Generating C++ enums from Rust enums

By default, a Rust enum is mapped to an opaque C++ type (see C++ bindings for Rust enums). However, Crubit can try to map Rust enums to C++ enums if requested using the #[crubit_annotate::cpp_enum(kind = "enum")] or #[crubit_annotate::cpp_enum(kind = "enum class")] attribute. C++ code can use such enums like any other C++ enum.

But #[cpp_enum] cannot be used with exhaustive Rust enums. It may only be used on non-exhaustive enums, such as those created with #[open_enum] from the open_enum crate. Therefore, to generate C++ enum bindings, you must annotate your Rust enum with #[cpp_enum(kind = ...)], #[repr(...)] (where ... is an integer type like i32), and #[open_enum].

C++ enums are non-exhaustive by default, meaning they can hold values other than the explicitly named enumerators. #[open_enum] generates a Rust enum that is similarly non-exhaustive. Additionally, C++ allows multiple enumerators to have the same value, which can be enabled in Rust by using #[open_enum(allow_alias)].

Example

Given the following Rust crate that uses #[cpp_enum] and #[open_enum(allow_alias)]:

#[open_enum(allow_alias)]
#[cpp_enum(kind = "enum class")]
#[repr(i32)]
#[derive(Copy, Clone, Default)]
pub enum Color {
    Red,
    Blue,
    Green = 5,
    Gray = 5,
    Magenta = 7,
}

Crubit will generate the following bindings:

// CRUBIT_ANNOTATE: cpp_enum=enum class
enum class CRUBIT_INTERNAL_RUST_TYPE(
    ":: example_crate_golden :: Color") Color : ::std::int32_t {
  Red = INT32_C(0),
  Blue = INT32_C(1),
  Green = INT32_C(5),
  Gray = INT32_C(5),
  Magenta = INT32_C(7),
};