If you want to get something done with microcontrollers, ARM Cortex-M tends to be much more powerful and is already great fun to write Rust for. It's a bit of work to get started--certainly a lot compared to Arduino which abstracts away even things that shouldn't be--but the code can be much more readable and portable than the corresponding C.

For example, get this $2 board http://wiki.stm32duino.com/index.php?title=Blue_Pill and use this library (see the examples!) https://github.com/japaric/stm32f103xx-hal

There probably will eventually be AVR implementations of https://github.com/japaric/embedded-hal so you could run mostly the same code on an Arduino board.