addl unit tests for PR #3970 (#3977)

* Add test_perf_accessors (to be merged into test_pytypes).

* Python < 3.8 f-string compatibility

* Use thread_local in inc_ref_counter()

* Intentional breakage, brute-force way to quickly find out how many platforms reach the PYBIND11_HANDLE_REF_DEBUG code, with and without threads.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Remove Intentional breakage

* Drop perf test, move inc_refs tests to test_pytypes

* Fold in PR #3970 with `#ifdef`s

* Complete test coverage for all newly added code.

* Condense new unit tests via a simple local helper macro.

* Remove PYBIND11_PR3970 define. See #3977 (comment)

* Move static keyword first (fixes silly oversight).

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

include/pybind11/pytypes.h

@@ -180,6 +180,10 @@ class object_api : public pyobject_tag {
 
 PYBIND11_NAMESPACE_END(detail)
 
+#if !defined(PYBIND11_HANDLE_REF_DEBUG) && !defined(NDEBUG)
+#    define PYBIND11_HANDLE_REF_DEBUG
+#endif
+
 /** \rst
     Holds a reference to a Python object (no reference counting)
 
@@ -209,6 +213,9 @@ class handle : public detail::object_api<handle> {
         this function automatically. Returns a reference to itself.
     \endrst */
     const handle &inc_ref() const & {
+#ifdef PYBIND11_HANDLE_REF_DEBUG
+        inc_ref_counter(1);
+#endif
         Py_XINCREF(m_ptr);
         return *this;
     }
@@ -244,6 +251,18 @@ class handle : public detail::object_api<handle> {
 
 protected:
     PyObject *m_ptr = nullptr;
+
+#ifdef PYBIND11_HANDLE_REF_DEBUG
+private:
+    static std::size_t inc_ref_counter(std::size_t add) {
+        thread_local std::size_t counter = 0;
+        counter += add;
+        return counter;
+    }
+
+public:
+    static std::size_t inc_ref_counter() { return inc_ref_counter(0); }
+#endif
 };
 
 /** \rst

tests/test_pytypes.cpp

@@ -297,6 +297,55 @@ TEST_SUBMODULE(pytypes, m) {
         return d;
     });
 
+    m.def("accessor_moves", []() { // See PR #3970
+        py::list return_list;
+#ifdef PYBIND11_HANDLE_REF_DEBUG
+        py::int_ py_int_0(0);
+        py::int_ py_int_42(42);
+        py::str py_str_count("count");
+
+        auto tup = py::make_tuple(0);
+
+        py::sequence seq(tup);
+
+        py::list lst;
+        lst.append(0);
+
+#    define PYBIND11_LOCAL_DEF(...)                                                               \
+        {                                                                                         \
+            std::size_t inc_refs = py::handle::inc_ref_counter();                                 \
+            __VA_ARGS__;                                                                          \
+            inc_refs = py::handle::inc_ref_counter() - inc_refs;                                  \
+            return_list.append(inc_refs);                                                         \
+        }
+
+        PYBIND11_LOCAL_DEF(tup[py_int_0])    // l-value (to have a control)
+        PYBIND11_LOCAL_DEF(tup[py::int_(0)]) // r-value
+
+        PYBIND11_LOCAL_DEF(tup.attr(py_str_count))     // l-value
+        PYBIND11_LOCAL_DEF(tup.attr(py::str("count"))) // r-value
+
+        PYBIND11_LOCAL_DEF(seq[py_int_0])    // l-value
+        PYBIND11_LOCAL_DEF(seq[py::int_(0)]) // r-value
+
+        PYBIND11_LOCAL_DEF(seq.attr(py_str_count))     // l-value
+        PYBIND11_LOCAL_DEF(seq.attr(py::str("count"))) // r-value
+
+        PYBIND11_LOCAL_DEF(lst[py_int_0])    // l-value
+        PYBIND11_LOCAL_DEF(lst[py::int_(0)]) // r-value
+
+        PYBIND11_LOCAL_DEF(lst.attr(py_str_count))     // l-value
+        PYBIND11_LOCAL_DEF(lst.attr(py::str("count"))) // r-value
+
+        auto lst_acc = lst[py::int_(0)];
+        lst_acc = py::int_(42);                    // Detaches lst_acc from lst.
+        PYBIND11_LOCAL_DEF(lst_acc = py_int_42)    // l-value
+        PYBIND11_LOCAL_DEF(lst_acc = py::int_(42)) // r-value
+#    undef PYBIND11_LOCAL_DEF
+#endif
+        return return_list;
+    });
+
     // test_constructors
     m.def("default_constructors", []() {
         return py::dict("bytes"_a = py::bytes(),

tests/test_pytypes.py

@@ -317,6 +317,15 @@ def func(self, x, *args):
     assert d["var"] == 99
 
 
+def test_accessor_moves():
+    inc_refs = m.accessor_moves()
+    if inc_refs:
+        # To be changed in PR #3970: [1, 0, 1, 0, ...]
+        assert inc_refs == [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
+    else:
+        pytest.skip("Not defined: PYBIND11_HANDLE_REF_DEBUG")
+
+
 def test_constructors():
     """C++ default and converting constructors are equivalent to type calls in Python"""
     types = [bytes, bytearray, str, bool, int, float, tuple, list, dict, set]