Location

https://doc.rust-lang.org/stable/std/macro.is_x86_feature_detected.html

Click the "Source" link.

Summary

The source link for is_x86_feature_detected is broken, it goes to https://docs.rs/std_detect/latest/src/std_detect/detect/arch/x86.rs.html#18-260 which redirects to https://docs.rs/crate/std_detect/latest#18-260

Which is the docs for the crate that implements this (presumably) but not the correct link for the source of the macro.

There is also a big banner: "std_detect-0.1.5 doesn't have any documentation." which may or may not be related. Either way the source link in the std docs is broken.