Given the “blazingly fast” branding, I too would have thought this would be in stable Rust by now.

However, like other commenters I assume it’s because it’s hard, not all that many users of Rust really need it, and the compiler team is small and only consists of volunteers.

Getting maximum performance out of SIMD requires rolling your own code with intrinsics. It is something a compiler can't do for you at a pretty fundamental level.

Most interesting performance optimizations from vector ISAs can't be done by the compiler.

I do scientific computing, and even I rarely have a situation where CPU SIMD is a clear win. Usually it's either not worth the added complexity, or the problem is so embarrassingly parallel that you should use a GPU.

Don’t forget that autovectorization does a lot too. This is only for when you want to ensure you get exactly what you want, for many applications, they just kinda get it for free sometimes.