Basic concept for customizing Component reference type

[chrisjuchem]

Looking for general feedback on this idea, and help solving a lifetime error:

error: lifetime may not live long enough
   --> crates/bevy_ecs/src/query/fetch.rs:535:9
    |
534 |     fn shrink<'wlong: 'wshort, 'wshort>(item: Self::Item<'wlong>) -> Self::Item<'wshort> {
    |               ------           ------- lifetime `'wshort` defined here
    |               |
    |               lifetime `'wlong` defined here
535 |         item
    |         ^^^^ associated function was supposed to return data with lifetime `'wlong` but it is returning data with lifetime `'wshort`
    |
    = help: consider adding the following bound: `'wshort: 'wlong`

Objective

Allow users to specify which reference types they want their components to be wrapped in when used in a Query. This would allow:

1. Disabling change detection, by specifying &T and &mut T instead of &T (or Ref<T>) and Mut<T>.
2. Enabling https://github.com/chrisjuchem/bevy_mod_index to update when components are changes by providing new Indexed reference types.

Solution

• Add an associated type to components through which the desired reference types can be accessed and used when creating WorldQuery::Items
• (todo) Allow setting this type with a new attribute in the component derive macro

---

Changelog

TODO

Migration Guide

TODO

[alice-i-cecile]

I quite like the basic idea: I can definitely see how this would be useful for #4513. This could also actually be used to make "read-only" component types, by only ever returning a &/Ref.

[Guvante]

I would take a look at #7344 and #6659 for some other work around allowing mutating the types here.

I believe the solve that #6659 did that I stole was to move the shrink function into a trait.

[chrisjuchem]

Thanks @Guvante, that was the trick I needed.