std::arch::* intrinsics for SIMD are stable and you can use them today. The situation is only slightly worse than C/C++ because the rust compilers cares a lot about undefined behavior, so there's some safe-but-technically-unsafe/annoying cfg stuff to make sure the intrinsics are actually emitted as you intend.

There is nothing blocking high quality SIMD libraries on stable in Rust today. The bar for inclusion in std is just much higher than the rest of the ecosystem.