Hack __text_signature__ back to working with abi3

src/pyclass.rs

@@ -256,6 +256,22 @@ fn tp_init_additional<T: PyClass>(type_object: *mut ffi::PyTypeObject) {
     // Just patch the type objects for the things there's no
     // PyType_FromSpec API for... there's no reason this should work,
     // except for that it does and we have tests.
+
+    // Running this causes PyPy to segfault.
+    #[cfg(not(PyPy))]
+    if T::DESCRIPTION != "\0" {
+        unsafe {
+            // Until CPython 3.10, tp_doc was treated specially for heap-types,
+            // and it removed the text_signature value from it. We go in after
+            // the fact and replace tp_doc with something that _does_ include
+            // the text_signature value!
+            ffi::PyObject_Free((*type_object).tp_doc as _);
+            let data = ffi::PyObject_Malloc(T::DESCRIPTION.len());
+            data.copy_from(T::DESCRIPTION.as_ptr() as _, T::DESCRIPTION.len());
+            (*type_object).tp_doc = data as _;
+        }
+    }
+
     if let Some(buffer) = T::buffer_methods() {
         unsafe {
             (*(*type_object).tp_as_buffer).bf_getbuffer = buffer.as_ref().bf_getbuffer;

tests/test_text_signature.rs

@@ -31,10 +31,8 @@ fn class_with_docs() {
     py_assert!(py, typeobj, "typeobj.__text_signature__ is None");
 }
 
-// Ignored because heap types don't have working __text_signature__:
-// https://github.com/python/cpython/blob/master/Objects/typeobject.c#L864-L870
 #[test]
-#[ignore]
+#[cfg_attr(Py_LIMITED_API, ignore)]
 fn class_with_docs_and_signature() {
     /// docs line1
     #[pyclass]
@@ -69,10 +67,8 @@ fn class_with_docs_and_signature() {
     );
 }
 
-// Ignored because heap types don't have working __text_signature__:
-// https://github.com/python/cpython/blob/master/Objects/typeobject.c#L864-L870
 #[test]
-#[ignore]
+#[cfg_attr(Py_LIMITED_API, ignore)]
 fn class_with_signature() {
     #[pyclass]
     #[text_signature = "(a, b=None, *, c=42)"]
@@ -92,7 +88,11 @@ fn class_with_signature() {
     let py = gil.python();
     let typeobj = py.get_type::<MyClass>();
 
-    py_assert!(py, typeobj, "typeobj.__doc__ is None");
+    py_assert!(
+        py,
+        typeobj,
+        "typeobj.__doc__ is None or typeobj.__doc__ == ''"
+    );
     py_assert!(
         py,
         typeobj,