--- a/mercurial/configitems.py	Tue Oct 17 21:15:31 2017 +0200
+++ b/mercurial/configitems.py	Mon Oct 16 17:41:27 2017 +0200
@@ -249,6 +249,9 @@
 coreconfigitem('devel', 'user.obsmarker',
     default=None,
 )
+coreconfigitem('devel', 'warn-config-unknown',
+    default=None,
+)
 coreconfigitem('diff', 'nodates',
     default=False,
 )

--- a/mercurial/ui.py	Tue Oct 17 21:15:31 2017 +0200
+++ b/mercurial/ui.py	Mon Oct 16 17:41:27 2017 +0200
@@ -477,6 +477,10 @@
 
         if item is not None:
             alternates.extend(item.alias)
+        else:
+            msg = ("accessing unregistered config item: '%s.%s'")
+            msg %= (section, name)
+            self.develwarn(msg, 2, 'warn-config-unknown')
 
         if default is _unset:
             if item is None:

--- a/tests/test-devel-warnings.t	Tue Oct 17 21:15:31 2017 +0200
+++ b/tests/test-devel-warnings.t	Mon Oct 16 17:41:27 2017 +0200
@@ -363,6 +363,8 @@
   >     repo.ui.config('test', 'some', 'foo')
   >     repo.ui.config('test', 'dynamic', 'some-required-default')
   >     repo.ui.config('test', 'dynamic')
+  >     repo.ui.config('test', 'unregistered')
+  >     repo.ui.config('unregistered', 'unregistered')
   > EOF
 
   $ hg --config "extensions.buggyconfig=${TESTTMP}/buggyconfig.py" buggyconfig
@@ -372,5 +374,7 @@
   devel-warn: specifying a default value for a registered config item: 'ui.interactive' 'None' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
   devel-warn: specifying a default value for a registered config item: 'test.some' 'foo' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
   devel-warn: config item requires an explicit default value: 'test.dynamic' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
+  devel-warn: accessing unregistered config item: 'test.unregistered' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
+  devel-warn: accessing unregistered config item: 'unregistered.unregistered' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
 
   $ cd ..

--- a/tests/test-trusted.py	Tue Oct 17 21:15:31 2017 +0200
+++ b/tests/test-trusted.py	Mon Oct 16 17:41:27 2017 +0200
@@ -67,6 +67,13 @@
                                      trusted))
 
     u = uimod.ui.load()
+    # disable the configuration registration warning
+    #
+    # the purpose of this test is to check the old behavior, not to validate the
+    # behavior from registered item. so we silent warning related to unregisted
+    # config.
+    u.setconfig('devel', 'warn-config-unknown', False, 'test')
+    u.setconfig('devel', 'all-warnings', False, 'test')
     u.setconfig('ui', 'debug', str(bool(debug)))
     u.setconfig('ui', 'report_untrusted', str(bool(report)))
     u.readconfig('.hg/hgrc')
@@ -157,6 +164,13 @@
 print()
 print("# read trusted, untrusted, new ui, trusted")
 u = uimod.ui.load()
+# disable the configuration registration warning
+#
+# the purpose of this test is to check the old behavior, not to validate the
+# behavior from registered item. so we silent warning related to unregisted
+# config.
+u.setconfig('devel', 'warn-config-unknown', False, 'test')
+u.setconfig('devel', 'all-warnings', False, 'test')
 u.setconfig('ui', 'debug', 'on')
 u.readconfig(filename)
 u2 = u.copy()

--- a/tests/test-ui-config.py	Tue Oct 17 21:15:31 2017 +0200
+++ b/tests/test-ui-config.py	Mon Oct 16 17:41:27 2017 +0200
@@ -6,6 +6,15 @@
 )
 
 testui = uimod.ui.load()
+
+# disable the configuration registration warning
+#
+# the purpose of this test is to check the old behavior, not to validate the
+# behavior from registered item. so we silent warning related to unregisted
+# config.
+testui.setconfig('devel', 'warn-config-unknown', False, 'test')
+testui.setconfig('devel', 'all-warnings', False, 'test')
+
 parsed = dispatch._parseconfig(testui, [
     'values.string=string value',
     'values.bool1=true',