pyo3-ffi: deprecate _PyLong_NumBits, use bit_length() for Python < 3.13

Author: codeguru42

newsfragments/10489.changed.md

@@ -1,2 +1,3 @@
 Removed use of the CPython-internal `_PyUnicode_COMPACT_DATA` symbol from PyO3's FFI test suite.
 Deprecated `_PySet_NextEntry` in `pyo3-ffi`; it is a CPython-internal API with no stability guarantee.
+Deprecated `_PyLong_NumBits` in `pyo3-ffi` and replaced its internal use with `int.bit_length()` for Python < 3.13 builds.

pyo3-ffi/src/longobject.rs

@@ -85,6 +85,7 @@ extern_libpython! {
 #[cfg(not(Py_LIMITED_API))]
 extern_libpython! {
     #[cfg_attr(PyPy, link_name = "_PyPyLong_NumBits")]
+    #[deprecated(note = "`_PyLong_NumBits` is a CPython-internal API; use `PyLong_AsNativeBytes` on Python 3.13+ or `int.bit_length()` instead")]
     pub fn _PyLong_NumBits(obj: *mut PyObject) -> size_t;
 }
 

src/conversions/num_bigint.rs

@@ -318,24 +318,9 @@ fn int_to_py_bytes<'py>(
 #[inline]
 #[cfg(any(not(Py_3_13), Py_LIMITED_API))]
 fn int_n_bits(long: &Bound<'_, PyInt>) -> PyResult<usize> {
-    let py = long.py();
-    #[cfg(not(Py_LIMITED_API))]
-    {
-        // fast path
-        let n_bits = unsafe { crate::ffi::_PyLong_NumBits(long.as_ptr()) };
-        if n_bits == (-1isize as usize) {
-            return Err(crate::PyErr::fetch(py));
-        }
-        Ok(n_bits)
-    }
-
-    #[cfg(Py_LIMITED_API)]
-    {
-        // slow path
-        use crate::types::PyAnyMethods;
-        long.call_method0(crate::intern!(py, "bit_length"))
-            .and_then(|any| any.extract())
-    }
+    use crate::types::PyAnyMethods;
+    long.call_method0(crate::intern!(long.py(), "bit_length"))
+        .and_then(|any| any.extract())
 }
 
 #[cfg(test)]