I think `core::simd` is probably not near, but 512-bit AVX SIMD will be out in a month or two! So, you could use f64x8, f32x16.
I've built my own f32xY, Vec3xY etc structs, since I want to use stable rust, but don't want to wait for core::SIMD. These all have syntaxes that mimic core::SIMD. I set up pack/unpack functions too, but they are still a hassle compared to non-SIMD operations.
THis begs the question: If core::simd doesn't end up ideal (For flexibility etc), how far can we get with a thin-wrapper lib? The ideal API (imo) is transparent, and supports the widest instructions available on your architecture, falling back to smaller ones or non-SIMD.
It also begs the question of if you should stop worrying and love the macro... We are talking f32/f64. Different widths. Different architectures. And now conflated with len 3 vecs, len 4 vecs etc (Where each vector/tensor item it a SIMD intrinsic). How does every vector (Not in the SIMD sense; the most popular one uses AoS SIMD which is IMO a mistake) handle this? Macros. This leads me to think we macro this too.