Make iterators covariant in element type

The internal Baseiter type underlies most of the ndarray iterators, and
it used `*mut A` for element type A. Update it to use `NonNull<A>` which
behaves identically except it's guaranteed to be non-null and is
covariant w.r.t the parameter A.

Add compile test from the issue.

Fixes #1290

src/impl_owned_array.rs

@@ -907,7 +907,7 @@ where D: Dimension
 
     // iter is a raw pointer iterator traversing the array in memory order now with the
     // sorted axes.
-    let mut iter = Baseiter::new(self_.ptr.as_ptr(), self_.dim, self_.strides);
+    let mut iter = Baseiter::new(self_.ptr, self_.dim, self_.strides);
     let mut dropped_elements = 0;
 
     let mut last_ptr = data_ptr;

src/impl_views/conversions.rs

@@ -199,7 +199,7 @@ where D: Dimension
     #[inline]
     pub(crate) fn into_base_iter(self) -> Baseiter<A, D>
     {
-        unsafe { Baseiter::new(self.ptr.as_ptr(), self.dim, self.strides) }
+        unsafe { Baseiter::new(self.ptr, self.dim, self.strides) }
     }
 }
 
@@ -209,7 +209,7 @@ where D: Dimension
     #[inline]
     pub(crate) fn into_base_iter(self) -> Baseiter<A, D>
     {
-        unsafe { Baseiter::new(self.ptr.as_ptr(), self.dim, self.strides) }
+        unsafe { Baseiter::new(self.ptr, self.dim, self.strides) }
     }
 }
 
@@ -220,7 +220,7 @@ where D: Dimension
     #[inline]
     pub(crate) fn into_base_iter(self) -> Baseiter<A, D>
     {
-        unsafe { Baseiter::new(self.ptr.as_ptr(), self.dim, self.strides) }
+        unsafe { Baseiter::new(self.ptr, self.dim, self.strides) }
     }
 
     #[inline]
@@ -262,7 +262,7 @@ where D: Dimension
     #[inline]
     pub(crate) fn into_base_iter(self) -> Baseiter<A, D>
     {
-        unsafe { Baseiter::new(self.ptr.as_ptr(), self.dim, self.strides) }
+        unsafe { Baseiter::new(self.ptr, self.dim, self.strides) }
     }
 
     #[inline]

src/iterators/into_iter.rs

@@ -33,16 +33,15 @@ impl<A, D> IntoIter<A, D>
 where D: Dimension
 {
     /// Create a new by-value iterator that consumes `array`
-    pub(crate) fn new(mut array: Array<A, D>) -> Self
+    pub(crate) fn new(array: Array<A, D>) -> Self
     {
         unsafe {
             let array_head_ptr = array.ptr;
-            let ptr = array.as_mut_ptr();
             let mut array_data = array.data;
             let data_len = array_data.release_all_elements();
             debug_assert!(data_len >= array.dim.size());
             let has_unreachable_elements = array.dim.size() != data_len;
-            let inner = Baseiter::new(ptr, array.dim, array.strides);
+            let inner = Baseiter::new(array_head_ptr, array.dim, array.strides);
 
             IntoIter {
                 array_data,

src/iterators/mod.rs

@@ -19,6 +19,7 @@ use alloc::vec::Vec;
 use std::iter::FromIterator;
 use std::marker::PhantomData;
 use std::ptr;
+use std::ptr::NonNull;
 
 use crate::Ix1;
 
@@ -38,7 +39,7 @@ use std::slice::{self, Iter as SliceIter, IterMut as SliceIterMut};
 #[derive(Debug)]
 pub struct Baseiter<A, D>
 {
-    ptr: *mut A,
+    ptr: NonNull<A>,
     dim: D,
     strides: D,
     index: Option<D>,
@@ -50,7 +51,7 @@ impl<A, D: Dimension> Baseiter<A, D>
     /// to be correct to avoid performing an unsafe pointer offset while
     /// iterating.
     #[inline]
-    pub unsafe fn new(ptr: *mut A, len: D, stride: D) -> Baseiter<A, D>
+    pub unsafe fn new(ptr: NonNull<A>, len: D, stride: D) -> Baseiter<A, D>
     {
         Baseiter {
             ptr,
@@ -74,7 +75,7 @@ impl<A, D: Dimension> Iterator for Baseiter<A, D>
         };
         let offset = D::stride_offset(&index, &self.strides);
         self.index = self.dim.next_for(index);
-        unsafe { Some(self.ptr.offset(offset)) }
+        unsafe { Some(self.ptr.offset(offset).as_ptr()) }
     }
 
     fn size_hint(&self) -> (usize, Option<usize>)
@@ -99,7 +100,7 @@ impl<A, D: Dimension> Iterator for Baseiter<A, D>
                 let mut i = 0;
                 let i_end = len - elem_index;
                 while i < i_end {
-                    accum = g(accum, row_ptr.offset(i as isize * stride));
+                    accum = g(accum, row_ptr.offset(i as isize * stride).as_ptr());
                     i += 1;
                 }
             }
@@ -145,7 +146,7 @@ impl<A> DoubleEndedIterator for Baseiter<A, Ix1>
             self.index = None;
         }
 
-        unsafe { Some(self.ptr.offset(offset)) }
+        unsafe { Some(self.ptr.offset(offset).as_ptr()) }
     }
 
     fn nth_back(&mut self, n: usize) -> Option<*mut A>
@@ -158,7 +159,7 @@ impl<A> DoubleEndedIterator for Baseiter<A, Ix1>
             if index == self.dim {
                 self.index = None;
             }
-            unsafe { Some(self.ptr.offset(offset)) }
+            unsafe { Some(self.ptr.offset(offset).as_ptr()) }
         } else {
             self.index = None;
             None
@@ -178,7 +179,8 @@ impl<A> DoubleEndedIterator for Baseiter<A, Ix1>
                     accum = g(
                         accum,
                         self.ptr
-                            .offset(Ix1::stride_offset(&self.dim, &self.strides)),
+                            .offset(Ix1::stride_offset(&self.dim, &self.strides))
+                            .as_ptr(),
                     );
                 }
             }

tests/iterators.rs

@@ -1,6 +1,4 @@
-#![allow(
-    clippy::many_single_char_names, clippy::deref_addrof, clippy::unreadable_literal, clippy::many_single_char_names
-)]
+#![allow(clippy::deref_addrof, clippy::unreadable_literal)]
 
 use ndarray::prelude::*;
 use ndarray::{arr3, indices, s, Slice, Zip};
@@ -1055,3 +1053,33 @@ impl Drop for DropCount<'_>
         self.drops.set(self.drops.get() + 1);
     }
 }
+
+#[test]
+fn test_impl_iter_compiles()
+{
+    // Requires that the iterators are covariant in the element type
+
+    // base case: std
+    fn slice_iter_non_empty_indices<'s, 'a>(array: &'a Vec<&'s str>) -> impl Iterator<Item = usize> + 'a
+    {
+        array
+            .iter()
+            .enumerate()
+            .filter(|(_index, elem)| !elem.is_empty())
+            .map(|(index, _elem)| index)
+    }
+
+    let _ = slice_iter_non_empty_indices;
+
+    // ndarray case
+    fn array_iter_non_empty_indices<'s, 'a>(array: &'a Array<&'s str, Ix1>) -> impl Iterator<Item = usize> + 'a
+    {
+        array
+            .iter()
+            .enumerate()
+            .filter(|(_index, elem)| !elem.is_empty())
+            .map(|(index, _elem)| index)
+    }
+
+    let _ = array_iter_non_empty_indices;
+}