allow `Bound<'_, T>` in #[pymethods] `self` position (#3896)

* allow `Bound<'_, T>` in #[pymethods] `self` position

* rename `TryFromPyCell` -> `TryFromBoundRef`

* remove unneccessary unsafe

pyo3-macros-backend/src/method.rs

@@ -151,7 +151,7 @@ impl FnType {
 #[derive(Clone, Debug)]
 pub enum SelfType {
     Receiver { mutable: bool, span: Span },
-    TryFromPyCell(Span),
+    TryFromBoundRef(Span),
 }
 
 #[derive(Clone, Copy)]
@@ -204,15 +204,14 @@ impl SelfType {
                     )
                 })
             }
-            SelfType::TryFromPyCell(span) => {
+            SelfType::TryFromBoundRef(span) => {
                 error_mode.handle_error(
                     quote_spanned! { *span =>
-                        #py.from_borrowed_ptr::<_pyo3::PyAny>(#slf).downcast::<_pyo3::PyCell<#cls>>()
+                        _pyo3::impl_::pymethods::BoundRef::ref_from_ptr(#py, &#slf).downcast::<#cls>()
                             .map_err(::std::convert::Into::<_pyo3::PyErr>::into)
                             .and_then(
-                                #[allow(clippy::useless_conversion)]  // In case slf is PyCell<Self>
                                 #[allow(unknown_lints, clippy::unnecessary_fallible_conversions)]  // In case slf is Py<Self> (unknown_lints can be removed when MSRV is 1.75+)
-                                |cell| ::std::convert::TryFrom::try_from(cell).map_err(::std::convert::Into::into)
+                                |bound| ::std::convert::TryFrom::try_from(bound).map_err(::std::convert::Into::into)
                             )
 
                     }
@@ -291,7 +290,7 @@ pub fn parse_method_receiver(arg: &syn::FnArg) -> Result<SelfType> {
             if let syn::Type::ImplTrait(_) = &**ty {
                 bail_spanned!(ty.span() => IMPL_TRAIT_ERR);
             }
-            Ok(SelfType::TryFromPyCell(ty.span()))
+            Ok(SelfType::TryFromBoundRef(ty.span()))
         }
     }
 }

pyo3-macros-backend/src/pyclass.rs

@@ -1188,7 +1188,7 @@ fn complex_enum_variant_field_getter<'a>(
 ) -> Result<MethodAndMethodDef> {
     let signature = crate::pyfunction::FunctionSignature::from_arguments(vec![])?;
 
-    let self_type = crate::method::SelfType::TryFromPyCell(field_span);
+    let self_type = crate::method::SelfType::TryFromBoundRef(field_span);
 
     let spec = FnSpec {
         tp: crate::method::FnType::Getter(self_type.clone()),

src/impl_/pymethods.rs

@@ -3,10 +3,12 @@ use crate::exceptions::PyStopAsyncIteration;
 use crate::gil::LockGIL;
 use crate::impl_::panic::PanicTrap;
 use crate::internal_tricks::extract_c_string;
+use crate::pycell::{PyBorrowError, PyBorrowMutError};
+use crate::pyclass::boolean_struct::False;
 use crate::types::{any::PyAnyMethods, PyModule, PyType};
 use crate::{
-    ffi, Bound, Py, PyAny, PyCell, PyClass, PyErr, PyObject, PyResult, PyTraverseError, PyVisit,
+    ffi, Bound, DowncastError, Py, PyAny, PyCell, PyClass, PyErr, PyObject, PyRef, PyRefMut,
-    Python,
+    PyResult, PyTraverseError, PyTypeCheck, PyVisit, Python,
 };
 use std::borrow::Cow;
 use std::ffi::CStr;
@@ -490,6 +492,10 @@ impl<'a, 'py> BoundRef<'a, 'py, PyAny> {
         Bound::ref_from_ptr_or_opt(py, ptr).as_ref().map(BoundRef)
     }
 
