derive asset for enums

[robtfm]

Objective

allow deriving Asset for enums, and for tuple structs.

Solution

add to the proc macro, generating visitor calls to the variant's data (if required).

supports unnamed or named field variants, and struct variants when the struct also derives Asset:

#[derive(Asset, TypePath)]
pub enum MyAssetEnum {
    Unnamed (
        #[dependency]
        Handle<Image>
    ),
    Named {
        #[dependency]
        array_handle: Handle<Image>,
        #[dependency]
        atlas_handles: Vec<Handle<Image>>,
    },
    StructStyle(
        #[dependency]
        VariantStruct
    ),
    Empty,
}

#[derive(Asset, TypePath)]
pub struct VariantStruct {
    // ...
}

also extend the struct implementation to support tuple structs:

#[derive(Asset, TypePath)]
pub struct MyImageNewtype(
    #[dependency]
    Handle<Image>
);

[nicopap]

I need this feature. (well, I don't, since I can easily wrap my enum into a struct, but it would be useful)

But the macro code is always very intimidating to newcomers, I think we should take care to use good quality code for proc macros. There are possible improvements.

[nicopap]

For interested 3rd parties: Additional feedback in robtfm#3

[robtfm]

@nicopap much cleaner, thank you.

i only added a tweak to remove warnings on tuple structs.

[SET001]

how about adding some simple test somewhere:

    #[derive(Asset, Reflect)]
    enum FooBar {
        Foo,
        Bar,
    }

[robtfm]

added some structs with the derive macro - it will test that the output compiles only.

[Shatur]

Looks like this PR doesn't break the API. Maybe we could apply it for 0.12.1?