Support aliasing in ArrayView::from_shape

[jturner314]

It is safe for ArrayView instances to have aliasing elements. It would be good to add support for this in ArrayView::from_shape. I'm not sure whether we should support all possible types of aliasing, but it would be good to at least add support for stride 0 for axes with length greater than 1. (Views like this are often the result of broadcasting.)

[LukeMathWalker]

What do you mean by aliasing elements? @jturner314

[jturner314]

I'm referring to the case where there exists an element such that multiple different indices can be used to access that element. This is situation is safe for ArrayView but not for ArrayViewMut. This typically occurs with axes that have zero stride as a result of broadcasting, but it would be nice to support aliasing in ArrayView::from_shape for manually-created views.

For example, consider

extern crate ndarray;

use ndarray::prelude::*;

fn main() {
    let a = ArrayView1::from(&[1, 2, 3]);
    let b = a.broadcast((2, 3)).unwrap();
    println!("{:?}", b);
}

which prints

[[1, 2, 3],
 [1, 2, 3]] shape=[2, 3], strides=[0, 1], layout=Custom (0x0), const ndim=2

The elements are aliased in the sense that e.g. b[(0,0)] and b[(1,0)] refer to the same element.

It would be nice to obtain b directly without broadcasting like this:

extern crate ndarray;

use ndarray::prelude::*;

fn main() {
    let b = ArrayView::from_shape((2, 3).strides((0, 1)), &[1, 2, 3]).unwrap();
    println!("{:?}", b);
}

but that currently exits with "thread 'main' panicked at 'called Result::unwrap() on an Err value: ShapeError/Unsupported: unsupported operation', src/libcore/result.rs:997:5".