`ArrayView` should be covariant over lifetime

[patengel]

Take a look at the following example:

fn reference_covariant<'a>(x: &'static f64) -> &'a f64 {
    x
}

fn arrayview_covariant<'a>(x: ArrayView1<'static, f64>) -> ArrayView1<'a, f64> {
    x
}

The first compiles fine, the second one does not compile and give the following error message:

fn arrayview_covariant<'a>(x: ArrayView1<'static, f64>) -> ArrayView1<'a, f64> {
                       -- lifetime `'a` defined here
   x
   ^ returning this value requires that `'a` must outlive `'static`

note: requirement occurs because of the type `ArrayBase<ViewRepr<&f64>, Dim<[usize; 1]>>`, which makes the generic argument `ViewRepr<&f64>` invariant
note: the struct `ArrayBase<S, D>` is invariant over the parameter `S`

The problem is caused by the usage of S::Elem inside the definition of ArrayBase.

I stumbled across this problem, since I used ArrayView deep inside a data structure which should be covariant.
As a user of ndarray I would assume that ArrayViews behave the same as Rust references, but I see fixing this bug is hard without interface changes.
Is there an easy work-around for such problems? I currently do not know of any...

[bluss]

Does the method reborrow help in your case? https://docs.rs/ndarray/latest/ndarray/type.ArrayView.html#method.reborrow

[patengel]

No sadly it does not. The functions in the code work on references to the complete data structure, and therefore using reborrow would require to duplicate the complete data structure.

[0xbe7a]

Edit: After looking at it, this is probably related to #1341

I have minimized it to this snippet: If either the Drop implementation is removed or ArrayBase does not have <S as MyTrait>::Elem and S as elements at the same time, the covariance works. Looks to me like a limitation of the Rust compiler

fn x<'s>(x: &'s ArrayBase<OwnedRepr<&'s str>>) {
    ()
}

pub struct ArrayBase<S>
where
    S: MyTrait, {
    pub data: S,
    pub ptr: <S as MyTrait>::Elem,
}

trait MyTrait {
    type Elem;
}

pub struct OwnedRepr<A>(A);

impl<A> Drop for OwnedRepr<A> {
    fn drop(&mut self) {
        todo!()
    }
}

impl<T> MyTrait for OwnedRepr<T> {
    type Elem = T;
}

fn main() {
    let elem = "abc";

    let data = OwnedRepr(elem);

    let input = ArrayBase {
        data,
        ptr: elem
    };

    x(&input);
}

[bluss]

put this in order after implementing ArrayRef #879, probably not going to be possible though (?) without new Rust features.

[akern40]

So to my surprise, there is a simple fix for this, as discussed in rust-lang/rust#115799 and rust-lang/rust#57440. I've just put in a PR for it. Since it's so small, I don't mind patching #1440 to include it once we merge to master.

[patengel]

Nice solution. I like it...