diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -1278,16 +1278,28 @@ def debuginstall(ui, **opts):
     fm.write('hgmodules', _("checking installed modules (%s)...\n"),
              os.path.dirname(pycompat.fsencode(__file__)))
 
-    if policy.policy in ('c', 'allow'):
+    rustandc = policy.policy in ('rust+c', 'rust+c-allow')
+    rustext = rustandc  # for now, that's the only case
+    cext = policy.policy in ('c', 'allow') or rustandc
+    nopure = cext or rustext
+    if nopure:
         err = None
         try:
-            from .cext import (
-                base85,
-                bdiff,
-                mpatch,
-                osutil,
-            )
-            dir(bdiff), dir(mpatch), dir(base85), dir(osutil) # quiet pyflakes
+            if cext:
+                from .cext import (
+                    base85,
+                    bdiff,
+                    mpatch,
+                    osutil,
+                )
+                # quiet pyflakes
+                dir(bdiff), dir(mpatch), dir(base85), dir(osutil)
+            if rustext:
+                from .rustext import (
+                    ancestor,
+                    dirstate,
+                )
+                dir(ancestor), dir(dirstate) # quiet pyflakes
         except Exception as inst:
             err = stringutil.forcebytestr(inst)
             problems += 1
diff --git a/mercurial/policy.py b/mercurial/policy.py
--- a/mercurial/policy.py
+++ b/mercurial/policy.py
@@ -13,6 +13,9 @@ import sys
 # Rules for how modules can be loaded. Values are:
 #
 #    c - require C extensions
+#    rust+c - require Rust and C extensions
+#    rust+c-allow - allow Rust and C extensions with fallback to pure Python
+#                   for each
 #    allow - allow pure Python implementation when C loading fails
 #    cffi - required cffi versions (implemented within pure module)
 #    cffi-allow - allow pure Python implementation if cffi version is missing
@@ -29,6 +32,9 @@ policy = b'allow'
     b'cffi': (r'cffi', None),
     b'cffi-allow': (r'cffi', r'pure'),
     b'py': (None, r'pure'),
+    # For now, rust policies impact importrust only
+    b'rust+c': (r'cext', None),
+    b'rust+c-allow': (r'cext', r'pure'),
 }
 
 try:
@@ -107,3 +113,34 @@ def importmod(modname):
                 raise
     pn, mn = _modredirects.get((purepkg, modname), (purepkg, modname))
     return _importfrom(pn, mn)
+
+def _isrustpermissive():
+    """Assuming the policy is a Rust one, tell if it's permissive."""
+    return policy.endswith(b'-allow')
+
+def importrust(modname, member=None, default=None):
+    """Import Rust module according to policy and availability.
+
+    If policy isn't a Rust one, this returns `default`.
+
+    If either the module or its member is not available, this returns `default`
+    if policy is permissive and raises `ImportError` if not.
+    """
+    if not policy.startswith(b'rust'):
+        return default
+
+    try:
+        mod = _importfrom(r'rustext', modname)
+    except ImportError:
+        if _isrustpermissive():
+            return default
+        raise
+    if member is None:
+        return mod
+
+    try:
+        return getattr(mod, member)
+    except AttributeError:
+        if _isrustpermissive():
+            return default
+        raise ImportError(r"Cannot import name %s" % member)
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -556,10 +556,17 @@ class hgbuildpy(build_py):
         if self.distribution.pure:
             modulepolicy = 'py'
         elif self.build_lib == '.':
-            # in-place build should run without rebuilding C extensions
-            modulepolicy = 'allow'
+            # in-place build should run without rebuilding C
+            # and Rust extensions
+            if hgrustext == 'cpython':
+                modulepolicy = 'rust+c-allow'
+            else:
+                modulepolicy = 'allow'
         else:
-            modulepolicy = 'c'
+            if hgrustext == 'cpython':
+                modulepolicy = 'rust+c'
+            else:
+                modulepolicy = 'c'
 
         content = b''.join([
             b'# this file is autogenerated by setup.py\n',