Remove the BoundObject impl for &Bound, use Borrowed instead

For rationale see #4467.

newsfragments/4487.changed.md

@@ -0,0 +1 @@
+Remove the `BoundObject` impl for `&Bound`, use `Borrowed` instead. For rationale see https://github.com/PyO3/pyo3/issues/4467.

src/conversion.rs

@@ -192,8 +192,8 @@ pub trait IntoPyObject<'py>: Sized {
     type Target;
     /// The smart pointer type to use.
     ///
-    /// This will usually be [`Bound<'py, Target>`], but can special cases `&'a Bound<'py, Target>`
-    /// or [`Borrowed<'a, 'py, Target>`] can be used to minimize reference counting overhead.
+    /// This will usually be [`Bound<'py, Target>`], but in special cases [`Borrowed<'a, 'py, Target>`] can be
+    /// used to minimize reference counting overhead.
     type Output: BoundObject<'py, Self::Target>;
     /// The type returned in the event of a conversion error.
     type Error;
@@ -273,11 +273,11 @@ impl<'py, T> IntoPyObject<'py> for Bound<'py, T> {
 
 impl<'a, 'py, T> IntoPyObject<'py> for &'a Bound<'py, T> {
     type Target = T;
-    type Output = &'a Bound<'py, Self::Target>;
+    type Output = Borrowed<'a, 'py, Self::Target>;
     type Error = Infallible;
 
     fn into_pyobject(self, _py: Python<'py>) -> Result<Self::Output, Self::Error> {
-        Ok(self)
+        Ok(self.as_borrowed())
     }
 }
 

src/instance.rs

@@ -16,7 +16,10 @@ use std::ops::Deref;
 use std::ptr::NonNull;
 
 /// Owned or borrowed gil-bound Python smart pointer
-pub trait BoundObject<'py, T>: bound_object_sealed::Sealed {
+/// 
+/// This is implemented for [`Bound`] and [`Borrowed`].
+// SAFETY: Type must be layout-compatible with `*mut ffi::PyObject`.
+pub unsafe trait BoundObject<'py, T>: bound_object_sealed::Sealed {
     /// Type erased version of `Self`
     type Any: BoundObject<'py, PyAny>;
     /// Borrow this smart pointer.
@@ -35,7 +38,6 @@ mod bound_object_sealed {
     pub trait Sealed {}
 
     impl<'py, T> Sealed for super::Bound<'py, T> {}
-    impl<'a, 'py, T> Sealed for &'a super::Bound<'py, T> {}
     impl<'a, 'py, T> Sealed for super::Borrowed<'a, 'py, T> {}
 }
 
@@ -590,7 +592,8 @@ unsafe impl<T> AsPyPointer for Bound<'_, T> {
     }
 }
 
-impl<'py, T> BoundObject<'py, T> for Bound<'py, T> {
+// SAFETY: `Bound` is layout-compatible with `*mut ffi::PyObject`.
+unsafe impl<'py, T> BoundObject<'py, T> for Bound<'py, T> {
     type Any = Bound<'py, PyAny>;
 
     fn as_borrowed(&self) -> Borrowed<'_, 'py, T> {
@@ -614,30 +617,6 @@ impl<'py, T> BoundObject<'py, T> for Bound<'py, T> {
     }
 }
 
-impl<'a, 'py, T> BoundObject<'py, T> for &'a Bound<'py, T> {
-    type Any = &'a Bound<'py, PyAny>;
-
-    fn as_borrowed(&self) -> Borrowed<'a, 'py, T> {
-        Bound::as_borrowed(self)
-    }
-
-    fn into_bound(self) -> Bound<'py, T> {
-        self.clone()
-    }
-
-    fn into_any(self) -> Self::Any {
-        self.as_any()
-    }
-
-    fn into_ptr(self) -> *mut ffi::PyObject {
-        self.clone().into_ptr()
-    }
-
-    fn unbind(self) -> Py<T> {
-        self.clone().unbind()
-    }
-}
-
 /// A borrowed equivalent to `Bound`.
 ///
 /// The advantage of this over `&Bound` is that it avoids the need to have a pointer-to-pointer, as Bound
@@ -820,7 +799,8 @@ impl<T> IntoPy<PyObject> for Borrowed<'_, '_, T> {
     }
 }
 
-impl<'a, 'py, T> BoundObject<'py, T> for Borrowed<'a, 'py, T> {
+// SAFETY: `Borrowed` is layout-compatible with `*mut ffi::PyObject`.
+unsafe impl<'a, 'py, T> BoundObject<'py, T> for Borrowed<'a, 'py, T> {
     type Any = Borrowed<'a, 'py, PyAny>;
 
     fn as_borrowed(&self) -> Borrowed<'a, 'py, T> {

src/pycell.rs

@@ -199,7 +199,7 @@ use crate::ffi_ptr_ext::FfiPtrExt;
 use crate::internal_tricks::{ptr_from_mut, ptr_from_ref};
 use crate::pyclass::{boolean_struct::False, PyClass};
 use crate::types::any::PyAnyMethods;
-use crate::{ffi, Bound, IntoPy, PyErr, PyObject, Python};
+use crate::{ffi, Borrowed, Bound, IntoPy, PyErr, PyObject, Python};
 use std::convert::Infallible;
 use std::fmt;
 use std::mem::ManuallyDrop;
@@ -479,11 +479,11 @@ impl<'py, T: PyClass> IntoPyObject<'py> for PyRef<'py, T> {
 
 impl<'a, 'py, T: PyClass> IntoPyObject<'py> for &'a PyRef<'py, T> {
     type Target = T;
-    type Output = &'a Bound<'py, T>;
+    type Output = Borrowed<'a, 'py, T>;
     type Error = Infallible;
 
     fn into_pyobject(self, _py: Python<'py>) -> Result<Self::Output, Self::Error> {
-        Ok(&self.inner)
+        Ok(self.inner.as_borrowed())
     }
 }
 
@@ -668,11 +668,11 @@ impl<'py, T: PyClass<Frozen = False>> IntoPyObject<'py> for PyRefMut<'py, T> {
 
 impl<'a, 'py, T: PyClass<Frozen = False>> IntoPyObject<'py> for &'a PyRefMut<'py, T> {
     type Target = T;
-    type Output = &'a Bound<'py, T>;
+    type Output = Borrowed<'a, 'py, T>;
     type Error = Infallible;
 
     fn into_pyobject(self, _py: Python<'py>) -> Result<Self::Output, Self::Error> {
-        Ok(&self.inner)
+        Ok(self.inner.as_borrowed())
     }
 }