Now dependent on command line option.

src/_pytest/assertion/__init__.py

@@ -24,6 +24,14 @@ def pytest_addoption(parser):
                             expression information.""",
     )
 
+    group = parser.getgroup("experimental")
+    group.addoption(
+        "--enable-assertion-pass-hook",
+        action="store_true",
+        help="Enables the pytest_assertion_pass hook."
+        "Make sure to delete any previously generated pyc cache files.",
+    )
+
 
 def register_assert_rewrite(*names):
     """Register one or more module names to be rewritten on import.

src/_pytest/assertion/rewrite.py

@@ -745,6 +745,7 @@ class AssertionRewriter(ast.NodeVisitor):
         format_dict = ast.Dict(keys, list(current.values()))
         form = ast.BinOp(expl_expr, ast.Mod(), format_dict)
         name = "@py_format" + str(next(self.variable_counter))
+        if getattr(self.config._ns, "enable_assertion_pass_hook", False):
             self.format_variables.append(name)
         self.expl_stmts.append(ast.Assign([ast.Name(name, ast.Store())], form))
         return ast.Name(name, ast.Load())
@@ -780,7 +781,10 @@ class AssertionRewriter(ast.NodeVisitor):
         self.statements = []
         self.variables = []
         self.variable_counter = itertools.count()
+
+        if getattr(self.config._ns, "enable_assertion_pass_hook", False):
             self.format_variables = []
+
         self.stack = []
         self.expl_stmts = []
         self.push_format_context()
@@ -793,6 +797,8 @@ class AssertionRewriter(ast.NodeVisitor):
                     top_condition, module_path=self.module_path, lineno=assert_.lineno
                 )
             )
+        if getattr(self.config._ns, "enable_assertion_pass_hook", False):
+            ### Experimental pytest_assertion_pass hook
             negation = ast.UnaryOp(ast.Not(), top_condition)
             msg = self.pop_format_context(ast.Str(explanation))
             if assert_.msg:
@@ -812,7 +818,10 @@ class AssertionRewriter(ast.NodeVisitor):
             orig = astor.to_source(assert_.test).rstrip("\n").lstrip("(").rstrip(")")
             hook_call_pass = ast.Expr(
                 self.helper(
-                "_call_assertion_pass", ast.Num(assert_.lineno), ast.Str(orig), fmt_pass
+                    "_call_assertion_pass",
+                    ast.Num(assert_.lineno),
+                    ast.Str(orig),
+                    fmt_pass,
                 )
             )
 
@@ -825,9 +834,31 @@ class AssertionRewriter(ast.NodeVisitor):
             self.statements.extend(self.expl_stmts)
             self.statements.append(main_test)
             if self.format_variables:
-            variables = [ast.Name(name, ast.Store()) for name in self.format_variables]
+                variables = [
+                    ast.Name(name, ast.Store()) for name in self.format_variables
+                ]
                 clear_format = ast.Assign(variables, _NameConstant(None))
                 self.statements.append(clear_format)
+        else:
+            ### Original assertion rewriting
+            # Create failure message.
+            body = self.expl_stmts
+            negation = ast.UnaryOp(ast.Not(), top_condition)
+            self.statements.append(ast.If(negation, body, []))
+            if assert_.msg:
+                assertmsg = self.helper("_format_assertmsg", assert_.msg)
+                explanation = "\n>assert " + explanation
+            else:
+                assertmsg = ast.Str("")
+                explanation = "assert " + explanation
+            template = ast.BinOp(assertmsg, ast.Add(), ast.Str(explanation))
+            msg = self.pop_format_context(template)
+            fmt = self.helper("_format_explanation", msg)
+            err_name = ast.Name("AssertionError", ast.Load())
+            exc = ast.Call(err_name, [fmt], [])
+            raise_ = ast.Raise(exc, None)
+
+            body.append(raise_)
         # Clear temporary variables by setting them to None.
         if self.variables:
             variables = [ast.Name(name, ast.Store()) for name in self.variables]

src/_pytest/config/__init__.py

@@ -746,8 +746,10 @@ class Config:
         and find all the installed plugins to mark them for rewriting
         by the importhook.
         """
-        ns, unknown_args = self._parser.parse_known_and_unknown_args(args)
-        mode = getattr(ns, "assertmode", "plain")
+        # Saving _ns so it can be used for other assertion rewriting purposes
+        # e.g. experimental assertion pass hook
+        self._ns, self._unknown_args = self._parser.parse_known_and_unknown_args(args)
+        mode = getattr(self._ns, "assertmode", "plain")
         if mode == "rewrite":
             try:
                 hook = _pytest.assertion.install_importhook(self)

src/_pytest/hookspec.py

@@ -503,6 +503,8 @@ def pytest_assertion_pass(item, lineno, orig, expl):
     This hook is still *experimental*, so its parameters or even the hook itself might
     be changed/removed without warning in any future pytest release.
 
+    It should be enabled using the `--enable-assertion-pass-hook` command line option.
+
     If you find this hook useful, please share your feedback opening an issue.
     """
 

testing/test_assertrewrite.py

@@ -1326,8 +1326,7 @@ class TestAssertionPass:
                 assert a+b == c+d
             """
         )
-        result = testdir.runpytest()
-        print(testdir.tmpdir)
+        result = testdir.runpytest("--enable-assertion-pass-hook")
         result.stdout.fnmatch_lines(
             "*Assertion Passed: a + b == c + d (1 + 2) == (3 + 0) at line 7*"
         )
@@ -1343,6 +1342,38 @@ class TestAssertionPass:
             _pytest.assertion.rewrite, "_call_assertion_pass", raise_on_assertionpass
         )
 
+        testdir.makepyfile(
+            """
+            def test_simple():
+                a=1
+                b=2
+                c=3
+                d=0
+
+                assert a+b == c+d
+            """
+        )
+        result = testdir.runpytest("--enable-assertion-pass-hook")
+        result.assert_outcomes(passed=1)
+
+    def test_hook_not_called_without_cmd_option(self, testdir, monkeypatch):
+        """Assertion pass should not be called (and hence formatting should
+        not occur) if there is no hook declared for pytest_assertion_pass"""
+
+        def raise_on_assertionpass(*_, **__):
+            raise Exception("Assertion passed called when it shouldn't!")
+
+        monkeypatch.setattr(
+            _pytest.assertion.rewrite, "_call_assertion_pass", raise_on_assertionpass
+        )
+
+        testdir.makeconftest(
+            """
+            def pytest_assertion_pass(item, lineno, orig, expl):
+                raise Exception("Assertion Passed: {} {} at line {}".format(orig, expl, lineno))
+            """
+        )
+
         testdir.makepyfile(
             """
             def test_simple():