Make aview* free functions be const fns

Author: jturner314

src/free_functions.rs

@@ -9,6 +9,7 @@
 use alloc::vec;
 use alloc::vec::Vec;
 use std::mem::{forget, size_of};
+use std::ptr::NonNull;
 
 use crate::imp_prelude::*;
 use crate::{dimension, ArcArray1, ArcArray2};
@@ -64,32 +65,105 @@ pub fn rcarr1<A: Clone>(xs: &[A]) -> ArcArray1<A> {
 }
 
 /// Create a zero-dimensional array view borrowing `x`.
-pub fn aview0<A>(x: &A) -> ArrayView0<'_, A> {
-    unsafe { ArrayView::from_shape_ptr(Ix0(), x) }
+pub const fn aview0<A>(x: &A) -> ArrayView0<'_, A> {
+    ArrayBase {
+        data: ViewRepr::new(),
+        // Safe because references are always non-null.
+        ptr: unsafe { NonNull::new_unchecked(x as *const A as *mut A) },
+        dim: Ix0(),
+        strides: Ix0(),
+    }
 }
 
 /// Create a one-dimensional array view with elements borrowing `xs`.
 ///
+/// **Panics** if the length of the slice overflows `isize`. (This can only
+/// occur if `A` is zero-sized, because slices cannot contain more than
+/// `isize::MAX` number of bytes.)
+///
 /// ```
-/// use ndarray::aview1;
+/// use ndarray::{aview1, ArrayView1};
 ///
 /// let data = [1.0; 1024];
 ///
 /// // Create a 2D array view from borrowed data
 /// let a2d = aview1(&data).into_shape((32, 32)).unwrap();
 ///
 /// assert_eq!(a2d.sum(), 1024.0);
+///
+/// // Create a const 1D array view
+/// const C: ArrayView1<'static, f64> = aview1(&[1., 2., 3.]);
+///
+/// assert_eq!(C.sum(), 6.);
 /// ```
-pub fn aview1<A>(xs: &[A]) -> ArrayView1<'_, A> {
-    ArrayView::from(xs)
+pub const fn aview1<A>(xs: &[A]) -> ArrayView1<'_, A> {
+    if size_of::<A>() == 0 {
+        assert!(
+            xs.len() <= isize::MAX as usize,
+            "Slice length must fit in `isize`.",
+        );
+    }
+    ArrayBase {
+        data: ViewRepr::new(),
+        // Safe because references are always non-null.
+        ptr: unsafe { NonNull::new_unchecked(xs.as_ptr() as *mut A) },
+        dim: Ix1(xs.len()),
+        strides: Ix1(1),
+    }
 }
 
 /// Create a two-dimensional array view with elements borrowing `xs`.
 ///
-/// **Panics** if the product of non-zero axis lengths overflows `isize` (This can only occur if A
-/// is zero-sized because slices cannot contain more than `isize::MAX` number of bytes).
-pub fn aview2<A, const N: usize>(xs: &[[A; N]]) -> ArrayView2<'_, A> {
-    ArrayView2::from(xs)
+/// **Panics** if the product of non-zero axis lengths overflows `isize` (This
+/// can only occur if A is zero-sized or if `N` is zero, because slices cannot
+/// contain more than `isize::MAX` number of bytes).
+///
+/// ```
+/// use ndarray::{aview2, ArrayView2};
+///
+/// let data = vec![[1., 2., 3.], [4., 5., 6.]];
+///
+/// let view = aview2(&data);
+/// assert_eq!(view.sum(), 21.);
+///
+/// // Create a const 2D array view
+/// const C: ArrayView2<'static, f64> = aview2(&[[1., 2., 3.], [4., 5., 6.]]);
+/// assert_eq!(C.sum(), 21.);
+/// ```
+pub const fn aview2<A, const N: usize>(xs: &[[A; N]]) -> ArrayView2<'_, A> {
+    let cols = N;
+    let rows = xs.len();
+    if size_of::<A>() == 0 {
+        if let Some(n_elems) = rows.checked_mul(cols) {
+            assert!(
+                rows <= isize::MAX as usize
+                    && cols <= isize::MAX as usize
+                    && n_elems <= isize::MAX as usize,
+                "Product of non-zero axis lengths must not overflow isize.",
+            );
+        } else {
+            panic!("Overflow in number of elements.");
+        }
+    } else if N == 0 {
+        assert!(
+            rows <= isize::MAX as usize,
+            "Product of non-zero axis lengths must not overflow isize.",
+        );
+    }
+    // Safe because references are always non-null.
+    let ptr = unsafe { NonNull::new_unchecked(xs.as_ptr() as *mut A) };
+    let dim = Ix2(rows, cols);
+    let strides = if rows == 0 || cols == 0 {
+        Ix2(0, 0)
+    } else {
+        Ix2(cols, 1)
+    };
+    ArrayBase {
+        data: ViewRepr::new(),
+        ptr,
+        dim,
+        strides,
+    }
 }
 
 /// Create a one-dimensional read-write array view with elements borrowing `xs`.