add a way to disable assertion rewriting for a module

_pytest/assertrewrite.py

@@ -98,7 +98,11 @@ class AssertionRewriter(ast.NodeVisitor):
         for item in mod.body:
             if (expect_docstring and isinstance(item, ast.Expr) and
                 isinstance(item.value, ast.Str)):
-                lineno += len(item.value.s.splitlines()) - 1
+                doc = item.value.s
+                if "PYTEST_DONT_REWRITE" in doc:
+                    # The module has disabled assertion rewriting.
+                    return
+                lineno += len(doc) - 1
                 expect_docstring = False
             elif (not isinstance(item, ast.ImportFrom) or item.level > 0 and
                   item.identifier != "__future__"):

testing/test_assertrewrite.py

@@ -76,6 +76,14 @@ class TestAssertionRewrite:
             assert imp.col_offset == 0
         assert isinstance(m.body[5], ast.Expr)
 
+    def test_dont_rewrite(self):
+        s = """'PYTEST_DONT_REWRITE'\nassert 14"""
+        m = rewrite(s)
+        assert len(m.body) == 2
+        assert isinstance(m.body[0].value, ast.Str)
+        assert isinstance(m.body[1], ast.Assert)
+        assert m.body[1].msg is None
+
     def test_name(self):
         def f():
             assert False