cpufeatures: requesting support for x86-64 microarchitecture levels

[indygreg]

There are now well-defined microarchitecture levels for x86-64: x86-64-v2, x86-64-v3, and x86-64-v4 (https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/low-level-sys-info.tex).

Each level is effectively a test for presence of various x86-64 CPU features.

Do you think it makes sense for cpufeatures to gain functionality for testing whether the current machine meets a given x86-64 microarchitecture level and/or resolving the current microarchitecture levels supported by the host CPU?

Thanks for this easy-to-use crate!

[newpavlov]

We could add it as a convenience shortcut for a set of features. But you still would have to list all features manually in your code using #[target_feature(enable = "..")], which is far from being ergonomic, so personally I think we should wait until support of microarchitecture levels will be added to Rust.