The AVR backend for LLVM was merged recently. So Rust support for AVR might happen if someone pitches in and does the work. Lots of Arduinos lying around, so it would make it more accessible.
The problem is HAL support. Some periphery is supported, other is not. ADC and DMAs for UARTs were merged just recently. Stuff like SPI / I2C slave modes is missing, I think HAL interface is not fleshed out yet. Stuff like usb stack would probably take a lot time to complete.
I think for a long time we will have to accept that Rust and C are going to co-exist on these devices. So that story should be workable.
My embedded code tends to have a strong split between application logic (in platform-independent, data-driven functional code, with automated tests) and underlying hardware-dependent "app host" (as little custom code as possible), with a data-based interface. I would be quite happy doing C for the hardware layer and Rust for the application logic layer.
Calling C would also have to touch the same structures which would, if nothing else, change the state of some peripherals to something that will break the rust guarantees.