--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-rust-discovery.py	Wed Feb 20 09:04:39 2019 +0100
@@ -0,0 +1,103 @@
+from __future__ import absolute_import
+import unittest
+
+try:
+    from mercurial import rustext
+    rustext.__name__  # trigger immediate actual import
+except ImportError:
+    rustext = None
+else:
+    # this would fail already without appropriate ancestor.__package__
+    from mercurial.rustext.discovery import (
+        PartialDiscovery,
+    )
+
+try:
+    from mercurial.cext import parsers as cparsers
+except ImportError:
+    cparsers = None
+
+# picked from test-parse-index2, copied rather than imported
+# so that it stays stable even if test-parse-index2 changes or disappears.
+data_non_inlined = (
+    b'\x00\x00\x00\x01\x00\x00\x00\x00\x00\x01D\x19'
+    b'\x00\x07e\x12\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\xff\xff'
+    b'\xff\xff\xff\xff\xd1\xf4\xbb\xb0\xbe\xfc\x13\xbd\x8c\xd3\x9d'
+    b'\x0f\xcd\xd9;\x8c\x07\x8cJ/\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+    b'\x00\x00\x00\x00\x00\x00\x01D\x19\x00\x00\x00\x00\x00\xdf\x00'
+    b'\x00\x01q\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\x00\xff'
+    b'\xff\xff\xff\xc1\x12\xb9\x04\x96\xa4Z1t\x91\xdfsJ\x90\xf0\x9bh'
+    b'\x07l&\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+    b'\x00\x01D\xf8\x00\x00\x00\x00\x01\x1b\x00\x00\x01\xb8\x00\x00'
+    b'\x00\x01\x00\x00\x00\x02\x00\x00\x00\x01\xff\xff\xff\xff\x02\n'
+    b'\x0e\xc6&\xa1\x92\xae6\x0b\x02i\xfe-\xe5\xbao\x05\xd1\xe7\x00'
+    b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01F'
+    b'\x13\x00\x00\x00\x00\x01\xec\x00\x00\x03\x06\x00\x00\x00\x01'
+    b'\x00\x00\x00\x03\x00\x00\x00\x02\xff\xff\xff\xff\x12\xcb\xeby1'
+    b'\xb6\r\x98B\xcb\x07\xbd`\x8f\x92\xd9\xc4\x84\xbdK\x00\x00\x00'
+    b'\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+    )
+
+
+@unittest.skipIf(rustext is None or cparsers is None,
+                 "rustext or the C Extension parsers module "
+                 "discovery relies on is not available")
+class rustdiscoverytest(unittest.TestCase):
+    """Test the correctness of binding to Rust code.
+
+    This test is merely for the binding to Rust itself: extraction of
+    Python variable, giving back the results etc.
+
+    It is not meant to test the algorithmic correctness of the provided
+    methods. Hence the very simple embedded index data is good enough.
+
+    Algorithmic correctness is asserted by the Rust unit tests.
+    """
+
+    def parseindex(self):
+        return cparsers.parse_index2(data_non_inlined, False)[0]
+
+    def testindex(self):
+        idx = self.parseindex()
+        # checking our assumptions about the index binary data:
+        self.assertEqual({i: (r[5], r[6]) for i, r in enumerate(idx)},
+                         {0: (-1, -1),
+                          1: (0, -1),
+                          2: (1, -1),
+                          3: (2, -1)})
+
+    def testaddcommonsmissings(self):
+        idx = self.parseindex()
+        disco = PartialDiscovery(idx, [3])
+        self.assertFalse(disco.hasinfo())
+        self.assertFalse(disco.iscomplete())
+
+        disco.addcommons([1])
+        self.assertTrue(disco.hasinfo())
+        self.assertFalse(disco.iscomplete())
+
+        disco.addmissings([2])
+        self.assertTrue(disco.hasinfo())
+        self.assertTrue(disco.iscomplete())
+
+        self.assertEqual(disco.commonheads(), {1})
+
+    def testaddinfocommonfirst(self):
+        idx = self.parseindex()
+        disco = PartialDiscovery(idx, [3])
+        disco.addinfo([(1, True), (2, False)])
+        self.assertTrue(disco.hasinfo())
+        self.assertTrue(disco.iscomplete())
+        self.assertEqual(disco.commonheads(), {1})
+
+    def testaddinfomissingfirst(self):
+        idx = self.parseindex()
+        disco = PartialDiscovery(idx, [3])
+        disco.addinfo([(2, False), (1, True)])
+        self.assertTrue(disco.hasinfo())
+        self.assertTrue(disco.iscomplete())
+        self.assertEqual(disco.commonheads(), {1})
+
+if __name__ == '__main__':
+    import silenttestrunner
+    silenttestrunner.main(__name__)