Add UI test to ensure only bare &self can be passed into traverse.

tests/test_compile_error.rs

@@ -45,6 +45,7 @@ fn _test_compile_errors() {
     t.compile_fail("tests/ui/invalid_pymethod_names.rs");
     t.compile_fail("tests/ui/invalid_pymodule_args.rs");
     t.compile_fail("tests/ui/reject_generics.rs");
+    t.compile_fail("tests/ui/traverse_bare_self.rs");
 
     tests_rust_1_49(&t);
     tests_rust_1_56(&t);

tests/ui/traverse_bare_self.rs

@@ -0,0 +1,12 @@
+use pyo3::prelude::*;
+use pyo3::PyVisit;
+
+#[pyclass]
+struct TraverseTriesToTakePyRef {}
+
+#[pymethods]
+impl TraverseTriesToTakePyRef {
+    fn __traverse__(slf: PyRef<Self>, visit: PyVisit) {}
+}
+
+fn main() {}

tests/ui/traverse_bare_self.stderr

@@ -0,0 +1,17 @@
+error[E0308]: mismatched types
+ --> tests/ui/traverse_bare_self.rs:8:6
+  |
+7 |   #[pymethods]
+  |   ------------ arguments to this function are incorrect
+8 |   impl TraverseTriesToTakePyRef {
+  |  ______^
+9 | |     fn __traverse__(slf: PyRef<Self>, visit: PyVisit) {}
+  | |___________________^ expected fn pointer, found fn item
+  |
+  = note: expected fn pointer `for<'a, 'b> fn(&'a TraverseTriesToTakePyRef, PyVisit<'b>) -> Result<(), PyTraverseError>`
+                found fn item `for<'a, 'b> fn(pyo3::PyRef<'a, TraverseTriesToTakePyRef>, PyVisit<'b>) {TraverseTriesToTakePyRef::__traverse__}`
+note: function defined here
+ --> src/impl_/pymethods.rs
+  |
+  | pub unsafe fn call_traverse_impl<T>(
+  |               ^^^^^^^^^^^^^^^^^^