--- a/mercurial/context.py	Fri Jan 28 11:33:01 2022 +0100
+++ b/mercurial/context.py	Fri Jan 28 11:35:41 2022 +0100
@@ -684,6 +684,14 @@
         """Return a list of byte bookmark names."""
         return self._repo.nodebookmarks(self._node)
 
+    def fast_rank(self):
+        repo = self._repo
+        if self._maybe_filtered:
+            cl = repo.changelog
+        else:
+            cl = repo.unfiltered().changelog
+        return cl.fast_rank(self._rev)
+
     def phase(self):
         return self._repo._phasecache.phase(self._repo, self._rev)
 

--- a/mercurial/templatekw.py	Fri Jan 28 11:33:01 2022 +0100
+++ b/mercurial/templatekw.py	Fri Jan 28 11:35:41 2022 +0100
@@ -304,6 +304,21 @@
     )
 
 
+@templatekeyword(b'_fast_rank', requires={b'ctx'})
+def fast_rank(context, mapping):
+    """the rank of a changeset if cached
+
+    The rank of a revision is the size of the sub-graph it defines as a head.
+    Equivalently, the rank of a revision `r` is the size of the set
+    `ancestors(r)`, `r` included.
+    """
+    ctx = context.resource(mapping, b'ctx')
+    rank = ctx.fast_rank()
+    if rank is None:
+        return None
+    return b"%d" % rank
+
+
 def _getfilestatus(context, mapping, listall=False):
     ctx = context.resource(mapping, b'ctx')
     revcache = context.resource(mapping, b'revcache')