I'm doing it for free!

Jokes aside I don't think it's a plausible example, because it's not like Rust is a product people directly buy. Whose interest would it be to astroturf Rust adoption? If people only pretend to use a programing language, it's not going anywhere and not going to generate revenue for anybody.

I agree that it's being done, but I don't think it's being done by a company paid by clients. If I really really loved Rust, or OCaml, or whatever, joining with a few people to promote it (by talking about it, writing articles, but also writing high quality libraries and contributing to the ecosystem) would be the logical thing to do. You often hear "I'd love to use $NEW_THING but the ecosystem isn't big enough.". That's something you can start fixing with a few dedicated people. If you really believe in a technology, or really love it, you can try to influence its course and popularity. You can promote it at your job, try it on a greenfield project, etc.

I use Arch, btw. It's even better than Slack!

Yeah, the phrase Developer Advocacy is still being used, and I think most 'big' libraries and tools have it - it's a form of marketing. Ideally they manage to astroturf themselves into becoming popular, so that the people work with the tools become advocates themselves.

I mean it works, to a point, as a sales / marketing tactic.

What's the problem with rust?