#[template(built_in)]

[cart]

Objective

Built in collection types like Option<T> and Vec<T> use the blanket FromTemplate impl for types that implement Default + Clone. This works in the common case where T is a normal Default + Clone type. This breaks down when T has custom template logic (ex: it is something like Handle<T>, which manually implements Template/FromTemplate, or it is something that derives FromTemplate). These cases currently require a solid chunk of boilerplate and knowledge about what is happening under the hood:

#[derive(Component, FromTemplate)]
struct Widget {
  #[template(OptionTemplate<HandleTemplate<Image>>)]
  image: Option<Handle<Image>>
}

Solution

This adds the BuiltInTemplate trait, which serves essentially the same function as FromTemplate, but does not have a blanket implementation for Default + Clone types. It requires a manual implementation.

This PR adds BuiltInTemplate implementations for Option and Vec.

When deriving FromTemplate, fields can opt in to using BuiltInTemplate instead of FromTemplate like this:

#[derive(Component, FromTemplate)]
struct Widget {
  #[template(built_in)]
  image: Option<Handle<Image>>
}

Of course ideally image: Option<Handle<Image>> would "just work". What we're seeing here is the "price" of transparently supporting any type that implements Default + Clone in BSN. That is a big win, and I consider the papercut in this case to be "worth it".

[alice-i-cecile]

I like this solution but it needs more docs. I left suggestions about where to put them.