Fix assertion rewrite to match module names correctly

Fix #2939

_pytest/assertion/rewrite.py

@@ -168,7 +168,7 @@ class AssertionRewritingHook(object):
                 return True
 
         for marked in self._must_rewrite:
-            if name.startswith(marked):
+            if name == marked or name.startswith(marked + '.'):
                 state.trace("matched marked file %r (from %r)" % (name, marked))
                 return True
 

changelog/2939.bugfix

@@ -0,0 +1 @@
+Fix issue in assertion rewriting which could lead it to rewrite modules which should not be rewritten.

testing/test_assertion.py

@@ -129,6 +129,24 @@ class TestImportHookInstallation(object):
         result = testdir.runpytest_subprocess('--assert=rewrite')
         assert result.ret == 0
 
+    def test_pytest_plugins_rewrite_module_names_correctly(self, testdir):
+        """Test that we match files correctly when they are marked for rewriting (#2939)."""
+        contents = {
+            'conftest.py': """
+                pytest_plugins = "ham"
+            """,
+            'ham.py': "",
+            'hamster.py': "",
+            'test_foo.py': """
+                def test_foo(pytestconfig):
+                    assert pytestconfig.pluginmanager.rewrite_hook.find_module('ham') is not None
+                    assert pytestconfig.pluginmanager.rewrite_hook.find_module('hamster') is None
+            """,
+        }
+        testdir.makepyfile(**contents)
+        result = testdir.runpytest_subprocess('--assert=rewrite')
+        assert result.ret == 0
+
     @pytest.mark.parametrize('mode', ['plain', 'rewrite'])
     @pytest.mark.parametrize('plugin_state', ['development', 'installed'])
     def test_installed_plugin_rewrite(self, testdir, mode, plugin_state):