The compiler now warns:

the feature const_mut_refs has been stable since 1.83.0 and no longer requires an attribute to enable

Recommendations

1. remove the feature from the blog posts
2. rephrase from:

you also need to add #![feature(const_mut_refs)] to the top of your lib.rs

to

you may need to add #![feature(const_mut_refs)] to the top of your lib.rs if you are running rust older than `1.83.0`

I can submit a PR if you want after I finish the series 👍🏻 thanks for this project, I've loved it so far