Cosmic DE is built using iced which is a rust gui library. As far as native, single-platform guis go, I'd say rust is plenty mature.
There's also Bevy, a rust game engine, which, if I'm not mistaken is built on egui(?), and I think supports multiple compile targets.
Between a desktop environment and a game engine, I'd say rust is in a pretty decent place when it comes to gui.
Saying “almost” or “yes” privileges the activity and hype around Rust GUI over actual results. Bevy is the ironic example: a game engine that produces more discussion and code than, so far, one notable & good game.
Not entirely correct, they have bevy_ui as the in-house example but many people use the third party bevy_egui crate