add `import_exception_bound!` macro (#4027)

* add `import_exception_bound!` macro

* newsfragment and tidy up

newsfragments/4027.added.md

@@ -0,0 +1 @@
+Add `import_exception_bound!` macro to import exception types without generating GIL Ref functionality for them.

src/exceptions.rs

@@ -29,18 +29,7 @@ macro_rules! impl_exception_boilerplate {
             }
         }
 
-        impl $name {
-            /// Creates a new [`PyErr`] of this type.
-            ///
-            /// [`PyErr`]: https://docs.rs/pyo3/latest/pyo3/struct.PyErr.html "PyErr in pyo3"
-            #[inline]
-            pub fn new_err<A>(args: A) -> $crate::PyErr
-            where
-                A: $crate::PyErrArguments + ::std::marker::Send + ::std::marker::Sync + 'static,
-            {
-                $crate::PyErr::new::<$name, A>(args)
-            }
-        }
+        $crate::impl_exception_boilerplate_bound!($name);
 
         impl ::std::error::Error for $name {
             fn source(&self) -> ::std::option::Option<&(dyn ::std::error::Error + 'static)> {
@@ -59,6 +48,25 @@ macro_rules! impl_exception_boilerplate {
     };
 }
 
+#[doc(hidden)]
+#[macro_export]
+macro_rules! impl_exception_boilerplate_bound {
+    ($name: ident) => {
+        impl $name {
+            /// Creates a new [`PyErr`] of this type.
+            ///
+            /// [`PyErr`]: https://docs.rs/pyo3/latest/pyo3/struct.PyErr.html "PyErr in pyo3"
+            #[inline]
+            pub fn new_err<A>(args: A) -> $crate::PyErr
+            where
+                A: $crate::PyErrArguments + ::std::marker::Send + ::std::marker::Sync + 'static,
+            {
+                $crate::PyErr::new::<$name, A>(args)
+            }
+        }
+    };
+}
+
 /// Defines a Rust type for an exception defined in Python code.
 ///
 /// # Syntax
@@ -105,34 +113,57 @@ macro_rules! import_exception {
 
         impl $name {
             fn type_object_raw(py: $crate::Python<'_>) -> *mut $crate::ffi::PyTypeObject {
-                use $crate::sync::GILOnceCell;
-                use $crate::prelude::PyTracebackMethods;
-                use $crate::prelude::PyAnyMethods;
-                static TYPE_OBJECT: GILOnceCell<$crate::Py<$crate::types::PyType>> =
-                    GILOnceCell::new();
+                use $crate::types::PyTypeMethods;
+                static TYPE_OBJECT: $crate::impl_::exceptions::ImportedExceptionTypeObject =
+                    $crate::impl_::exceptions::ImportedExceptionTypeObject::new(stringify!($module), stringify!($name));
+                TYPE_OBJECT.get(py).as_type_ptr()
+            }
+        }
+    };
+}
 
-                TYPE_OBJECT
-                    .get_or_init(py, || {
-                        let imp = py
-                            .import_bound(stringify!($module))
-                            .unwrap_or_else(|err| {
-                                let traceback = err
-                                    .traceback_bound(py)
-                                    .map(|tb| tb.format().expect("raised exception will have a traceback"))
-                                    .unwrap_or_default();
-                                ::std::panic!("Can not import module {}: {}\n{}", stringify!($module), err, traceback);
-                            });
-                        let cls = imp.getattr(stringify!($name)).expect(concat!(
-                            "Can not load exception class: ",
-                            stringify!($module),
-                            ".",
-                            stringify!($name)
-                        ));
+/// Variant of [`import_exception`](crate::import_exception) that does not emit code needed to
+/// use the imported exception type as a GIL Ref.
+///
+/// This is useful only during migration as a way to avoid generating needless code.
+#[macro_export]
+macro_rules! import_exception_bound {
+    ($module: expr, $name: ident) => {
+        /// A Rust type representing an exception defined in Python code.
+        ///
+        /// This type was created by the [`pyo3::import_exception_bound!`] macro - see its documentation
+        /// for more information.
+        ///
+        /// [`pyo3::import_exception_bound!`]: https://docs.rs/pyo3/latest/pyo3/macro.import_exception.html "import_exception in pyo3"
+        #[repr(transparent)]
+        #[allow(non_camel_case_types)] // E.g. `socket.herror`
+        pub struct $name($crate::PyAny);
 
