diff -r ccab61d84ea7 -r 8c7d8d5e1e0f mercurial/commands.py
--- a/mercurial/commands.py	Thu Sep 24 10:15:37 2015 +0300
+++ b/mercurial/commands.py	Thu Sep 10 16:53:07 2015 +0200
@@ -19,7 +19,7 @@
 import merge as mergemod
 import minirst, revset, fileset
 import dagparser, context, simplemerge, graphmod, copies
-import random
+import random, operator
 import setdiscovery, treediscovery, dagutil, pvec, localrepo
 import phases, obsolete, exchange, bundle2, repair, lock as lockmod
 import ui as uimod
@@ -2171,6 +2171,45 @@
         localrevs = opts.get('local_head')
         doit(localrevs, remoterevs)
 
+@command('debugextensions', formatteropts, [], norepo=True)
+def debugextensions(ui, **opts):
+    '''show information about active extensions'''
+    exts = extensions.extensions(ui)
+    fm = ui.formatter('debugextensions', opts)
+    for extname, extmod in sorted(exts, key=operator.itemgetter(0)):
+        extsource = extmod.__file__
+        exttestedwith = getattr(extmod, 'testedwith', None)
+        if exttestedwith is not None:
+            exttestedwith = exttestedwith.split()
+        extbuglink = getattr(extmod, 'buglink', None)
+
+        fm.startitem()
+
+        if ui.quiet or ui.verbose:
+            fm.write('name', '%s\n', extname)
+        else:
+            fm.write('name', '%s', extname)
+            if not exttestedwith:
+                fm.plain(_(' (untested!)\n'))
+            else:
+                if exttestedwith == ['internal'] or \
+                                util.version() in exttestedwith:
+                    fm.plain('\n')
+                else:
+                    lasttestedversion = exttestedwith[-1]
+                    fm.plain(' (%s!)\n' % lasttestedversion)
+
+        fm.condwrite(ui.verbose and extsource, 'source',
+                 _('  location: %s\n'), extsource or "")
+
+        fm.condwrite(ui.verbose and exttestedwith, 'testedwith',
+                 _('  tested with: %s\n'), ' '.join(exttestedwith or []))
+
+        fm.condwrite(ui.verbose and extbuglink, 'buglink',
+                 _('  bug reporting: %s\n'), extbuglink or "")
+
+    fm.end()
+
 @command('debugfileset',
     [('r', 'rev', '', _('apply the filespec on this revision'), _('REV'))],
     _('[-r REV] FILESPEC'))