diff --git a/src/pip/_internal/operations/prepare.py b/src/pip/_internal/operations/prepare.py
index 5f5505cd7..0b61f2052 100644
--- a/src/pip/_internal/operations/prepare.py
+++ b/src/pip/_internal/operations/prepare.py
@@ -420,6 +420,8 @@ class RequirementPreparer(object):
             # editable in a req, a non deterministic error
             # occurs when the script attempts to unpack the
             # build directory
+            # Since source_dir is only set for editable requirements.
+            assert req.source_dir is None
             req.ensure_has_source_dir(self.build_dir)
             # If a checkout exists, it's unwise to keep going.  version
             # inconsistencies are logged later, but do not fail the
diff --git a/src/pip/_internal/req/req_install.py b/src/pip/_internal/req/req_install.py
index 58746cd2a..22ac24b96 100644
--- a/src/pip/_internal/req/req_install.py
+++ b/src/pip/_internal/req/req_install.py
@@ -36,9 +36,9 @@ from pip._internal.utils.logging import indent_log
 from pip._internal.utils.marker_files import (
     PIP_DELETE_MARKER_FILENAME,
     has_delete_marker_file,
+    write_delete_marker_file,
 )
 from pip._internal.utils.misc import (
-    _make_build_dir,
     ask_path_exists,
     backup_dir,
     display_path,
@@ -370,7 +370,8 @@ class InstallRequirement(object):
         # need this)
         if not os.path.exists(build_dir):
             logger.debug('Creating directory %s', build_dir)
-            _make_build_dir(build_dir)
+            os.makedirs(build_dir)
+            write_delete_marker_file(build_dir)
         return os.path.join(build_dir, name)
 
     def _set_requirement(self):
diff --git a/src/pip/_internal/utils/misc.py b/src/pip/_internal/utils/misc.py
index a7ee5398b..4a5816019 100644
--- a/src/pip/_internal/utils/misc.py
+++ b/src/pip/_internal/utils/misc.py
@@ -39,7 +39,6 @@ from pip._internal.utils.compat import (
     stdlib_pkgs,
     str_to_display,
 )
-from pip._internal.utils.marker_files import write_delete_marker_file
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING, cast
 from pip._internal.utils.virtualenv import (
     running_under_virtualenv,
@@ -523,11 +522,6 @@ def write_output(msg, *args):
     logger.info(msg, *args)
 
 
-def _make_build_dir(build_dir):
-    os.makedirs(build_dir)
-    write_delete_marker_file(build_dir)
-
-
 class FakeFile(object):
     """Wrap a list of lines in an object with readline() to make
     ConfigParser happy."""