To add on to what bri3d said, these Rust register definitions are being auto-generated using SVD* files published by the chip vendors. For stm32 for example there are the auto-generated register definitions** and then the HAL layers*** on top that try to build easy to use tools on top of the registers (e.g. an SPI or USART type with write and read functions).

* https://www.keil.com/pack/doc/CMSIS/SVD/html/index.html

** https://github.com/stm32-rs/stm32-rs

*** https://github.com/stm32-rs/stm32f4xx-hal for the stm32f4xx line