bevy_image: derive TypePath when Reflect is not available

[mockersf]

Objective

• bevy_image fails to build without default features:

error[E0277]: `image::Image` does not implement `TypePath` so cannot provide static type path information
   --> crates/bevy_image/src/image.rs:341:12
    |
341 | pub struct Image {
    |            ^^^^^ the trait `bevy_reflect::type_path::TypePath` is not implemented for `image::Image`
    |
    = note: consider annotating `image::Image` with `#[derive(Reflect)]` or `#[derive(TypePath)]`
    = help: the following other types implement trait `bevy_reflect::type_path::TypePath`:
              &'static Location<'static>
              &'static T
              &'static mut T
              ()
              (P,)
              (P1, P0)
              (P1, P2, P0)
              (P1, P2, P3, P0)
            and 146 others
note: required by a bound in `Asset`
   --> /home/runner/work/bevy-releasability/bevy-releasability/crates/bevy_asset/src/lib.rs:415:43
    |
415 | pub trait Asset: VisitAssetDependencies + TypePath + Send + Sync + 'static {}
    |                                           ^^^^^^^^ required by this bound in `Asset`
    = note: `Asset` is a "sealed trait", because to implement it you also need to implement `bevy_reflect::type_path::TypePath`, which is not accessible; this is usually done to force you to use one of the provided types that already implement it
    = help: the following types implement the trait:
              bevy_asset::AssetIndex
              bevy_asset::LoadedUntypedAsset
              bevy_asset::AssetEvent<A>
              bevy_asset::LoadedFolder
              bevy_asset::StrongHandle
              bevy_asset::Handle<A>
              bevy_asset::AssetId<A>
              bevy_asset::AssetPath<'a>
            and 148 others

error[E0277]: `image::Image` does not implement `TypePath` so cannot provide static type path information
   --> crates/bevy_image/src/image_loader.rs:121:18
    |
121 |     type Asset = Image;
    |                  ^^^^^ the trait `bevy_reflect::type_path::TypePath` is not implemented for `image::Image`
    |
    = note: consider annotating `image::Image` with `#[derive(Reflect)]` or `#[derive(TypePath)]`
    = help: the following other types implement trait `bevy_reflect::type_path::TypePath`:
              &'static Location<'static>
              &'static T
              &'static mut T
              ()
              (P,)
              (P1, P0)
              (P1, P2, P0)
              (P1, P2, P3, P0)
            and 146 others
    = note: required for `<ImageLoader as AssetLoader>::Asset` to implement `Asset`
note: required by a bound in `bevy_asset::AssetLoader::Asset`
   --> /home/runner/work/bevy-releasability/bevy-releasability/crates/bevy_asset/src/loader.rs:33:17
    |
33  |     type Asset: Asset;
    |                 ^^^^^ required by this bound in `AssetLoader::Asset`

error[E0277]: `texture_atlas::TextureAtlasLayout` does not implement `TypePath` so cannot provide static type path information
   --> crates/bevy_image/src/texture_atlas.rs:100:12
    |
100 | pub struct TextureAtlasLayout {
    |            ^^^^^^^^^^^^^^^^^^ the trait `bevy_reflect::type_path::TypePath` is not implemented for `texture_atlas::TextureAtlasLayout`
    |
    = note: consider annotating `texture_atlas::TextureAtlasLayout` with `#[derive(Reflect)]` or `#[derive(TypePath)]`
    = help: the following other types implement trait `bevy_reflect::type_path::TypePath`:
              &'static Location<'static>
              &'static T
              &'static mut T
              ()
              (P,)
              (P1, P0)
              (P1, P2, P0)
              (P1, P2, P3, P0)
            and 146 others
note: required by a bound in `Asset`
   --> /home/runner/work/bevy-releasability/bevy-releasability/crates/bevy_asset/src/lib.rs:415:43
    |
415 | pub trait Asset: VisitAssetDependencies + TypePath + Send + Sync + 'static {}
    |                                           ^^^^^^^^ required by this bound in `Asset`
    = note: `Asset` is a "sealed trait", because to implement it you also need to implement `bevy_reflect::type_path::TypePath`, which is not accessible; this is usually done to force you to use one of the provided types that already implement it
    = help: the following types implement the trait:
              bevy_asset::AssetIndex
              bevy_asset::LoadedUntypedAsset
              bevy_asset::AssetEvent<A>
              bevy_asset::LoadedFolder
              bevy_asset::StrongHandle
              bevy_asset::Handle<A>
              bevy_asset::AssetId<A>
              bevy_asset::AssetPath<'a>
            and 148 others

• Asset trait depends on TypePath which is in bevy_reflect. it's usually implemented by the Reflect derive

Solution

• make bevy_reflect not an optional dependency
• when feature bevy_reflect is not enabled, derive TypePath directly

[MrGVSV]

Nit: Maybe a comment here? I think normally we use bevy_reflect features to enable the dependency, so the purpose of this feature might not be exactly clear. I think it's to opt into deriving all the reflection traits?

Alternatively we could rename the feature to derive_reflect or something.

[MrGVSV]

Eventually it might be worth splitting TypePath out into its own crate so we don't have to continue pulling in bevy_reflect when it's not needed 🤔