Make cqrs::TypedEvent static

[50U10FCA7]

For optimization purposes result of cqrs::TypedEvent::event_types() should be generates once.

There's two ways to do implement this:

Add additional trait cqrs::TypedEventStatic which provides static version of event_types;
Replace implementation of cqrs::TypedEvent to make it static.

[50U10FCA7]

@tyranron Reimpl way has problem with generic params. once_cell::sync::Lazy can't pass generics from outer function. I think we can use something like this to solve this problem.

[tyranron]

@50U10FCA7 please, remind me, why we don't use arrays and slices and const?

[50U10FCA7]

@tyranron Most traits implemented on Vec<T> but not for &[T]. In our case its postgres_types::ToSql.

Also checked the variant with constant slices and it doesn't work with generics too. At now it's impossible to pass generic to constant. Compiler throws these errors on attempt to pass generic/associated item to constant:

• can't use generic parameters from outer function;
• generic Self types are currently not permitted in anonymous constants.

Here's a MRE: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d206aef139f79bfc27c94e18c4bd0274

[50U10FCA7]

@ilslv ping

[ilslv]

@50U10FCA7 I believe you can hack around it by generating separate non-generic struct and implement Event on it: playground

In juniper we also do similar things: link

[50U10FCA7]

@ilslv There still problem with const evaluation: playground. Const block is required because of nested event types concatenation. Concatenation uses this solution.

[ilslv]

@50U10FCA7 please provide more exact MRE, because I don't understand why are you using inline const in Len implementation and why using <Self as Len>::LEN isn't possible.

[50U10FCA7]

@ilslv

Full MRE looks like this. I tried to avoid the problem with:

• Nested assoc type
• Self's assoc type

[ilslv]

@50U10FCA7 I've got pretty close to make it work, but compiler warned about this issue

As we only allowed generic constants where the actual type did not matter, it should always be possible to replace the generic parameters with dummy types. For example [0; std::mem::size_of::<*mut T>()] can be changed to [0; std::mem::size_of::<*mut ()>()].

So I think we can simply replace every generic parameter with (), because thats what compiler will eventually try to do.

[50U10FCA7]

FCM

Replace `TypedEvent::event_types` with associated constant (#21)