Interesting might not be the same as useful.

If I'm trying to figure out which language to learn next, knowing what I can get paid for might be more useful, even if it's not that "interesting".

If lots of projects are starting up in Rust, but I can't get interviews because nobody is advertising, how useful is learning Rust?

Well, we have to define what a language's popularity mean. Because Rust is surely more 'hyped', than Java, but Java has at least an order of more developers/software written, etc.

So in which meaning do you use 'popular'?

Ideally we'd like to know both, as they tell us different things.

    > we used Rust quite a lot at my previous company but we didn't advertise for Rust developers at all.

How did you find Rust developers when you needed to hire?

Find developers. Tell them they get to use Rust. You now have Rust developers.

Find a CS program that teaches Rust and hire their graduates.

Existing C++ developers learned Rust.