Improve error message when Model doesn't implement Default

[MartinKavik]

"... and AfterMount doesn't create Model instance explicitly".

Should we add something like assert somewhere to give the user a reasonable error message?
Ideas?

[familyfriendlymikey]

Here's the error:

error[E0599]: no method named `build_and_start` found for type `seed::app::builder::Builder<Msg, Model, seed::virtual_dom::node::Node<Msg>, seed::app::UndefinedGMsg, seed::app::builder::init::UndefinedInitAPI>` in the current scope
  --> src/lib.rs:35:32
   |
35 |     App::builder(update, view).build_and_start();
   |                                ^^^^^^^^^^^^^^^ method not found in `seed::app::builder::Builder<Msg, Model, seed::virtual_dom::node::Node<Msg>, seed::app::UndefinedGMsg, seed::app::builder::init::UndefinedInitAPI>`
   |
   = note: the method `build_and_start` exists but the following trait bounds were not satisfied:
           `seed::app::builder::init::UndefinedInitAPI : seed::app::builder::InitAPI<Msg, Model, seed::virtual_dom::node::Node<Msg>, seed::app::UndefinedGMsg>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.

Solved with:

#[derive(Default)]
struct Model {
    data: Data,
    ...
}

#[derive(Default)]
struct Data {
    ...
}

[TatriX]

I'm sure I've seen this error before, but right now I'm not sure how can I reproduce it. Could you please add an example?

[MartinKavik]

@TatriX You were asking me about it some time ago if I remember correctly :) How to repro - make sure your Model doesn't impl Default and don't use after_mount.

---

I did quick research.
I recommend to remove the code that represents the deprecated app builder API as the first step, because it makes code pretty complex.
2nd step: Implement two build_and_start functions. It's not possible directly on Builder so we have to add an extra trait for it. The first function will build and start the app in the standard way, the second one will contain compile_error!("Hey, I don't know how to create Model!").
(I'm not sure if it's doable and the best way, other opinions welcome.)

[MartinKavik]

New API for App initialization - #382 - implemented in PR #388. It's much simpler so it should eliminate all cryptic compiler errors.