Improve docs for Py::as_ref() and Py::into_ref()

Co-authored-by: Georg Brandl <georg@python.org>

guide/src/migration.md

@@ -60,9 +60,12 @@ This should change very little from a usage perspective. If you implemented trai
 `PyObject` and `Py<T>`, you may find you can just remove the `PyObject` implementation.
 
 ### `AsPyRef` has been removed
-The only implementor of `AsPyRef` was `Py<T>`, so the `AsPyRef::as_ref` method has been moved to
-`Py::as_ref`. This should require no code changes except removing the old `use` for code which
-did not use `pyo3::prelude`.
+As `PyObject` has been changed to be just a type alias, the only remaining implementor of `AsPyRef`
+was `Py<T>`. This removed the need for a trait, so the `AsPyRef::as_ref` method has been moved to
+`Py::as_ref`.
+
+This should require no code changes except removing `use pyo3::AsPyRef` for code which did not use
+`pyo3::prelude::*`.
 
 Before:
 ```rust,ignore

guide/src/types.md

@@ -140,11 +140,18 @@ Can be cloned using Python reference counts with `.clone()`.
 # let py = gil.python();
 let list: Py<PyList> = PyList::empty(py).into();
 
-// Access the native type using Py::as_ref(py) or Py::into_ref(py)
-// (For #[pyclass] types, these will return &PyCell<T>)
-let _: &PyList = list.as_ref(py);
-let _: &PyList = list.clone().into_ref(py);
+// Access to the native type using Py::as_ref(py) or Py::into_ref(py)
+// (For #[pyclass] types T, these will return &PyCell<T>)
 
+// Py::as_ref() borrows the object
+let _: &PyList = list.as_ref(py);
+
+# let list_clone = list.clone(); // Just so that the .into() example for PyObject compiles.
+// Py::into_ref() moves the reference into pyo3's "object storage"; useful for making APIs
+// which return gil-bound references.
+let _: &PyList = list.into_ref(py);
+
+# let list = list_clone;
 // Convert to PyObject with .into()
 let _: PyObject = list.into();
 ```

src/instance.rs

@@ -37,7 +37,7 @@ pub unsafe trait PyNativeType: Sized {
 ///
 /// Accessing this object is thread-safe, since any access to its API requires a `Python<'py>` GIL
 /// token. There are a few different ways to use the Python object contained:
-///  - [`Py::as_ref`](#method.as_ref) to borrow a GIL-bound reference to the contained object,
+///  - [`Py::as_ref`](#method.as_ref) to borrow a GIL-bound reference to the contained object.
 ///  - [`Py::borrow`](#method.borrow), [`Py::try_borrow`](#method.try_borrow),
 ///    [`Py::borrow_mut`](#method.borrow_mut), or [`Py::try_borrow_mut`](#method.try_borrow_mut),
 ///    to directly access a `#[pyclass]` value (which has RefCell-like behavior, see
@@ -48,9 +48,6 @@ pub unsafe trait PyNativeType: Sized {
 ///
 /// See [the guide](https://pyo3.rs/master/types.html) for an explanation
 /// of the different Python object types.
-///
-/// `Py<T>` is implemented as a safe wrapper around `NonNull<ffi::PyObject>` with
-/// specified type information.
 #[repr(transparent)]
 pub struct Py<T>(NonNull<ffi::PyObject>, PhantomData<T>);
 
@@ -112,32 +109,23 @@ where
         unsafe { PyNativeType::unchecked_downcast(&*any) }
     }
 
-    /// Similar to [`as_ref`](#method.as_ref), but instead consumes this `Py` and registers the
+    /// Similar to [`as_ref`](#method.as_ref), and also consumes this `Py` and registers the
     /// Python object reference in PyO3's object storage. The reference count for the Python
     /// object will not be decreased until the GIL lifetime ends.
     ///
-    /// # Examples
-    /// Create `&PyList` from `Py<PyList>`:
-    /// ```
-    /// # use pyo3::prelude::*;
-    /// # use pyo3::types::PyList;
-    /// # Python::with_gil(|py| {
-    /// let list: Py<PyList> = PyList::empty(py).into();
-    /// let list: &PyList = list.into_ref(py);
-    /// assert_eq!(list.len(), 0);
-    /// # });
-    /// ```
+    /// # Example
     ///
-    /// Create `&PyCell<MyClass>` from `Py<MyClass>`:
+    /// Useful when returning GIL-bound references from functions. In the snippet below, note that
+    /// the `'py` lifetime of the input GIL lifetime is also given to the returned reference:
     /// ```
     /// # use pyo3::prelude::*;
-    /// #[pyclass]
-    /// struct MyClass { }
-    /// # Python::with_gil(|py| {
-    /// let my_class: Py<MyClass> = Py::new(py, MyClass { }).unwrap();
-    /// let my_class_cell: &PyCell<MyClass> = my_class.into_ref(py);
-    /// assert!(my_class_cell.try_borrow().is_ok());
-    /// # });
+    /// fn new_py_any<'py>(py: Python<'py>, value: impl IntoPy<PyObject>) -> &'py PyAny {
+    ///     let obj: PyObject = value.into_py(py);
+    ///
+    ///     // .as_ref(py) would not be suitable here, because a reference to `obj` may not be
+    ///     // returned from the function.
+    ///     obj.into_ref(py)
+    /// }
     /// ```
     pub fn into_ref(self, py: Python) -> &T::AsRefTarget {
         unsafe { py.from_owned_ptr(self.into_ptr()) }