[mrec]

The author discusses this in the corresponding /r/rust thread:

https://www.reddit.com/r/rust/comments/mvc6o5/incredibly_fas...

tl;dr: it's not straightforward, mostly because the standard functionality lives in `core` which doesn't have access to OS support for CPU feature detection.

[bombcar]

Does Rust allow for libraries to “override” core functionality when available?

[Kinrany]

If it was all userspace Rust, it would have been possible to choose the implementation at compile time conditional on the presence of std.