-                        cls.extract()
-                            .expect("Imported exception should be a type object")
-                    })
-                    .as_ptr() as *mut _
+        $crate::impl_exception_boilerplate_bound!($name);
+
+        // FIXME remove this: was necessary while `PyTypeInfo` requires `HasPyGilRef`,
+        // should change in 0.22.
+        unsafe impl $crate::type_object::HasPyGilRef for $name {
+            type AsRefTarget = $crate::PyAny;
+        }
+
+        $crate::pyobject_native_type_info!(
+            $name,
+            $name::type_object_raw,
+            ::std::option::Option::Some(stringify!($module))
+        );
+
+        impl $crate::types::DerefToPyAny for $name {}
+
+        impl $name {
+            fn type_object_raw(py: $crate::Python<'_>) -> *mut $crate::ffi::PyTypeObject {
+                use $crate::types::PyTypeMethods;
+                static TYPE_OBJECT: $crate::impl_::exceptions::ImportedExceptionTypeObject =
+                    $crate::impl_::exceptions::ImportedExceptionTypeObject::new(
+                        stringify!($module),
+                        stringify!($name),
+                    );
+                TYPE_OBJECT.get(py).as_type_ptr()
             }
         }
     };
@@ -849,8 +880,8 @@ mod tests {
     use crate::types::{IntoPyDict, PyDict};
     use crate::{PyErr, PyNativeType};
 
-    import_exception!(socket, gaierror);
-    import_exception!(email.errors, MessageError);
+    import_exception_bound!(socket, gaierror);
+    import_exception_bound!(email.errors, MessageError);
 
     #[test]
     fn test_check_exception() {

src/impl_.rs

@@ -9,6 +9,7 @@
 #[cfg(feature = "experimental-async")]
 pub mod coroutine;
 pub mod deprecations;
+pub mod exceptions;
 pub mod extract_argument;
 pub mod freelist;
 pub mod frompyobject;

src/impl_/exceptions.rs

@@ -0,0 +1,28 @@
+use crate::{sync::GILOnceCell, types::PyType, Bound, Py, Python};
+
+pub struct ImportedExceptionTypeObject {
+    imported_value: GILOnceCell<Py<PyType>>,
+    module: &'static str,
+    name: &'static str,
+}
+
+impl ImportedExceptionTypeObject {
+    pub const fn new(module: &'static str, name: &'static str) -> Self {
+        Self {
+            imported_value: GILOnceCell::new(),
+            module,
+            name,
+        }
+    }
+
+    pub fn get<'py>(&self, py: Python<'py>) -> &Bound<'py, PyType> {
+        self.imported_value
+            .get_or_try_init_type_ref(py, self.module, self.name)
+            .unwrap_or_else(|e| {
+                panic!(
+                    "failed to import exception {}.{}: {}",
+                    self.module, self.name, e
+                )
+            })
+    }
+}

src/sync.rs

@@ -201,13 +201,17 @@ impl GILOnceCell<Py<PyType>> {
     ///
     /// This is a shorthand method for `get_or_init` which imports the type from Python on init.
     pub(crate) fn get_or_try_init_type_ref<'py>(
-        &'py self,
+        &self,
         py: Python<'py>,
         module_name: &str,
         attr_name: &str,
     ) -> PyResult<&Bound<'py, PyType>> {
         self.get_or_try_init(py, || {
-            py.import_bound(module_name)?.getattr(attr_name)?.extract()
+            let type_object = py
+                .import_bound(module_name)?
+                .getattr(attr_name)?
+                .downcast_into()?;
+            Ok(type_object.unbind())
         })
         .map(|ty| ty.bind(py))
     }