diff --git a/ext/-test-/testutil/extconf.rb b/ext/-test-/testutil/extconf.rb
new file mode 100644
index 0000000000..157a3d62b8
--- /dev/null
+++ b/ext/-test-/testutil/extconf.rb
@@ -0,0 +1,8 @@
+# frozen_string_literal: false
+$INCFLAGS << " -I$(topdir) -I$(top_srcdir)"
+$srcs = Dir[File.join($srcdir, "*.{#{SRC_EXT.join(%q{,})}}")]
+inits = $srcs.map {|s| File.basename(s, ".*")}
+inits.delete("init")
+inits.map! {|s|"X(#{s})"}
+$defs << "-DTEST_INIT_FUNCS(X)=\"#{inits.join(' ')}\""
+create_makefile("-test-/testutil")
diff --git a/ext/-test-/testutil/init.c b/ext/-test-/testutil/init.c
new file mode 100644
index 0000000000..cf2804df4f
--- /dev/null
+++ b/ext/-test-/testutil/init.c
@@ -0,0 +1,11 @@
+#include "ruby.h"
+
+#define init(n) {void Init_##n(VALUE klass); Init_##n(klass);}
+
+void
+Init_testutil(void)
+{
+    VALUE mBug = rb_define_module("Bug");
+    VALUE klass = rb_define_class_under(mBug, "TestUtil", rb_cObject);
+    TEST_INIT_FUNCS(init);
+}
diff --git a/ext/-test-/testutil/integer.c b/ext/-test-/testutil/integer.c
new file mode 100644
index 0000000000..da5812c616
--- /dev/null
+++ b/ext/-test-/testutil/integer.c
@@ -0,0 +1,29 @@
+#include "internal.h"
+
+static VALUE
+int_bignum_p(VALUE self)
+{
+    return RB_TYPE_P(self, T_BIGNUM) ? Qtrue : Qfalse;
+}
+
+static VALUE
+int_fixnum_p(VALUE self)
+{
+    return FIXNUM_P(self) ? Qtrue : Qfalse;
+}
+
+static VALUE
+rb_int_to_bignum(VALUE x)
+{
+    if (FIXNUM_P(x))
+        x = rb_int2big(FIX2LONG(x));
+    return x;
+}
+
+void
+Init_integer(VALUE klass)
+{
+    rb_define_method(rb_cInteger, "bignum?", int_bignum_p, 0);
+    rb_define_method(rb_cInteger, "fixnum?", int_fixnum_p, 0);
+    rb_define_method(rb_cInteger, "to_bignum", rb_int_to_bignum, 0);
+}