diff --git a/Lib/imp.py b/Lib/imp.py
index e02aaef344..fc42c15765 100644
--- a/Lib/imp.py
+++ b/Lib/imp.py
@@ -9,7 +9,7 @@
 from _imp import (lock_held, acquire_lock, release_lock,
                   get_frozen_object, is_frozen_package,
                   init_frozen, is_builtin, is_frozen,
-                  _fix_co_filename)
+                  _fix_co_filename, _frozen_module_names)
 try:
     from _imp import create_dynamic
 except ImportError:
@@ -226,7 +226,7 @@ def load_module(name, file, filename, details):
 
     """
     suffix, mode, type_ = details
-    if mode and (not mode.startswith(('r', 'U')) or '+' in mode):
+    if mode and (not mode.startswith('r') or '+' in mode):
         raise ValueError('invalid file open mode {!r}'.format(mode))
     elif file is None and type_ in {PY_SOURCE, PY_COMPILED}:
         msg = 'file object required for import (type code {})'.format(type_)
diff --git a/Lib/test/test_imp.py b/Lib/test/test_imp.py
index 9b602582e2..1ccd072b3f 100644
--- a/Lib/test/test_imp.py
+++ b/Lib/test/test_imp.py
@@ -1,3 +1,4 @@
+import gc
 import importlib
 import importlib.util
 import os
@@ -8,19 +9,21 @@
 from test.support import import_helper
 from test.support import os_helper
 from test.support import script_helper
+from test.support import warnings_helper
 import unittest
 import warnings
-with warnings.catch_warnings():
-    warnings.simplefilter('ignore', DeprecationWarning)
-    import imp
+imp = warnings_helper.import_deprecated('imp')
 import _imp
 
 
+OS_PATH_NAME = os.path.__name__
+
+
 def requires_load_dynamic(meth):
     """Decorator to skip a test if not running under CPython or lacking
     imp.load_dynamic()."""
     meth = support.cpython_only(meth)
-    return unittest.skipIf(not hasattr(imp, 'load_dynamic'),
+    return unittest.skipIf(getattr(imp, 'load_dynamic', None) is None,
                            'imp.load_dynamic() required')(meth)
 
 
@@ -216,15 +219,17 @@ def test_load_from_source(self):
         # state after reversion. Reinitialising the module contents
         # and just reverting os.environ to its previous state is an OK
         # workaround
-        orig_path = os.path
-        orig_getenv = os.getenv
-        with os_helper.EnvironmentVarGuard():
-            x = imp.find_module("os")
-            self.addCleanup(x[0].close)
-            new_os = imp.load_module("os", *x)
-            self.assertIs(os, new_os)
-            self.assertIs(orig_path, new_os.path)
-            self.assertIsNot(orig_getenv, new_os.getenv)
+        with import_helper.CleanImport('os', 'os.path', OS_PATH_NAME):
+            import os
+            orig_path = os.path
+            orig_getenv = os.getenv
+            with os_helper.EnvironmentVarGuard():
+                x = imp.find_module("os")
+                self.addCleanup(x[0].close)
+                new_os = imp.load_module("os", *x)
+                self.assertIs(os, new_os)
+                self.assertIs(orig_path, new_os.path)
+                self.assertIsNot(orig_getenv, new_os.getenv)
 
     @requires_load_dynamic
     def test_issue15828_load_extensions(self):
@@ -351,8 +356,8 @@ def test_issue_35321(self):
     # TODO: RUSTPYTHON
     @unittest.expectedFailure
     def test_source_hash(self):
-        self.assertEqual(_imp.source_hash(42, b'hi'), b'\xc6\xe7Z\r\x03:}\xab')
-        self.assertEqual(_imp.source_hash(43, b'hi'), b'\x85\x9765\xf8\x9a\x8b9')
+        self.assertEqual(_imp.source_hash(42, b'hi'), b'\xfb\xd9G\x05\xaf$\x9b~')
+        self.assertEqual(_imp.source_hash(43, b'hi'), b'\xd0/\x87C\xccC\xff\xe2')
 
     def test_pyc_invalidation_mode_from_cmdline(self):
         cases = [
@@ -384,6 +389,35 @@ def test_find_and_load_checked_pyc(self):
         self.assertEqual(mod.x, 42)
 
 
+    @support.cpython_only
+    def test_create_builtin_subinterp(self):
+        # gh-99578: create_builtin() behavior changes after the creation of the
+        # first sub-interpreter. Test both code paths, before and after the
+        # creation of a sub-interpreter. Previously, create_builtin() had
+        # a reference leak after the creation of the first sub-interpreter.
+
+        import builtins
+        create_builtin = support.get_attribute(_imp, "create_builtin")
+        class Spec:
+            name = "builtins"
+        spec = Spec()
+
+        def check_get_builtins():
+            refcnt = sys.getrefcount(builtins)
+            mod = _imp.create_builtin(spec)
+            self.assertIs(mod, builtins)
+            self.assertEqual(sys.getrefcount(builtins), refcnt + 1)
+            # Check that a GC collection doesn't crash
+            gc.collect()
+
+        check_get_builtins()
+
+        ret = support.run_in_subinterp("import builtins")
+        self.assertEqual(ret, 0)
+
+        check_get_builtins()
+
+
 class ReloadTests(unittest.TestCase):
 
     """Very basic tests to make sure that imp.reload() operates just like
diff --git a/vm/src/stdlib/imp.rs b/vm/src/stdlib/imp.rs
index 773afed6db..e2e735acd1 100644
--- a/vm/src/stdlib/imp.rs
+++ b/vm/src/stdlib/imp.rs
@@ -154,6 +154,11 @@ mod _imp {
         // TODO:
     }
 
+    #[pyfunction]
+    fn _frozen_module_names(_code: PyObjectRef) {
+        // TODO:
+    }
+
     #[allow(clippy::type_complexity)]
     #[pyfunction]
     fn find_frozen(