configitems: use standard "dynamicdefault" approach in edge case

This makes for fewer edge cases, which will help a future patch.

mercurial/configitems.py

@@ -1473,22 +1473,10 b' coreconfigitem('
     b'usestore',
     default=True,
 )
-
-
-def _persistent_nodemap_default():
-    """compute `use-persistent-nodemap` default value
-
-    The feature is disabled unless a fast implementation is available.
-    """
-    from . import policy
-
-    return policy.importrust('revlog') is not None
-
-
 coreconfigitem(
     b'format',
     b'use-persistent-nodemap',
-    default=_persistent_nodemap_default,
+    default=dynamicdefault,
 )
 coreconfigitem(
     b'format',

mercurial/localrepo.py

@@ -58,6 +58,7 b' from . import ('
     obsolete,
     pathutil,
     phases,
+    policy,
     pushkey,
     pycompat,
     rcutil,
@@ -3763,7 +3764,11 b' def newreporequirements(ui, createopts):'
     if ui.configbool(b'format', b'bookmarks-in-store'):
         requirements.add(requirementsmod.BOOKMARKS_IN_STORE_REQUIREMENT)
 
-    if ui.configbool(b'format', b'use-persistent-nodemap'):
+    # The feature is disabled unless a fast implementation is available.
+    persistent_nodemap_default = policy.importrust('revlog') is not None
+    if ui.configbool(
+        b'format', b'use-persistent-nodemap', persistent_nodemap_default
+    ):
         requirements.add(requirementsmod.NODEMAP_REQUIREMENT)
 
     # if share-safe is enabled, let's create the new repository with the new