[BenjaminLucier]

Fixes #1610.

Bug

Since #1480, a bare ArrayView or CowArray in a function signature shortens fine, but a struct containing one of the type aliases is still invariant, over both the view lifetime and the element type:

use ndarray::{Array1, CowArray, Ix1};

struct Holder<'a>(CowArray<'a, f64, Ix1>);
fn shorten<'a: 'b, 'b>(h: Holder<'a>) -> Holder<'b> { h } // error: lifetime may not live long enough

struct Test<T>(Array1<T>);
fn shorten_elem<'a: 'b, 'b>(t: Test<&'a u8>) -> Test<&'b u8> { t } // same error

This affects Array, ArcArray, CowArray, ArrayView, ArrayViewMut and RawArrayView, and every dimension alias built on them, on 0.17.0 through master. It is also the original use case of #1332, whose reporter hit the problem with a view "deep inside a data structure".

Root cause

#1480 made ArrayBase itself covariant by adding a defaulted third parameter, ArrayBase<S, D, A = <S as RawData>::Elem>, but the public aliases kept the two-parameter form, so they still expand to a type containing the projection <S as RawData>::Elem.

rustc computes a struct's variance from its field types without normalizing projections, and treats every parameter inside a projection as invariant (rust-lang/rust#115799). In a function signature the alias is normalized before the subtyping check, which is why #1480's test passes while a struct field stays invariant.

Fix

Spell out the element type in the seven base aliases:

-pub type CowArray<'a, A, D> = ArrayBase<CowRepr<'a, A>, D>;
+pub type CowArray<'a, A, D> = ArrayBase<CowRepr<'a, A>, D, A>;

The bound S: RawData<Elem = A> forces A == S::Elem, so each alias names exactly the same type as before — not a breaking change, suitable for a patch release.

A struct field written as ArrayBase<S, D> directly still gets the default and stays invariant; ndarray can't fix that without removing the default (breaking). The ArrayBase docs now explain the third parameter, with a doc-tested example of a covariant generic wrapper struct.

Tests

tests/variance.rs gains a struct-wrapped shorten_* case per alias, which compiles only if the struct is covariant, plus a runtime test calling them. RawArrayViewMut is omitted because, like *mut T, it is correctly invariant. Without the alias change all seven new cases fail to compile; with it they pass.

Checked locally on stable 1.99.1 (aarch64-apple-darwin) with RUSTFLAGS=-D warnings: clippy (-F approx,serde,rayon), the scripts/all-tests.sh steps except BLAS integration and benches (no features / approx / all features + quickcheck / release / --features blas lib / blas-mock / examples / doc tests), and RUSTDOCFLAGS=-Dwarnings cargo doc --no-deps --all-features. Not run locally: nightly cargo fmt, MSRV 1.87, miri, nostd, cross, cargo-careful, BLAS integration.

AI assisted.