Depends on what parts of the environment you're looking at. The compositor that's the basis of the entire environment will be written entirely in Rust, based on

Depends on what parts of the environment you're looking at. The compositor that's the basis of the entire environment will be written entirely in Rust, based on smithay. But applications written for COSMIC are currently based on GTK4. GTK is currently the best GUI toolkit for Rust application developers. It's the only toolkit that's fully functional with first class and official bindings. It's been pretty well endorsed by GNOME for most of their new applications lately. There are some Rust GUI toolkits out there that are shaping up, such as some former Qt developers actively developing sixtyfps, but it'll be a while before we have a GUI toolkit in the Rust space that's truly ready for complex application development.