I am really looking forward to this support. Currently, STM32 is one of the best hobbyist embedded platforms for Rust, it is well-supported with various HAL crates. It is debuggable with OpenOCD + gdb or Black Magic Probe. Moreover, you can buy Blue Pills (cheap STM32 board) for a couple of bucks. However, the downside is that the cheap STM32 board such as the Blue Pill do not have Bluetooth or WiFi support. So you have to hook up an ESP8266 (quite pointless to use another full MCU), some Bluetooth module, W5500 ethernet, or perhaps an NRF24l01+.
The ESP32 has built-in support for WiFi and Bluetooth. Having an LLVM backend would make Rust support possible and would give a very affordable Rust-capable MCU with connectivity out of the box.