Firefox should be the poster child for this, with the Oxidation project now in full swing. I'm hoping to see similar work appearing elsewhere though. There's no reason why any large C or C++ project couldn't decide to write some new component in Rust, if they felt it would benefit them.

But they would need to have a contributor and maintainer base sufficiently motivated to learn Rust and stick with it. Which will happen more and more if Firefox can be visibly shown to benefit from including Rust in their toolkit.