`ArrayViewMut::into_view()` should be public

[jiawen]

There is no public API to convert an owned ArrayViewMut<'a, T, D> to ArrayView<'a, T, D> while preserving the original lifetime 'a. The method into_view() exists internally but is pub(crate).

ArrayViewMut has a .view() method, but it cannot be used here because its signature borrows self. A simplified example of my use case where I have an ArrayViewMut inside a struct:

use ndarray::{ArrayView2, ArrayViewMut2};

struct Foo<'a> {
    data: ArrayViewMut2<'a, f32>,
}

impl<'a> Foo<'a> {
    /// Downgrade to a shared view. This fails to compile.
    fn into_shared(self) -> ArrayView2<'a, f32> {
        self.data.view() // error[E0515]: returns value referencing local `self.data`
    }
}

The compiler complains:

error[E0515]: cannot return value referencing local data `self.data`
  --> src/main.rs:10:9
   |
10 |         self.data.view()
   |         ^^^^^^^^^------
   |         |        |
   |         |        `self.data` is borrowed here
   |         returns a value referencing data owned by the current function

The data pointed to by self.data is valid for 'a, but .view() can only express "valid for the duration of the &self.data borrow", which expires when self is dropped at the end of into_shared. The internal ArrayViewMut::into_view(self) -> ArrayView<'a, A, D> handles this correctly but is pub(crate).

I can work around this with unsafe { std::mem::transmute(...) } as a workaround which is valid but gross.

Request

Make into_view(self) -> ArrayView<'a, A, D> public, or add a From<ArrayViewMut<'a, A, D>> for ArrayView<'a, A, D> impl.