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ArrayView
should be covariant over lifetime
#1332
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Does the method reborrow help in your case? https://docs.rs/ndarray/latest/ndarray/type.ArrayView.html#method.reborrow |
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. |
Edit: After looking at it, this is probably related to #1341 I have minimized it to this snippet: If either the 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);
} |
put this in order after implementing ArrayRef #879, probably not going to be possible though (?) without new Rust features. |
Take a look at the following example:
The first compiles fine, the second one does not compile and give the following error message:
The problem is caused by the usage of
S::Elem
inside the definition ofArrayBase
.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
ArrayView
s 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...
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