Refactor: Ergonomic State definition

[azriel91]

In GitLab by @azriel91 on Mar 26, 2019, 15:22

Currently impl State for MyState is written as:

• For states that are not aliases to AppState:

  impl<'a, 'b, S> State<GameData<'a, 'b>, AppEvent> for LoadingState<'a, 'b, S>

• For states that are aliases to AppState:

  pub type CharacterSelectionState<'a, 'b, F, S> =
      AppState<'a, 'b, CharacterSelectionStateDelegate<'a, 'b, F, S>, CharacterSelectionEntityId>;
  
  impl<'a, 'b, F, S> State<GameData<'a, 'b>, AppEvent>
      for CharacterSelectionStateDelegate<'a, 'b, F, S>

This is very verbose. Also, we have this:

pub trait AutexState<'a, 'b>: State<GameData<'a, 'b>, AppEvent> {}

// Blanket impl.
impl<'a, 'b, T> AutexState<'a, 'b> for T where T: State<GameData<'a, 'b>, AppEvent> {}

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To make it ergonomic to implement states, it would be much better if that could change to:

impl<S> State for LoadingState<S>

This possibly means all states having the behaviour of the current AppState.