Rust doesn't just run on arduino boards, but most AVR chips. (you need some memory though)
Also those AVR chips come as DIP, take 5V and programmers are cheap so no need to buy expensive arduino boards really.
But if you wan't to do embedded Rust you should really get some ARM or RISC-V chips, they are better supported, run faster, have more memory and peripherals and some have quite extensive HAL crates for fast prototyping.
The day RISC-V replaces proprietary PIC, AVR, and ESP as the microcontroller architecture of choice will make me, and the hobby electronics community, extremely happy.
Too many microcontrollers, especially the cheaper ones, can only be programmed with the company's proprietary IDE and their proprietary version of C or assembly. AVR actually seems to be an exception to this which is why the Arduino and similar products chose them for their open source board.
I've been using RPPAL to work on a Pi 4 and it works pretty well. Has good defaults & support for all the interfaces I'm using (GPIO, SPI). Good experience overall (once I figured out cross-compiling).