Embassy STM32 HAL
The Embassy STM32 HAL is based on the stm32-metapac project.
The infinite variant problem
STM32 microcontrollers comes in many families and flavors, and supporting all of them is a big undertaking. Embassy has taken advantage of the fact that the STM32 peripheral versions are shared across chip families. Instead of re-implementing the SPI peripheral for every STM32 chip family, embassy have a single SPI implementation that depends on code-generated register types that are identical for STM32 families with the same version of a given peripheral.
The metapac
The stm32-metapac module uses pre-generated chip and register definitions for STM32 chip families to generate register types. This is done at compile time based on Cargo feature flags.
The chip and register definitions are located in a separate module, stm32-data, which is modified whenever a bug is found in the definitions, or when adding support for new chip families.