The C++/Qt/QML code calls into Rust. The Rust parts are only calling to C++ with callbacks passed into Rust. Those callbacks are small and typical Qt. They do construction, destruction and (thread-safe) signaling.

So as per custom in Rust the safety rules of the C/C++ code is encoded in the Rust API which makes that part safe.

The C++ part calling Rust has the normal C++ safety guarantees, which are less strict than Rust. To have concurrency safety, it is recommended to only do the one GUI thread for C++ and do the multi-threading in Rust.

The safety guarantees are precisely those of a program which calls reams of legacy C++ cruft.

Would appreciate pointers to pure rust GUI libraries from any rust experts in this thread.

The current state of UI in rust is very early.

Most of the mature UI libs are bindings to another language.

However there are some native UI libs

https://github.com/PistonDevelopers/conrod/blob/master/READM...