diff --git a/tests/hghave b/tests/hghave
--- a/tests/hghave
+++ b/tests/hghave
@@ -4,7 +4,7 @@ if all features are there, non-zero othe
 prefixed with "no-", the absence of feature is tested.
 """
 import optparse
-import sys
+import os, sys
 import hghave
 
 checks = hghave.checks
@@ -33,8 +33,30 @@ parser.add_option("--list-features", act
 parser.add_option("-q", "--quiet", action="store_true",
                   help="check features silently")
 
+def _loadaddon(quiet):
+    if 'TESTDIR' in os.environ:
+        # loading from '.' isn't needed, because `hghave` should be
+        # running at TESTTMP in this case
+        path = os.environ['TESTDIR']
+    else:
+        path = '.'
+
+    if not os.path.exists(os.path.join(path, 'hghaveaddon.py')):
+        return
+
+    sys.path.insert(0, path)
+    try:
+        import hghaveaddon
+    except BaseException, inst:
+        if not quiet:
+            sys.stderr.write('failed to import hghaveaddon.py from %r: %s\n'
+                             % (path, inst))
+        sys.exit(2)
+    sys.path.pop(0)
+
 if __name__ == '__main__':
     options, args = parser.parse_args()
+    _loadaddon(options.quiet)
     if options.list_features:
         list_features()
         sys.exit(0)
diff --git a/tests/test-hghave.t b/tests/test-hghave.t
--- a/tests/test-hghave.t
+++ b/tests/test-hghave.t
@@ -1,3 +1,39 @@
 Testing that hghave does not crash when checking features
 
   $ hghave --test-features 2>/dev/null
+
+Testing hghave extensibility for third party tools
+
+  $ cat > hghaveaddon.py <<EOF
+  > import hghave
+  > @hghave.check("custom", "custom hghave feature")
+  > def has_custom():
+  >     return True
+  > EOF
+
+(invocation via run-tests.py)
+
+  $ cat > test-hghaveaddon.t <<EOF
+  > #require custom
+  >   $ echo foo
+  >   foo
+  > EOF
+  $ run-tests.py test-hghaveaddon.t
+  .
+  # Ran 1 tests, 0 skipped, 0 warned, 0 failed.
+
+(invocation via command line)
+
+  $ unset TESTDIR
+  $ hghave custom
+
+(terminate with exit code 2 at failure of importing hghaveaddon.py)
+
+  $ rm hghaveaddon.*
+  $ cat > hghaveaddon.py <<EOF
+  > importing this file should cause syntax error
+  > EOF
+
+  $ hghave custom
+  failed to import hghaveaddon.py from '.': invalid syntax (hghaveaddon.py, line 1)
+  [2]