Shared Enums

#[cxx::bridge] mod ffi { enum Suit { Clubs, Diamonds, Hearts, Spades, } }

Generated Rust:

#![allow(unused)] fn main() { #[derive(Copy, Clone, PartialEq, Eq)] #[repr(transparent)] pub struct Suit { pub repr: u8, } #[allow(non_upper_case_globals)] impl Suit { pub const Clubs: Self = Suit { repr: 0 }; pub const Diamonds: Self = Suit { repr: 1 }; pub const Hearts: Self = Suit { repr: 2 }; pub const Spades: Self = Suit { repr: 3 }; } }

Generated C++:

enum class Suit : uint8_t { Clubs = 0, Diamonds = 1, Hearts = 2, Spades = 3, };

Speaker Notes

  • On the Rust side, the code generated for shared enums is actually a struct wrapping a numeric value. This is because it is not UB in C++ for an enum class to hold a value different from all of the listed variants, and our Rust representation needs to have the same behavior.