Remove length 3 repr_simd vectors

[calebzulawski]

In rust-lang/rust#80652 we are removing support for non-power-of-two vector lengths with #[repr(simd)] from nightly Rust, and vek is one of the crates that will be effected. Just wanted to give a heads up before we make the change. If you have any questions feel free to ask!

[yoanlcq]

Hi, thanks for letting me know!

Fortunately this will be easy to fix on my side, I'll just make the non-power-of-two vectors in my repr_simd modules be actually #[repr(C)] instead, so this wouldn't break the API, although this has minor consequences related to alignment and stuff, but I doubt anybody would complain. If they do care, they should be using length 4 vectors instead anyway.

That said, I liked the fact that it "worked" and that the compiler would silently round up the length to the next power of two. Very often I like to use a Vec3 because it describes my intent exactly, but still have it backed by an SIMD register for efficiency.
Compared to biting the bullet and using a Vec4, this has positive consequences such as :

• Completely preventing (erroneous) use of the 4th element, since there is no such member in the struct;
• Some operations such as the cross product only make sense for Vec3. Of course I could implement these for Vec4 as well (and I probably will have to), but it was nice to have the type system help me enforce this while backing that with SIMD for me.

I wish it could keep "working" as it did, but I understand that this regression may make life significantly easier for the backend, so let's roll with it!

[calebzulawski]

I agree with all of your reasoning and I'll open an issue with stdsimd to make sure we get to it eventually. Unfortunately the cranelift backend does not support unusual vector lengths yet, and we are trying to bring std::simd and cranelift simultaneously. Once cranelift supports it, we can reenable it. Thanks for the quick fix!