fsmonitor: increase the threshold before we recommend it, when using rust

50k files works just fine with the rust status, and it's annoying to
get told "consider using fsmonitor" when we have another solution to
the status speed.

400k files was not chosen in any rigorous way. I know 200k files is
fine. Twice as many files should still be fine.

Differential Revision: https://phab.mercurial-scm.org/D9021

hgext/fsmonitor/__init__.py

@@ -73,6 +73,8 b" beneficial to performance but isn't enab"
 
     [fsmonitor]
     warn_update_file_count = (integer)
+    # or when mercurial is built with rust support
+    warn_update_file_count_rust = (integer)
 
 If ``warn_when_unused`` is set and fsmonitor isn't enabled, a warning will
 be printed during working directory updates if this many files will be

mercurial/configitems.py

@@ -793,6 +793,9 b' coreconfigitem('
     b'fsmonitor', b'warn_update_file_count', default=50000,
 )
 coreconfigitem(
+    b'fsmonitor', b'warn_update_file_count_rust', default=400000,
+)
+coreconfigitem(
     b'help', br'hidden-command\..*', default=False, generic=True,
 )
 coreconfigitem(

mercurial/merge.py

@@ -1698,6 +1698,15 b' def _advertisefsmonitor(repo, num_gets, '
     fsmonitorthreshold = repo.ui.configint(
         b'fsmonitor', b'warn_update_file_count'
     )
+    # avoid cycle dirstate -> sparse -> merge -> dirstate
+    from . import dirstate
+
+    if dirstate.rustmod is not None:
+        # When using rust status, fsmonitor becomes necessary at higher sizes
+        fsmonitorthreshold = repo.ui.configint(
+            b'fsmonitor', b'warn_update_file_count_rust',
+        )
+
     try:
         # avoid cycle: extensions -> cmdutil -> merge
         from . import extensions

tests/test-clone.t

@@ -1241,6 +1241,7 b' Lower the warning threshold to simulate '
   $ cat >> $HGRCPATH << EOF
   > [fsmonitor]
   > warn_update_file_count = 2
+  > warn_update_file_count_rust = 2
   > EOF
 
 We should see a warning about no fsmonitor on supported platforms