Drop PyArray::as_cell_slice due to its unsound interactions with PyReadonlyArray.

Author: adamreichold

CHANGELOG.md

@@ -5,6 +5,7 @@
   - Support borrowing arrays that are part of other Python objects via `PyArray::borrow_from_array` ([#230](https://github.com/PyO3/rust-numpy/pull/216))
   - `PyArray::new` is now `unsafe`, as it produces uninitialized arrays ([#220](https://github.com/PyO3/rust-numpy/pull/220))
   - `PyArray::from_exact_iter` does not unsoundly trust `ExactSizeIterator::len` any more ([#262](https://github.com/PyO3/rust-numpy/pull/262))
+  - `PyArray::as_cell_slice` was removed as it unsoundly interacts with `PyReadonlyArray` allowing safe code to violate aliasing rules ([#260](https://github.com/PyO3/rust-numpy/pull/260))
   - `rayon` feature is now removed, and directly specifying the feature via `ndarray` dependency is recommended ([#250](https://github.com/PyO3/rust-numpy/pull/250))
   - Descriptors rework and related changes ([#256](https://github.com/PyO3/rust-numpy/pull/256)):
     - Remove `DataType`

src/array.rs

@@ -1,6 +1,5 @@
 //! Safe interface for NumPy ndarray
 use std::{
-    cell::Cell,
     marker::PhantomData,
     mem,
     os::raw::{c_int, c_void},
@@ -603,9 +602,7 @@ impl<T: Element, D: Dimension> PyArray<T, D> {
 
     /// Returns the immutable view of the internal data of `PyArray` as slice.
     ///
-    /// Please consider the use of safe alternatives
-    /// ([`PyReadonlyArray::as_slice`](../struct.PyReadonlyArray.html#method.as_slice)
-    /// , [`as_cell_slice`](#method.as_cell_slice) or [`to_vec`](#method.to_vec)) instead of this.
+    /// Please consider the use of the safe alternative [`PyReadonlyArray::as_slice`].
     ///
     /// # Safety
     /// If the internal array is not readonly and can be mutated from Python code,
@@ -618,22 +615,11 @@ impl<T: Element, D: Dimension> PyArray<T, D> {
         }
     }
 
-    /// Returns the view of the internal data of `PyArray` as `&[Cell<T>]`.
-    pub fn as_cell_slice(&self) -> Result<&[Cell<T>], NotContiguousError> {
-        if !self.is_contiguous() {
-            Err(NotContiguousError)
-        } else {
-            Ok(unsafe { slice::from_raw_parts(self.data() as _, self.len()) })
-        }
-    }
-
     /// Returns the view of the internal data of `PyArray` as mutable slice.
     ///
     /// # Safety
     /// If another reference to the internal data exists(e.g., `&[T]` or `ArrayView`),
     /// it might cause undefined behavior.
-    ///
-    /// In such case, please consider the use of [`as_cell_slice`](#method.as_cell_slice),
     pub unsafe fn as_slice_mut(&self) -> Result<&mut [T], NotContiguousError> {
         if !self.is_contiguous() {
             Err(NotContiguousError)