Remove event type and keyword

[Y-Nak]

Currently, we use event type defined with event keyword to emit an event.
But actually, we could handle event types in the same way as types defined with the struct keyword, i.e., normal user-defined struct.
The only difference between event and struct would be topic, so it would be necessary to introduce some special syntax or language features that enable it to be specified.

Currently, I'd like to propose allowing @[indexed] or @[topic] attribute in the field like below.

struct MyEvent:
    @[indexed]
    x: i32

    y: i32

In the short-term plan, we could treat @[indexed] attribute specially, then generalize the attribute feature in the long-term plan.

[distractedm1nd]

What effect does this have (or would it have in the future) on indexed fields?

[cburgdorf]

In the short-term plan, we could treat @[indexed] attribute specially, then generalize the attribute feature in the long-term plan.

I agree we should do this. This is basically the only thing holding us back from adding an emit<T: Event>(val: T) {..} message directly on context.fe in the std lib.

[cburgdorf]

What effect does this have (or would it have in the future) on indexed fields?

I think the main advantages of treating events as structs are:

1. Less special syntax
2. Code re-use (e.g. being able to use the a Foo struct for foo things and just happen to also emit it as an event)
3. Flexibility. If events are just structs, chances are people can come up with nice little helper libraries down the road

[Y-Nak]

#772