SmallVec<[T; N]> is invariant over T

[zakarumych]

SmallVec<[T; N]> is invariant over T.
Although [T; N] is covariant over T as expected.

This is due to SmallVec<A> having field of type <A as Array>::Item in underlying union.
I propose to change that field type to A and remove A: Array bound from type declaration.

[mbrubeck]

For reference, this is similar to bluss/arrayvec#96, and is caused by rust-lang/rust#21726.

[Nadrieril]

The following trick (can't remember where I found it):

enum SmallVec<A: Array<Item=Item>, Item=<A as Array>::Item> {
    Inline(MaybeUninit<A>),
    Heap((NonNull<Item>, usize)),
}

fixes the variance issue (playground link).
Would that be possible to use or would it be a breaking change to the API ?

[bkolobara]

I just spent a few hours debugging a lifetime compilation error related to this after replacing Vec with SmallVec. It took me a long time to figure out that the error originated in SmallVec. This can be really a tricky issue.

[mbrubeck]

Fortunately, I expect this issue will be fixed in smallvec 2.0 when we replace the Array trait with const generics (#240).

That work depends on the min_const_generics feature, which the Rust team is planning to ship in early 2021: rust-lang/rust#79135

[tesujimath]

Having stumbled upon a lifetime problem caused by this, I found the very new 2.0.0-alpha.1 version, tried it, and found it fixed my problem, so yay! And thanks!

[Nadrieril]

min_const_generics was stabilized 3 years ago: rust-lang/rust#79135, are there any plans to provide a SmallVec<T, N> alternative that has the right variance?

[Nadrieril]

Oh wait, it looks like master has that already, I guess it's not finalized yet?