+    pub fn downcast<T: PyTypeCheck>(self) -> Result<BoundRef<'a, 'py, T>, DowncastError<'a, 'py>> {
+        self.0.downcast::<T>().map(BoundRef)
+    }
+
     pub unsafe fn downcast_unchecked<T>(self) -> BoundRef<'a, 'py, T> {
         BoundRef(self.0.downcast_unchecked::<T>())
     }
@@ -511,6 +517,36 @@ impl<'a> From<BoundRef<'a, 'a, PyModule>> for &'a PyModule {
     }
 }
 
+impl<'a, 'py, T: PyClass> From<BoundRef<'a, 'py, T>> for &'a PyCell<T> {
+    #[inline]
+    fn from(bound: BoundRef<'a, 'py, T>) -> Self {
+        bound.0.as_gil_ref()
+    }
+}
+
+impl<'a, 'py, T: PyClass> TryFrom<BoundRef<'a, 'py, T>> for PyRef<'py, T> {
+    type Error = PyBorrowError;
+    #[inline]
+    fn try_from(value: BoundRef<'a, 'py, T>) -> Result<Self, Self::Error> {
+        value.0.clone().into_gil_ref().try_into()
+    }
+}
+
+impl<'a, 'py, T: PyClass<Frozen = False>> TryFrom<BoundRef<'a, 'py, T>> for PyRefMut<'py, T> {
+    type Error = PyBorrowMutError;
+    #[inline]
+    fn try_from(value: BoundRef<'a, 'py, T>) -> Result<Self, Self::Error> {
+        value.0.clone().into_gil_ref().try_into()
+    }
+}
+
+impl<'a, 'py, T> From<BoundRef<'a, 'py, T>> for Bound<'py, T> {
+    #[inline]
+    fn from(bound: BoundRef<'a, 'py, T>) -> Self {
+        bound.0.clone()
+    }
+}
+
 impl<'a, 'py, T> From<BoundRef<'a, 'py, T>> for &'a Bound<'py, T> {
     #[inline]
     fn from(bound: BoundRef<'a, 'py, T>) -> Self {

src/pycell.rs

@@ -654,7 +654,7 @@ impl<T: PyClass + fmt::Debug> fmt::Debug for PyCell<T> {
 /// }
 /// # Python::with_gil(|py| {
 /// #     let sub = Py::new(py, Child::new()).unwrap();
-/// #     pyo3::py_run!(py, sub, "assert sub.format() == 'Caterpillar(base: Butterfly, cnt: 3)'");
+/// #     pyo3::py_run!(py, sub, "assert sub.format() == 'Caterpillar(base: Butterfly, cnt: 4)'");
 /// # });
 /// ```
 ///

tests/ui/invalid_pymethod_receiver.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `i32: From<&PyCell<MyClass>>` is not satisfied
+error[E0277]: the trait bound `i32: From<BoundRef<'_, '_, MyClass>>` is not satisfied
  --> tests/ui/invalid_pymethod_receiver.rs:8:43
   |
 8 |     fn method_with_invalid_self_type(slf: i32, py: Python<'_>, index: u32) {}
-  |                                           ^^^ the trait `From<&PyCell<MyClass>>` is not implemented for `i32`
+  |                                           ^^^ the trait `From<BoundRef<'_, '_, MyClass>>` is not implemented for `i32`
   |
   = help: the following other types implement trait `From<T>`:
             <i32 as From<bool>>
@@ -11,5 +11,5 @@ error[E0277]: the trait bound `i32: From<&PyCell<MyClass>>` is not satisfied
             <i32 as From<u8>>
             <i32 as From<u16>>
             <i32 as From<NonZeroI32>>
-  = note: required for `&PyCell<MyClass>` to implement `Into<i32>`
+  = note: required for `BoundRef<'_, '_, MyClass>` to implement `Into<i32>`
-  = note: required for `i32` to implement `TryFrom<&PyCell<MyClass>>`
+  = note: required for `i32` to implement `TryFrom<BoundRef<'_, '_, MyClass>>`