improve error message

pyo3-macros-backend/src/pymethod.rs

@@ -434,19 +434,21 @@ fn impl_traverse_slot(
     let Ctx { pyo3_path } = ctx;
     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."));
+            Usually, an implementation of `__traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError>` \
+            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."));
     }
 
+    // check that the receiver does not try to smuggle an (implicit) `Python` token into here
     if let FnType::Fn(SelfType::TryFromBoundRef(span))
     | FnType::Fn(SelfType::Receiver {
         mutable: true,
         span,
     }) = spec.tp
     {
         bail_spanned! { span =>
-            "__traverse__ may not take a receiver other than `&self`. Usually, an implementation of `__traverse__` \
+            "__traverse__ may not take a receiver other than `&self`. Usually, an implementation of \
+            `__traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError>` \
             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.stderr

@@ -1,28 +1,28 @@
-error: __traverse__ may not take a receiver other than `&self`. 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.
+error: __traverse__ may not take a receiver other than `&self`. Usually, an implementation of `__traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError>` 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:10:26
    |
 10 |     fn __traverse__(slf: PyRef<Self>, visit: PyVisit) -> Result<(), PyTraverseError> {
    |                          ^^^^^
 
-error: __traverse__ may not take a receiver other than `&self`. 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.
+error: __traverse__ may not take a receiver other than `&self`. Usually, an implementation of `__traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError>` 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:20:26
    |
 20 |     fn __traverse__(slf: PyRefMut<Self>, visit: PyVisit) -> Result<(), PyTraverseError> {
    |                          ^^^^^^^^
 
-error: __traverse__ may not take a receiver other than `&self`. 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.
+error: __traverse__ may not take a receiver other than `&self`. Usually, an implementation of `__traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError>` 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:30:26
    |
 30 |     fn __traverse__(slf: Bound<'_, Self>, visit: PyVisit) -> Result<(), PyTraverseError> {
    |                          ^^^^^
 
-error: __traverse__ may not take a receiver other than `&self`. 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.
+error: __traverse__ may not take a receiver other than `&self`. Usually, an implementation of `__traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError>` 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:40:21
    |
 40 |     fn __traverse__(&mut self, visit: PyVisit) -> Result<(), PyTraverseError> {
    |                     ^
 
-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.
+error: __traverse__ may not take `Python`. Usually, an implementation of `__traverse__(&self, visit: PyVisit<'_>) -> Result<(), PyTraverseError>` 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:60:32
    |
 60 |     fn __traverse__(&self, py: Python<'_>, visit: PyVisit<'_>) -> Result<(), PyTraverseError> {