You are manually vectorizing, but it lets the optimizer know you don't care about safe rounding behavior so it ends up using the simd instructions. And this way it is portable still vs using intrinsics. Floating point addition is the only one the optimizer isn't allowed to do, so if you just need multiplication or only use integers it all autovectorizes fine. The f32xN stuff is just a way to tell it you don't care about the rounding. There are better ways to do that that could be added, like a FastF32 type, but I don't know if llvm could support that.

Edit: go to godbolt and load the rust aligned sum and play around with types. If you see addps that is the packed scalar simd instruction. The more you get packed, the higher your score! You'll need to pass some extra arguments they don't list to get avx512 sized registers vs the xmm or ymm ones. And not all the instances it uses support avx512 so sometimes you have to try a couple times.