API: Add requirement of non-negative strides to raw ptr constructors

For raw views and array views, require non-negative strides and check
this with a debug assertion.

src/dimension/mod.rs

@@ -280,6 +280,19 @@ pub fn stride_offset_checked(dim: &[Ix], strides: &[Ix], index: &[Ix]) -> Option
     Some(offset)
 }
 
+/// Checks if strides are non-negative.
+pub fn strides_non_negative<D>(strides: &D) -> Result<(), ShapeError>
+where
+    D: Dimension,
+{
+    for &stride in strides.slice() {
+        if (stride as isize) < 0 {
+            return Err(from_kind(ErrorKind::Unsupported));
+        }
+    }
+    Ok(())
+}
+
 /// Implementation-specific extensions to `Dimension`
 pub trait DimensionExt {
     // note: many extensions go in the main trait if they need to be special-

src/impl_raw_views.rs

@@ -62,6 +62,11 @@ where
     ///     [`.offset()`] regardless of the starting point due to past offsets.
     ///
     /// * The product of non-zero axis lengths must not exceed `isize::MAX`.
+    /// 
+    /// * Strides must be non-negative.
+    ///
+    /// This function can use debug assertions to check some of these requirements,
+    /// but it's not a complete check.
     ///
     /// [`.offset()`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.offset
     pub unsafe fn from_shape_ptr<Sh>(shape: Sh, ptr: *const A) -> Self
@@ -73,6 +78,7 @@ where
         if cfg!(debug_assertions) {
             assert!(!ptr.is_null(), "The pointer must be non-null.");
             if let Strides::Custom(strides) = &shape.strides {
+                dimension::strides_non_negative(strides).unwrap();
                 dimension::max_abs_offset_check_overflow::<A, _>(&dim, &strides).unwrap();
             } else {
                 dimension::size_of_shape_checked(&dim).unwrap();
@@ -202,6 +208,11 @@ where
     ///     [`.offset()`] regardless of the starting point due to past offsets.
     ///
     /// * The product of non-zero axis lengths must not exceed `isize::MAX`.
+    /// 
+    /// * Strides must be non-negative.
+    ///
+    /// This function can use debug assertions to check some of these requirements,
+    /// but it's not a complete check.
     ///
     /// [`.offset()`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.offset
     pub unsafe fn from_shape_ptr<Sh>(shape: Sh, ptr: *mut A) -> Self
@@ -213,6 +224,7 @@ where
         if cfg!(debug_assertions) {
             assert!(!ptr.is_null(), "The pointer must be non-null.");
             if let Strides::Custom(strides) = &shape.strides {
+                dimension::strides_non_negative(strides).unwrap();
                 dimension::max_abs_offset_check_overflow::<A, _>(&dim, &strides).unwrap();
             } else {
                 dimension::size_of_shape_checked(&dim).unwrap();

src/impl_views/constructors.rs

@@ -96,6 +96,11 @@ where
     ///
     /// * The product of non-zero axis lengths must not exceed `isize::MAX`.
     ///
+    /// * Strides must be non-negative.
+    ///
+    /// This function can use debug assertions to check some of these requirements,
+    /// but it's not a complete check.
+    ///
     /// [`.offset()`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.offset
     pub unsafe fn from_shape_ptr<Sh>(shape: Sh, ptr: *const A) -> Self
     where
@@ -188,6 +193,11 @@ where
     ///
     /// * The product of non-zero axis lengths must not exceed `isize::MAX`.
     ///
+    /// * Strides must be non-negative.
+    ///
+    /// This function can use debug assertions to check some of these requirements,
+    /// but it's not a complete check.
+    ///
     /// [`.offset()`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.offset
     pub unsafe fn from_shape_ptr<Sh>(shape: Sh, ptr: *mut A) -> Self
     where

tests/raw_views.rs

@@ -81,3 +81,18 @@ fn raw_view_deref_into_view_misaligned() {
     let data: [u16; 2] = [0x0011, 0x2233];
     misaligned_deref(&data);
 }
+
+#[test]
+#[cfg(debug_assertions)]
+#[should_panic = "Unsupported"]
+fn raw_view_negative_strides() {
+    fn misaligned_deref(data: &[u16; 2]) -> ArrayView1<'_, u16> {
+        let ptr: *const u16 = data.as_ptr();
+        unsafe {
+            let raw_view = RawArrayView::from_shape_ptr(1.strides((-1isize) as usize), ptr);
+            raw_view.deref_into_view()
+        }
+    }
+    let data: [u16; 2] = [0x0011, 0x2233];
+    misaligned_deref(&data);
+}