fix: do not set docstring for function when empty (#2745)

* Do not set docstring for function when it's empty

* No need to check pointer for `free`

* Use ternary operator to conditionally set `ml_doc`

include/pybind11/pybind11.h

@@ -439,9 +439,9 @@ protected:
 
         /* Install docstring */
         auto *func = (PyCFunctionObject *) m_ptr;
-        if (func->m_ml->ml_doc)
-            std::free(const_cast<char *>(func->m_ml->ml_doc));
-        func->m_ml->ml_doc = strdup(signatures.c_str());
+        std::free(const_cast<char *>(func->m_ml->ml_doc));
+        // Install docstring if it's non-empty (when at least one option is enabled)
+        func->m_ml->ml_doc = signatures.empty() ? nullptr : strdup(signatures.c_str());
 
         if (rec->is_method) {
             m_ptr = PYBIND11_INSTANCE_METHOD_NEW(m_ptr, rec->scope.ptr());

tests/test_docstring_options.cpp

@@ -48,6 +48,14 @@ TEST_SUBMODULE(docstring_options, m) {
     {
         py::options options;
         options.disable_user_defined_docstrings();
+        options.disable_function_signatures();
+
+        m.def("test_function8", []() {});
+    }
+
+    {
+        py::options options;
+        options.disable_user_defined_docstrings();
 
         struct DocstringTestFoo {
             int value;

tests/test_docstring_options.py

@@ -34,6 +34,9 @@ def test_docstring_options():
     assert m.test_function7.__doc__.startswith("test_function7(a: int, b: int) -> None")
     assert m.test_function7.__doc__.endswith("A custom docstring\n")
 
+    # when all options are disabled, no docstring (instead of an empty one) should be generated
+    assert m.test_function8.__doc__ is None
+
     # Suppression of user-defined docstrings for non-function objects
     assert not m.DocstringTestFoo.__doc__
     assert not m.DocstringTestFoo.value_prop.__doc__