reports(signame, pid, now)
+          if signame
+            sigdesc = "SIG#{signame} (#{sigdesc})"
+          end
+          if status.coredump?
+            sigdesc = "#{sigdesc} (core dumped)"
+          end
+          full_message = ''.dup
+          message = message.call if Proc === message
+          if message and !message.empty?
+            full_message << message << "\n"
+          end
+          full_message << "pid #{pid}"
+          full_message << " exit #{status.exitstatus}" if status.exited?
+          full_message << " killed by #{sigdesc}" if sigdesc
+          if out and !out.empty?
+            full_message << "\n" << out.b.gsub(/^/, '| ')
+            full_message.sub!(/(?<!\n)\z/, "\n")
+          end
+          if log
+            full_message << "Diagnostic reports:\n" << log.b.gsub(/^/, '| ')
+          end
+          full_message
+        end
+        faildesc
+      end
+
+      def assert_in_out_err(args, test_stdin = "", test_stdout = [], test_stderr = [], message = nil,
+                            success: nil, **opt)
+        args = Array(args).dup
+        args.insert((Hash === args[0] ? 1 : 0), '--disable=gems')
+        stdout, stderr, status = EnvUtil.invoke_ruby(args, test_stdin, true, true, **opt)
+        if signo = status.termsig
+          EnvUtil.diagnostic_reports(Signal.signame(signo), status.pid, Time.now)
+        end
+        if block_given?
+          raise "test_stdout ignored, use block only or without block" if test_stdout != []
+          raise "test_stderr ignored, use block only or without block" if test_stderr != []
+          yield(stdout.lines.map {|l| l.chomp }, stderr.lines.map {|l| l.chomp }, status)
+        else
+          all_assertions(message) do |a|
+            [["stdout", test_stdout, stdout], ["stderr", test_stderr, stderr]].each do |key, exp, act|
+              a.for(key) do
+                if exp.is_a?(Regexp)
+                  assert_match(exp, act)
+                elsif exp.all? {|e| String === e}
+                  assert_equal(exp, act.lines.map {|l| l.chomp })
+                else
+                  assert_pattern_list(exp, act)
+                end
+              end
+            end
+            unless success.nil?
+              a.for("success?") do
+                if success
+                  assert_predicate(status, :success?)
+                else
+                  assert_not_predicate(status, :success?)
+                end
+              end
+            end
+          end
+          status
+        end
+      end
+
+      ABORT_SIGNALS = Signal.list.values_at(*%w"ILL ABRT BUS SEGV TERM")
+
+      def assert_separately(args, file = nil, line = nil, src, ignore_stderr: nil, **opt)
+        unless file and line
+          loc, = caller_locations(1,1)
+          file ||= loc.path
+          line ||= loc.lineno
+        end
+        src = <<eom
+# -*- coding: #{line += __LINE__; src.encoding}; -*-
+  require #{__dir__.dump};include Test::Unit::Assertions
+  END {
+    puts [Marshal.dump($!)].pack('m'), "assertions=\#{self._assertions}"
+  }
+#{line -= __LINE__; src}
+  class Test::Unit::Runner
+    @@stop_auto_run = true
+  end
+eom
+        args = args.dup
+        args.insert((Hash === args.first ? 1 : 0), "-w", "--disable=gems", *$:.map {|l| "-I#{l}"})
+        stdout, stderr, status = EnvUtil.invoke_ruby(args, src, true, true, **opt)
+        abort = status.coredump? || (status.signaled? && ABORT_SIGNALS.include?(status.termsig))
+        assert(!abort, FailDesc[status, nil, stderr])
+        self._assertions += stdout[/^assertions=(\d+)/, 1].to_i
+        begin
+          res = Marshal.load(stdout.unpack("m")[0])
+        rescue => marshal_error
+          ignore_stderr = nil
+        end
+        if res
+          if bt = res.backtrace
+            bt.each do |l|
+              l.sub!(/\A-:(\d+)/){"#{file}:#{line + $1.to_i}"}
+            end
+            bt.concat(caller)
+          else
+            res.set_backtrace(caller)
+          end
+          raise res unless SystemExit === res
+        end
+
+        # really is it succeed?
+        unless ignore_stderr
+          # the body of assert_separately must not output anything to detect error
+          assert(stderr.empty?, FailDesc[status, "assert_separately failed with error message", stderr])
+        end
+        assert(status.success?, FailDesc[status, "assert_separately failed", stderr])
+        raise marshal_error if marshal_error
+      end
+
+      class << (AssertFile = Struct.new(:failure_message).new)
+        include CoreAssertions
+        def assert_file_predicate(predicate, *args)
+          if /\Anot_/ =~ predicate
+            predicate = $'
+            neg = " not"
+          end
+          result = File.__send__(predicate, *args)
+          result = !result if neg
+          mesg = "Expected file ".dup << args.shift.inspect
+          mesg << "#{neg} to be #{predicate}"
+          mesg << mu_pp(args).sub(/\A\[(.*)\]\z/m, '(\1)') unless args.empty?
+          mesg << " #{failure_message}" if failure_message
+          assert(result, mesg)
+        end
+        alias method_missing assert_file_predicate
+
+        def for(message)
+          clone.tap {|a| a.failure_message = message}
+        end
+      end
+
+      class AllFailures
+        attr_reader :failures
+
+        def initialize
+          @count = 0
+          @failures = {}
+        end
+
+        def for(key)
+          @count += 1
+          yield
+        rescue Exception => e
+          @failures[key] = [@count, e]
+        end
+
+        def foreach(*keys)
+          keys.each do |key|
+            @count += 1
+            begin
+              yield key
+            rescue Exception => e
+              @failures[key] = [@count, e]
+            end
+          end
+        end
+
+        def message
+          i = 0
+          total = @count.to_s
+          fmt = "%#{total.size}d"
+          @failures.map {|k, (n, v)|
+            v = v.message
+            "\n#{i+=1}. [#{fmt%n}/#{total}] Assertion for #{k.inspect}\n#{v.b.gsub(/^/, '   | ').force_encoding(v.encoding)}"
+          }.join("\n")
+        end
+
+        def pass?
+          @failures.empty?
+        end
+      end
+
+      def assert_all_assertions(msg = nil)
+        all = AllFailures.new
+        yield all
+      ensure
+        assert(all.pass?, message(msg) {all.message.chomp(".")})
+      end
+      alias all_assertions assert_all_assertions
+
+    end
+  end
+end