Users can no longer get a Res from &World

[ethereumdegen]

Bevy version

0.15

I made this [systemparam] to make it simpler for me to access loaded assets related to Dialog .

#[derive(SystemParam)]
pub struct DialogDataLoadExt<'w> {
    dialog_assets_resource: Res<'w, DialogAssets>,
    dialog_assets: Res<'w, Assets<Dialog>>,
    lore_node_assets: Res<'w,Assets<LoreNode>>,
    lore_set_assets: Res<'w, Assets<LoreSet>>, 
}

I want to be able to derive this from World. So i made this fn and it worked great in 0.14

  pub fn from_world<'w>(world: &'w World) -> Option< DialogDataLoadExt<'w> > {
        let dialog_assets_resource = world.get_resource_ref::<DialogAssets>()?;
        let dialog_assets = world.get_resource_ref::<Assets<Dialog>>()?;
        let lore_node_assets = world.get_resource_ref::<Assets<LoreNode>>()?;
        let lore_set_assets = world.get_resource_ref::<Assets<LoreSet>>()?;

        Some( DialogDataLoadExt {
            dialog_assets_resource,
            dialog_assets,
            lore_node_assets,
            lore_set_assets,
        } )
     }

in 0.15, it throws this compilation error

error[E0308]: mismatched types
  --> src/file_system_interaction/load_extensions/dialog_load_extension.rs:36:13
   |
36 |             dialog_assets_resource,
   |             ^^^^^^^^^^^^^^^^^^^^^^ expected `Res<'_, DialogAssets>`, found `Ref<'_, DialogAssets>`
   |
   = note: expected struct `bevy::prelude::Res<'_, DialogAssets>`
              found struct `bevy::prelude::Ref<'_, DialogAssets>`

error[E0308]: mismatched types
  --> src/file_system_interaction/load_extensions/dialog_load_extension.rs:37:13
   |
37 |             dialog_assets,
   |             ^^^^^^^^^^^^^ expected `Res<'_, Assets<Dialog>>`, found `Ref<'_, Assets<Dialog>>`
   |
   = note: expected struct `bevy::prelude::Res<'_, bevy::prelude::Assets<file_system_interaction::dialog::Dialog>>`
              found struct `bevy::prelude::Ref<'_, bevy::prelude::Assets<file_system_interaction::dialog::Dialog>>`

error[E0308]: mismatched types
  --> src/file_system_interaction/load_extensions/dialog_load_extension.rs:38:13
   |
38 |             lore_node_assets,
   |             ^^^^^^^^^^^^^^^^ expected `Res<'_, Assets<LoreNode>>`, found `Ref<'_, Assets<LoreNode>>`
   |
   = note: expected struct `bevy::prelude::Res<'_, bevy::prelude::Assets<LoreNode>>`
              found struct `bevy::prelude::Ref<'_, bevy::prelude::Assets<LoreNode>>`

error[E0308]: mismatched types
  --> src/file_system_interaction/load_extensions/dialog_load_extension.rs:39:13
   |
39 |             lore_set_assets,
   |             ^^^^^^^^^^^^^^^ expected `Res<'_, Assets<LoreSet>>`, found `Ref<'_, Assets<LoreSet>>`
   |
   = note: expected struct `bevy::prelude::Res<'_, bevy::prelude::Assets<LoreSet>>`
              found struct `bevy::prelude::Ref<'_, bevy::prelude::Assets<LoreSet>>`

what is the purpose of this error in 0.15 ? how can i access this systemparam from World in 0.15 ? (NOT &mut World, just readonly World )

Also migration docs dont mention this breaking change

[ethereumdegen]

to be more clear, I can get &Res from world, but i cannot use a SystemParam struct in a way to allow me to build helper functions on that SystemParam struct while also being able to generate it from & world .

[alice-i-cecile]

In order to construct arbitrary SystemParam from the world, the easiest and most reliable way is to use SystemState. Unfortunately, as you pointed out, that requires &mut World. That would save you from attempting to manually construct this though!

As a workaround, you could transmute from Ref to Res. Double-check the definitions, but their layout should be identical. Requires unsafe though, so that's less than great.

For an actual solution, I think that we should allow users to get arbitrary system param from the world more directly, and only require immutable access for read only system param. This is closely related to #3774.

[ethereumdegen]

yes this is related to the exact thing I pointed out in #3774 (except now it is burning me harder ) and in fact #3774 would help me with even more than just this since I am commonly trying to access lots of data using &World and not &mut World and so the fact that i cannot do a SystemState from &World is painful too .

I imagine it may be possible to do a SystemState with &World and then only be able to read stuff? Ideally... idk. A pipe dream perhaps . Or some other soln