If you are not afraid of unsafe blocks and extern "C" functions, it is possible to call a any C api directly including Win32 API for GUI graphical user interfaces. On Linux, one can call Gtk directly or call Qt through a custom C wrapper. The only problem is that both methods are not cross-platform. However, one with enough motivation can develop a cross-platform unified wrapper or interface that abstracts Gtk, X11 and/or Win32 API and MacOSX cocoa API.

Calling Qt5 widgets through a custom C-wrapper may be a better approach than reinventing the whell since Qt already abstracts Win32 API, Unix X11 (X Windows Systems), Unix-Wayland, and MacOSX Cocoa .

There aren't any mature solutions yet, but you can find a list of GUI frameworks here:

http://www.areweguiyet.com/

I'm pretty sure Rust can run in the browser via WASM.