Add a CI build that uses mypyc

.travis.yml

@@ -10,6 +10,7 @@ dist: xenial
 env:
   TOXENV=py
   EXTRA_ARGS="-n 12"
+  TEST_MYPYC=0
 
 jobs:
   include:
@@ -34,6 +35,12 @@ jobs:
     - TOXENV=py
     - EXTRA_ARGS="-n 12"
     - NEWSEMANAL=1
+  - name: "run test suite with python 3.7 (compiled with mypyc)"
+    python: 3.7
+    env:
+    - TOXENV=py
+    - EXTRA_ARGS="-n 12"
+    - TEST_MYPYC=1
   - name: "run test suite with python 3.8-dev"
     python: 3.8-dev
   - name: "type check our own code"
@@ -64,6 +71,11 @@ install:
 - pip install -U pip setuptools
 - pip install -U tox==3.9.0
 - tox --notest
+# This is a big hack and only works because the layout of our directories
+# means that tox picks up the mypy from the source directories instead of
+# the version it installed into a venv. This is also *why* we need to do this,
+# since if we arranged for tox to build with mypyc, pytest wouldn't use it.
+- if [[ $TEST_MYPYC == 1 ]]; then pip install -r mypyc-requirements.txt; CC=clang MYPYC_OPT_LEVEL=0 python3 setup.py --use-mypyc build_ext --inplace; fi
 
 script:
 - tox -- $EXTRA_ARGS

mypy/test/testmypyc.py

@@ -0,0 +1,12 @@
+"""A basic check to make sure that we are using a mypyc-compiled version when expected."""
+
+import mypy
+
+from unittest import TestCase
+import os
+
+
+class MypycTest(TestCase):
+    def test_using_mypyc(self) -> None:
+        if os.getenv('TEST_MYPYC', None) == '1':
+            assert not mypy.__file__.endswith('.py'), "Expected to find a mypyc-compiled version"