Are we ndarray yet?

Start with the checklist in issue #597 and review each linked ndarray issue to determine whether it remains open and immediately actionable. Compare the listed categories and statuses with the referenced issues and pull requests; done means the tracker accurately reflects current work and provides useful starting points for contributors.

Purpose

The idea behind this collection is to provide an index to easily navigate all currently open ndarray's issues which are immediately actionable.
This is meant to be a good starting point for new contributors (e.g. what should I work on?) and it can also help existing contributors to identify trends and hot areas. I have pinned it using GitHub's new feature, so that it doesn't get lost (and stale).

Given that we have ~100 open issues (and more are opened every day), you are very welcome contributing to this taxonomy effort either commenting on this issue or editing it directly (if you have permissions to do so).
I am only adding to this tracker things I can easily understand/where enough context is provided in the issue - if I left something along the way, feel free to add it and to provide more info on it.

New functionality

Documentation

• Guidelines on how to use ndarray's types in a public API (Similar to Vec<T> vs &[T] considerations)

Feature parity

• Equivalent of numpy.where or numpy.nonzero (Issue: https://github.com/rust-ndarray/ndarray/issues/466)
• Kronecker product (or tensor product) (Reference: np.kron) (Issue: https://github.com/rust-ndarray/ndarray/issues/652)(Ongoing PR: #690)
• Scalar versions of standard deviation and variance (Issue: https://github.com/rust-ndarray/ndarray/issues/655)
• Add dstack, vstack, and hstack (Issue: https://github.com/rust-ndarray/ndarray/issues/667)
• Sorting (Issue: https://github.com/rust-ndarray/ndarray/issues/195)

Interop / Finer-grained control

• Implement ascontiguousarray() or contiguous() method (Issue: https://github.com/rust-ndarray/ndarray/issues/532)
• Add shrink_to_fit method (Issue: https://github.com/rust-ndarray/ndarray/issues/427)

Ergonomics

• Implement multislice_axis! macro (Issue: https://github.com/rust-ndarray/ndarray/issues/593)
• New constructor method for 2D arrays from an iterator of 1D arrays/vectors (Issue: https://github.com/rust-ndarray/ndarray/issues/539) (https://github.com/rust-ndarray/ndarray/issues/609)
• ArrawView as custom Dynamically Sized Type (Issue: https://github.com/rust-ndarray/ndarray/issues/538)
• Use #[track_caller] to improve panic info #972

Quality of life

• Implement proptest's Arbitrary trait for Array (Issue: https://github.com/rust-ndarray/ndarray/issues/596)
• Add new type aliases: ArcArray1 and ArcArray2 (Issue: https://github.com/rust-ndarray/ndarray/issues/661)
• Run rustfmt on the project and add it to the CI pipeline (PR: https://github.com/rust-ndarray/ndarray/pull/608)
• Run clippy on the project and take care of the linter warnings (PR: https://github.com/rust-ndarray/ndarray/pull/642)

Other

• Add in-place variants of dimension-changing operations for dynamic-dimensional arrays (Issue: https://github.com/rust-ndarray/ndarray/issues/428)
• Support Clone elements in stack and select (Issue: #269)

Improvements

Documentation

• Add a new example to ndarray-examples
• Provide more details on Axis NewType pattern rationale (Issue: https://github.com/rust-ndarray/ndarray/issues/564)
• Document ndarray's equivalent to NumPy's astype (Issues: https://github.com/rust-ndarray/ndarray/issues/493 , https://github.com/rust-ndarray/ndarray/issues/525)
• Improve doc examples for Zip/azip with failing examples (Issue: https://github.com/rust-ndarray/ndarray/issues/453)

Error messages / Debugging

• Better messages for incompatible shapes errors (Issue: https://github.com/rust-ndarray/ndarray/issues/449).
• Better formatting with Debug for arrays (Issue: https://github.com/rust-ndarray/ndarray/issues/398, PR: https://github.com/rust-ndarray/ndarray/pull/606)

Sharp API edges/corner cases

• Avoid panicking for zero-length axis in map_axis/map_axis_mut (Issue: https://github.com/rust-ndarray/ndarray/issues/579)
• Refactor all dimension-related traits (Issues: https://github.com/rust-ndarray/ndarray/issues/519 https://github.com/rust-ndarray/ndarray/issues/367)

Core

• Change ArrayBase.ptr to NonNull type (Issue: https://github.com/rust-ndarray/ndarray/issues/434)(Ongoing PR: #683)
• Provide more direct mutable access to shape, strides, and owned data (Issues: https://github.com/rust-ndarray/ndarray/issues/429 https://github.com/rust-ndarray/ndarray/issues/592)

Performance

• Have a look at sum_3_azip (Issue: https://github.com/rust-ndarray/ndarray/issues/561)
• Faster, arbitrary-order iterators (Issue: https://github.com/rust-ndarray/ndarray/issues/469)
• Co-broadcasting/two-sided broadcasting performance fixes #936