"Free" dynamic dispatch for core SIMD?

Hi! I'm very excited by the work on portable/core simd. Thank you for all of your work :)

I was looking over the API, and I was thinking it should be possible to get "free" runtime dispatch with a proc macro on top of portable SIMD, which would make portability really easy!

I'm thinking something like (more of a sketch than concrete proposal):

#[runtime_dispatch(sse,avx,avx2)]
fn some_simd_fn(...) {
    // uses portable simd API
}

This would generate code much like the example in the core::arch docs here: https://doc.rust-lang.org/core/arch/index.html#dynamic-cpu-feature-detection, where it would generate versions of the function for each target_feature, then make the actual main function dynamically dispatch to each implementation.

Anyway, I thought I'd open this and get your thoughts.

Start by reviewing the portable/core SIMD API and the dynamic CPU feature detection example in the core::arch documentation. The issue is a sketch seeking feedback rather than a defined implementation task; done would require an agreed design and scope for runtime dispatch.