Give a better error message for Python in traverse

pyo3-macros-backend/src/pymethod.rs

@@ -210,7 +210,7 @@ pub fn gen_py_method(
                     GeneratedPyMethod::Proto(impl_call_slot(cls, method.spec)?)
                 }
                 PyMethodProtoKind::Traverse => {
-                    GeneratedPyMethod::Proto(impl_traverse_slot(cls, spec.name))
+                    GeneratedPyMethod::Proto(impl_traverse_slot(cls, spec)?)
                 }
                 PyMethodProtoKind::SlotFragment(slot_fragment_def) => {
                     let proto = slot_fragment_def.generate_pyproto_fragment(cls, spec)?;
@@ -404,7 +404,16 @@ fn impl_call_slot(cls: &syn::Type, mut spec: FnSpec<'_>) -> Result<MethodAndSlot
     })
 }
 
-fn impl_traverse_slot(cls: &syn::Type, rust_fn_ident: &syn::Ident) -> MethodAndSlotDef {
+fn impl_traverse_slot(cls: &syn::Type, spec: &FnSpec<'_>) -> syn::Result<MethodAndSlotDef> {
+    if let (Some(py_arg), _) = split_off_python_arg(&spec.signature.arguments) {
+        return Err(syn::Error::new_spanned(py_arg.ty, "__traverse__ may not take `Python`. \
+            Usually, an implementation of `__traverse__` should do nothing but calls to `visit.call`. \
+            Most importantly, safe access to the GIL is prohibited inside implementations of `__traverse__`, \
+            i.e. `Python::with_gil` will panic."));
+    }
+
+    let rust_fn_ident = spec.name;
+
     let associated_method = quote! {
         pub unsafe extern "C" fn __pymethod_traverse__(
             slf: *mut _pyo3::ffi::PyObject,
@@ -420,10 +429,10 @@ fn impl_traverse_slot(cls: &syn::Type, rust_fn_ident: &syn::Ident) -> MethodAndS
             pfunc: #cls::__pymethod_traverse__ as _pyo3::ffi::traverseproc as _
         }
     };
-    MethodAndSlotDef {
+    Ok(MethodAndSlotDef {
         associated_method,
         slot_def,
-    }
+    })
 }
 
 fn impl_py_class_attribute(cls: &syn::Type, spec: &FnSpec<'_>) -> syn::Result<MethodAndMethodDef> {

tests/test_compile_error.rs

@@ -36,5 +36,5 @@ fn test_compile_errors() {
     t.compile_fail("tests/ui/not_send.rs");
     t.compile_fail("tests/ui/not_send2.rs");
     t.compile_fail("tests/ui/get_set_all.rs");
-    t.compile_fail("tests/ui/traverse_bare_self.rs");
+    t.compile_fail("tests/ui/traverse.rs");
 }

tests/ui/traverse.rs

@@ -0,0 +1,27 @@
+use pyo3::prelude::*;
+use pyo3::PyVisit;
+use pyo3::PyTraverseError;
+
+#[pyclass]
+struct TraverseTriesToTakePyRef {}
+
+#[pymethods]
+impl TraverseTriesToTakePyRef {
+    fn __traverse__(slf: PyRef<Self>, visit: PyVisit) {}
+}
+
+#[pyclass]
+struct Class;
+
+#[pymethods]
+impl Class {
+    fn __traverse__(&self, py: Python<'_>, visit: PyVisit<'_>) -> Result<(), PyTraverseError> {
+        Ok(())
+    }
+
+    fn __clear__(&mut self) {
+    }
+}
+
+
+fn main() {}

tests/ui/traverse.stderr

@@ -0,0 +1,23 @@
+error: __traverse__ may not take `Python`. Usually, an implementation of `__traverse__` should do nothing but calls to `visit.call`. Most importantly, safe access to the GIL is prohibited inside implementations of `__traverse__`, i.e. `Python::with_gil` will panic.
+  --> tests/ui/traverse.rs:18:32
+   |
+18 |     fn __traverse__(&self, py: Python<'_>, visit: PyVisit<'_>) -> Result<(), PyTraverseError> {
+   |                                ^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> tests/ui/traverse.rs:9:6
+   |
+8  |   #[pymethods]
+   |   ------------ arguments to this function are incorrect
+9  |   impl TraverseTriesToTakePyRef {
+   |  ______^
+10 | |     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>(
+   |               ^^^^^^^^^^^^^^^^^^

tests/ui/traverse_bare_self.rs

@@ -1,12 +0,0 @@
-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

@@ -1,17 +0,0 @@
-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>(
-  |               ^^^^^^^^^^^^^^^^^^