Consider how to support enums through to QML

[ahayzen-kdab]

We need to support enums to allow for complex objects like QAbstractListModel etc.

• Enums on QObjects from Rust
• Enums in a namespace from Rust
• Existing enums (via CXX)

[ahayzen-kdab]

Now you can define an enum in CXX, then register that enum to QML manually in C++.

[Be-ing]

Now you can define an enum in CXX, then register that enum to QML manually in C++.

Right, CXX takes care of most of the work already. But it would be helpful to be able to add a #[q_enum(QObjectClassName)] annotation to a cxx shared enum type and have our C++ generator add a Q_ENUM(enum_name) macro to the generated C++.

[ahayzen-kdab]

I need to try this again but i almost got it working before using Q_ENUM_NS from C++ as a workaround until we do add support with an attribute to shared enums. (eg #402 will hopefully have helped with that)

[LeonMatthesKDAB]

We should be able to support both Q_ENUM, as well as Q_ENUM_NS using CXX Extern enums.

e.g.:

#pragma once

#include <cstdint>

struct MyObject {
  enum class MyEnum : std::int8_t { A, B };
};

namespace rust {
namespace MyObject {
using MyEnum = ::MyObject::MyEnum;
}
} // namespace rust

#[cxx::bridge]
pub mod ffi {

    #[cxx_name = "MyEnum"]
    #[namespace = "rust::MyObject"]
    enum MyEnum {
        A,
        B,
    }

    unsafe extern "C++" {
        include!("src/myobject.h");

        #[cxx_name = "MyEnum"]
        #[namespace = "rust::MyObject"]
        type MyEnum;
    }
}

Then we would support these two kinds of declarations:

#[qenum]
#[namespace="MyNamespace"]
enum MyEnum { ... }

#[qenum(MyObject)]
enum MyEnum { ... }

extern "RustQt" {
    type MyObject = ...;
}

[LeonMatthesKDAB]

It turns out we can use QML_ELEMENT with Q_NAMESPACE to expose a Q_ENUM_NS it to QML.

See: https://doc.qt.io/qt-6/qqmlengine.html#QML_ELEMENT

[LeonMatthesKDAB]

Done

[ahayzen-kdab]

This is done.