Example in new book uses invalid trait object `Box<dyn Command + Clone>`

[Seldom-SE]

Describe the issue

In the Designing Components chapter, under Storing Functions Inside of Components, the example uses Box<dyn Command + Clone>. Unfortunately, this type doesn't compile:

error[E0225]: only auto traits can be used as additional traits in a trait object
 --> src/main.rs:5:31
  |
5 | fn bar(bar: Box<dyn Command + Clone>) {}
  |                     -------   ^^^^^ additional non-auto trait
  |                     |
  |                     first non-auto trait
  |
  = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: bevy::prelude::Command + Clone {}`
  = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>

The error's proposed solution doesn't work either:

trait NewTrait: Command + Clone {}

fn bar(bar: Box<dyn NewTrait>) {}

error[E0038]: the trait `NewTrait` is not dyn compatible
 --> src/main.rs:7:17
  |
7 | fn bar(bar: Box<dyn NewTrait>) {}
  |                 ^^^^^^^^^^^^ `NewTrait` is not dyn compatible
  |
note: for a trait to be dyn compatible it needs to allow building a vtable
      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
 --> src/main.rs:5:27
  |
5 | trait NewTrait: Command + Clone {}
  |       --------            ^^^^^ ...because it requires `Self: Sized`
  |       |
  |       this trait is not dyn compatible...

Proposed Solution

At work, when I ran into this problem, I'd make a subtrait like ClonableCommand: Command with a method fn clone(&self) -> Box<dyn ClonableCommand>. That would detract from the focus of the example, though. I guess this example needs a use case where each command is only used once, so doesn't need Clone. Maybe something like

#[derive(Component)]
struct Projectile {
    on_hit: Vec<Box<dyn Command>>,
}

In this case, when the projectile hits, you'd drain on_hit, running each command, and then despawn the entity.

Another solution would be to not encourage this pattern. Ime, it's rarely better than one-shot systems, but your mileage may vary.

[alice-i-cecile]

We can probably just cut this tbh.