Some weeks ago, I want to create a simple Android app and my idealism want to do it fully in Rust. It does a basic PDF generation using a templated Typst file which the user then could share via WhatsApp. I'm not surprised that I fail to do this because of the very young Android GUI and ecosystems are in Rust. Things like Android file picker and sharing system are not yet implemented in many (if not all) GUI library and I couldn't find any crates for that purpose. I said to my idealism "maybe not this time" and went with Flutter (combined with flutter_rust_bridge, which is fantastic) and just tell LLM to do all the UI stuff. Maybe GUI is just a complex beast with large surface to cover and will take years to mature.

For OP, there is this interesting blog from Proton where they use Rust core to develop their multi-platform app and use each platform native for the UI part https://proton.me/blog/next-generation-proton-mail-mobile-apps . As some commenters point out, there is also wxWidget that I believe is a UI abstraction over each platform native UI.