Playground link is mangled as well: I needed to manually replace &s with & to make it load the gist.

I asked in the Rust Discord channels and it seems like my approach is fine. Even the mutable version is okay and doesn’t need to be unsafe if it returns the reference with self’s lifetime.

Then maybe publish it as a mini-crate (e.g. any-slice), ideally with a link to the discussion that proves that it is sound.

Note: both code fragments and links contain things like & which makes them inoperable as is.

Is anyvec crate of relevance?

Is (or will) Ferrocene be open source?

... exclusive early access to Ferrocene releases ...
exclusive

This phrase look suspicious, as "open source" and "exclusive" typically does not mix well.

Will it be a special version of rustc with private bugfixes, like in ~"we have fixed 19 bugs in rustc that can lead to miscompilations in your missing-critical program, so you better pay us for better private rustc instead of using buggy upstream version"~?

Or will publicly available source code be exactly the same as in certified version of the compiler, with the difference only being the legal warranties?

Or will it follow something like grsecurity model, i.e. technically open source, but please do not distribute (lest lose the subscription).