`#[implement]` macro for Rust enums

[BlueGlassBlock]

Suggestion

I am writing a Rust SDK for PowerToys Command Palette, and it has an interface like below:

enum CommandResultKind {
    Dismiss,    // Reset the palette to the main page and dismiss
    GoHome,     // Go back to the main page, but keep it open
    GoBack,     // Go back one level
    Hide,       // Keep this page open, but hide the palette.
    KeepOpen,   // Do nothing.
    GoToPage,   // Go to another page. GoToPageArgs will tell you where.
    ShowToast,  // Display a transient message to the user
    Confirm,    // Display a confirmation dialog
};
[uuid("f9d6423b-bd5e-44bb-a204-2f5c77a72396")]
[contract(Microsoft.CommandPalette.Extensions.ExtensionsContract, 1)]
interface ICommandResultArgs{};
[contract(Microsoft.CommandPalette.Extensions.ExtensionsContract, 1)]
interface IGoToPageArgs requires ICommandResultArgs{
    String PageId { get; };
    NavigationMode NavigationMode { get; };
}
[contract(Microsoft.CommandPalette.Extensions.ExtensionsContract, 1)]
interface IToastArgs requires ICommandResultArgs{
    String Message { get; };
    ICommandResult Result { get; };
}
[contract(Microsoft.CommandPalette.Extensions.ExtensionsContract, 1)]
interface IConfirmationArgs requires ICommandResultArgs{
    String Title { get; };
    String Description { get; };
    ICommand PrimaryCommand { get; };
    Boolean IsPrimaryCommandCritical { get; };
}
[contract(Microsoft.CommandPalette.Extensions.ExtensionsContract, 1)]
interface ICommandResult {
    CommandResultKind Kind { get; };
    ICommandResultArgs Args { get; };
}

Currently, I write

#[implement(ICommandResult)]
pub struct CommandResult {
}

Then manually copy everything macro generated and change definition to:

// #[implement(ICommandResult)]
pub enum CommandResult {
    Dismiss,
    GoHome,
    GoBack,
    Hide,
    #[default]
    KeepOpen,
    GoToPage(ComObject<GoToPageArgs>),
    ShowToast(ComObject<ToastArgs>),
    Confirm(ComObject<ConfirmationArgs>),
}

It would be helpful if I can use #[implement(ICommandResult)] directly.

[kennykerr]

That's a good idea - I hope to get that working soon.

Then manually copy everything macro generated and change definition to...

I would suggest that copying the code generated by the macro is unnecessarily complex. You could for example workaround this restriction as follows:

use windows_core::*;

#[interface("818f2fd1-d479-4398-b286-a93c4c7904d1")]
unsafe trait INumber: IUnknown {
    fn number(&self) -> u32;
}

#[implement(INumber)]
struct Outer(Inner);

enum Inner {
    One,
    Two,
    Three,
}

impl INumber_Impl for Outer_Impl {
    unsafe fn number(&self) -> u32 {
        match self.0 {
            Inner::One => 1,
            Inner::Two => 2,
            Inner::Three => 3,
        }
    }
}

#[test]
fn test() {
    unsafe {
        let number: INumber = Outer(Inner::Two).into();
        assert_eq!(number.number(), 2);
    }
}

[riverar]

I would also consider using upstream Win32 Metadata tooling to handle this conversion (mostly) automatically rather than doing it manually. https://withinrafael.com/2023/01/18/generating-metadata-for-the-windows-crate/