Function tp_traverse 

#[doc(hidden)]
pub unsafe extern "C" fn tp_traverse<T>(
    slf: *mut PyObject,
    visit: visitproc,
    arg: *mut c_void,
) -> c_int
where
    T: PyClass,

Calls an implementation of traverse for tp_traverse

NB cannot accept 'static visitor, this is a sanity check below:

use pyo3::prelude::*;
use pyo3::pyclass::{PyTraverseError, PyVisit};

#[pyclass]
struct Foo;

#[pymethods]
impl Foo {
    fn __traverse__(&self, _visit: PyVisit<'static>) -> Result<(), PyTraverseError> {
        Ok(())
    }
}

Elided lifetime should compile ok:

use pyo3::prelude::*;
use pyo3::pyclass::{PyTraverseError, PyVisit};

#[pyclass]
struct Foo;

#[pymethods]
impl Foo {
    fn __traverse__(&self, _visit: PyVisit<'_>) -> Result<(), PyTraverseError> {
        Ok(())
    }
}