Lifetime issues with `slice` methods

Start by reproducing the minimal Rust example from the playground link, comparing the slice_axis behavior in foo with slice_axis_inplace in bar and the explicit lifetimes in baz. Trace the slice and slice_axis APIs to determine which lifetime is tied to the returned view; done means the intended foo pattern is either supported with appropriate coverage or documented as requiring explicit lifetimes.

Hello folks,

I think I found an issue in the way lifetimes are inferred when using slice or slice_axis on array views.

Here is a minimal example with three functions, only bar and baz compile, while foo doesn't. I would argue that foo should be able to compile.

use ndarray; // 0.15.6

use ndarray::{ArrayBase, ViewRepr, Axis, Dimension, Slice, ArrayView};

fn main() {
    let a = ndarray::array![[1, 2, 3], [4, 5, 6]];
    foo(a.view(), ndarray::Axis(1));
}

fn foo<T, D>(arrayview: ArrayView<T, D>, axis: ndarray::Axis) -> ArrayView<T, D>
where
    D: Dimension,
{
    arrayview.slice_axis(axis, Slice::from(1..2))
}

fn bar<T, D>(arrayview: &mut ArrayView<T, D>, axis: Axis)
where
    D: Dimension,
{
    arrayview.slice_axis_inplace(axis, Slice::from(1..2));
}

fn baz<'a, 'data: 'a, T, D>(
    arrayview: &'a ArrayBase<ViewRepr<&'data T>, D>,
    axis: Axis,
) -> ArrayBase<ViewRepr<&'a T>, D>
where
    D: Dimension,
{
    arrayview.slice_axis(axis, Slice::from(1..2))
}

(playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7d62b20eec45d257e629dae07840544b)

The error is the following:

error[[E0311]](https://doc.rust-lang.org/stable/error-index.html#E0311): the parameter type `D` may not live long enough
  --> src/main.rs:14:5
   |
14 |     arrayview.slice_axis(axis, Slice::from(1..2))
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: the parameter type `D` must be valid for the anonymous lifetime defined here...
  --> src/main.rs:10:25
   |
10 | fn foo<T, D>(arrayview: ArrayView<T, D>, axis: ndarray::Axis) -> ArrayView<T, D>
   |                         ^^^^^^^^^^^^^^^
note: ...so that the type `D` will meet its required lifetime bounds
  --> src/main.rs:14:5
   |
14 |     arrayview.slice_axis(axis, Slice::from(1..2))
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: consider adding an explicit lifetime bound...
   |
12 |     D: Dimension + 'a,
   |                  ++++

error[[E0515]](https://doc.rust-lang.org/stable/error-index.html#E0515): cannot return reference to function parameter `arrayview`
  --> src/main.rs:14:5
   |
14 |     arrayview.slice_axis(axis, Slice::from(1..2))
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returns a reference to data owned by the current function

For more information about this error, try `rustc --explain E0515`.
error: could not compile `playground` due to 2 previous errors

My understanding of the problem is the following: the lifetime of the data represented by the array view passed in as argument to foo is not propagated correctly to the one obtained by slicing it, and the output of the slice function is actually depending on the lifetime of the view passed as argument and not the lifetime of the data referenced by the view.
In bar I show how to circumnavigate the issue by slicing in place. Given that views are cheap to create and clone, this is the alternative I'm going with for now.
In baz, I show a version where I explicitly annotate lifetimes to make it work, note how I have to constrain 'a to 'data and how I need to make the argument a reference to avoid having the function drop it while the returned value still references it.

Also, I'm not sure why the error message is angry about the lifetime of D in the first place, I don't see where it's being defined as a reference or a pointer.

I'm not super good at understanding lifetimes but it feels like it should be possible to slice a view and obtain something that doesn't depend on the view itself but on the underlying data only. Or am I missing something here ?