I'm more interested in an alternative to write desktop apps in, rather than small one off scripts.

I'm interested in this from a vision point of view. What are going to be your first "written-using-Rust" apps ? Firefox is gargantuan, so it cant be your first. Ergo, the question about platform toolkit compatibility. If you build that one first, you will start spawning an ecosystem almost instantly... if you want.

Mono, Vala, etc. have nice apps that people love. I am wondering on how can you bridge the gap between javascript/Qtscript and Rust to make it easier to build apps in.