Another option would be to define a type alias: #[cfg(feature = "splitapple")] type Port = nrf52840_hal::pac::P1; #[cfg(feature = "keytron")]

Another option would be to define a type alias: #[cfg(feature = "splitapple")] type Port = nrf52840_hal::pac::P1; #[cfg(feature = "keytron")] type Port = nrf52833_hal::pac::P0; fn read_keys(port: Port) -> Packet {} but I think the whole problem here is that author tried to do what would have been #ifdef in C, but that doesn't get you far in Rust when macros are AST-based and types are more specific than `int` and `char*`. A more "rustic" solution would be to use a trait, e.g. trait ReadKeys { type Port; fn read_keys(Self::Port) -> Packet {} } impl ReadKeys for Splitapple {…} impl ReadKeys for Keytron {…}