`ndarray::stack` changed behavior w.r.t layout between 0.15.1 and 0.15.2

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
rust
Domain
data

Research direction

Start by running the issue's demonstration program with ndarray pinned to 0.15.2 and 0.15.3, then inspect the ndarray::stack implementation and its documentation. Determine whether the layout change is intended and whether the docs specify the returned layout. Done means the behavior is clarified through a fix or documentation update, with the reproduction covered.

Written by the indexing model from the issue text.

Description

docs

Before 0.15.3, stack would return an array with C-layout when given C-layout views. But with 0.15.3, this behavior changed and now we get F-layout outputs. This demonstration program will succeed if you pin ndarray to "=0.15.2" and fail if you pin to "=0.15.3":

use ndarray::{arr2, stack, Axis};

fn main() {
    // Here are some 2x3 arrays.
    let a = arr2(&[[1, 2, 3], [4, 5, 6]]);
    let b = &a * 2;
    assert_eq!(a.shape(), b.shape());
    assert_eq!(a.shape(), [2, 3]);

    // When we stack them with Axis(2), we'll get a 2x3x4 array
    // with the last dimension stored continuously.
    let c = stack(Axis(2), &[a.view(), b.view(), a.view(), b.view()]).unwrap();
    assert_eq!(c.shape(), [2, 3, 4]);
    assert!(c.is_standard_layout());
    dbg!(&c);
}

I don't see anything about the layout of stack's return value in the docs so I assumed that it would match the inputs. So even if no code behavior change is warranted, the docs should really include a note explaining that you might get a different layout than your inputs.

Dominant language
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