diff -r ab58098bebed -r b4c2a2af25e2 mercurial/requirements.py
--- a/mercurial/requirements.py	Wed Mar 03 12:35:29 2021 +0100
+++ b/mercurial/requirements.py	Wed Mar 03 12:37:13 2021 +0100
@@ -10,6 +10,7 @@
 GENERALDELTA_REQUIREMENT = b'generaldelta'
 DOTENCODE_REQUIREMENT = b'dotencode'
 STORE_REQUIREMENT = b'store'
+FNCACHE_REQUIREMENT = b'fncache'
 
 # When narrowing is finalized and no longer subject to format changes,
 # we should move this to just "narrow" or similar.