Derive default for enums where possible (#5158)

# Objective

Fixes #5153

## Solution

Search for all enums and manually check if they have default impls that can use this new derive.

By my reckoning:

| enum | num |
|-|-|
| total | 159 |
| has default impl | 29 |
| default is unit variant | 23 |

crates/bevy_asset/src/handle.rs

@@ -109,18 +109,14 @@ where
     marker: PhantomData<fn() -> T>,
 }
 
+// FIXME: Default is only needed because `Handle`'s field `handle_type` is currently ignored for reflection
+#[derive(Default)]
 enum HandleType {
+    #[default]
     Weak,
     Strong(Sender<RefChange>),
 }
 
-// FIXME: This only is needed because `Handle`'s field `handle_type` is currently ignored for reflection
-impl Default for HandleType {
-    fn default() -> Self {
-        Self::Weak
-    }
-}
-
 impl Debug for HandleType {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {

crates/bevy_core_pipeline/src/clear_color.rs

@@ -4,20 +4,15 @@ use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
 use bevy_render::{color::Color, extract_resource::ExtractResource};
 use serde::{Deserialize, Serialize};
 
-#[derive(Reflect, Serialize, Deserialize, Clone, Debug)]
+#[derive(Reflect, Serialize, Deserialize, Clone, Debug, Default)]
 #[reflect_value(Serialize, Deserialize)]
 pub enum ClearColorConfig {
+    #[default]
     Default,
     Custom(Color),
     None,
 }
 
-impl Default for ClearColorConfig {
-    fn default() -> Self {
-        ClearColorConfig::Default
-    }
-}
-
 /// When used as a resource, sets the color that is used to clear the screen between frames.
 ///
 /// This color appears as the "background" color for simple apps, when

crates/bevy_ecs/src/component.rs

@@ -145,21 +145,16 @@ mod sealed {
 /// #[component(storage = "SparseSet")]
 /// struct A;
 /// ```
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[derive(Debug, Copy, Clone, Default, Eq, PartialEq)]
 pub enum StorageType {
     /// Provides fast and cache-friendly iteration, but slower addition and removal of components.
     /// This is the default storage type.
+    #[default]
     Table,
     /// Provides fast addition and removal of components, but slower iteration.
     SparseSet,
 }
 
-impl Default for StorageType {
-    fn default() -> Self {
-        StorageType::Table
-    }
-}
-
 #[derive(Debug)]
 pub struct ComponentInfo {
     id: ComponentId,

crates/bevy_pbr/src/alpha.rs

@@ -4,9 +4,10 @@ use bevy_reflect::Reflect;
 
 // FIXME: This should probably be part of bevy_render2!
 /// Alpha mode
-#[derive(Component, Debug, Reflect, Copy, Clone, PartialEq)]
+#[derive(Component, Debug, Default, Reflect, Copy, Clone, PartialEq)]
 #[reflect(Component, Default)]
 pub enum AlphaMode {
+    #[default]
     Opaque,
     /// An alpha cutoff must be supplied where alpha values >= the cutoff
     /// will be fully opaque and < will be fully transparent
@@ -15,9 +16,3 @@ pub enum AlphaMode {
 }
 
 impl Eq for AlphaMode {}
-
-impl Default for AlphaMode {
-    fn default() -> Self {
-        AlphaMode::Opaque
-    }
-}

crates/bevy_reflect/bevy_reflect_derive/src/container_attributes.rs

@@ -24,9 +24,10 @@ const HASH_ATTR: &str = "Hash";
 pub(crate) const REFLECT_DEFAULT: &str = "ReflectDefault";
 
 /// A marker for trait implementations registered via the `Reflect` derive macro.
-#[derive(Clone)]
+#[derive(Clone, Default)]
 pub(crate) enum TraitImpl {
     /// The trait is not registered as implemented.
+    #[default]
     NotImplemented,
     /// The trait is registered as implemented.
     Implemented,
@@ -35,13 +36,6 @@ pub(crate) enum TraitImpl {
     /// The trait is registered with a custom function rather than an actual implementation.
     Custom(Ident),
 }
-
-impl Default for TraitImpl {
-    fn default() -> Self {
-        Self::NotImplemented
-    }
-}
-
 /// A collection of traits that have been registered for a reflected type.
 ///
 /// This keeps track of a few traits that are utilized internally for reflection

crates/bevy_reflect/bevy_reflect_derive/src/field_attributes.rs

@@ -22,8 +22,10 @@ pub(crate) struct ReflectFieldAttr {
 }
 
 /// Controls how the default value is determined for a field.
+#[derive(Default)]
 pub(crate) enum DefaultBehavior {
     /// Field is required.
+    #[default]
     Required,
     /// Field can be defaulted using `Default::default()`.
     Default,
@@ -34,12 +36,6 @@ pub(crate) enum DefaultBehavior {
     Func(syn::ExprPath),
 }
 
-impl Default for DefaultBehavior {
-    fn default() -> Self {
-        Self::Required
-    }
-}
-
 /// Parse all field attributes marked "reflect" (such as `#[reflect(ignore)]`).
 pub(crate) fn parse_field_attrs(attrs: &[Attribute]) -> Result<ReflectFieldAttr, syn::Error> {
     let mut args = ReflectFieldAttr::default();

crates/bevy_render/src/camera/camera.rs

@@ -307,21 +307,16 @@ impl RenderTarget {
     }
 }
 
-#[derive(Debug, Clone, Copy, Reflect, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, Default, Reflect, Serialize, Deserialize)]
 #[reflect_value(Serialize, Deserialize)]
 pub enum DepthCalculation {
     /// Pythagorean distance; works everywhere, more expensive to compute.
+    #[default]
     Distance,
     /// Optimization for 2D; assuming the camera points towards -Z.
     ZDifference,
 }
 
-impl Default for DepthCalculation {
-    fn default() -> Self {
-        DepthCalculation::Distance
-    }
-}
-
 pub fn camera_system<T: CameraProjection + Component>(
     mut window_resized_events: EventReader<WindowResized>,
     mut window_created_events: EventReader<WindowCreated>,

crates/bevy_render/src/mesh/shape/capsule.rs

@@ -31,10 +31,11 @@ impl Default for Capsule {
     }
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Default, Clone, Copy)]
 /// Manner in which UV coordinates are distributed vertically.
 pub enum CapsuleUvProfile {
     /// UV space is distributed by how much of the capsule consists of the hemispheres.
+    #[default]
     Aspect,
     /// Hemispheres get UV space according to the ratio of latitudes to rings.
     Uniform,
@@ -43,12 +44,6 @@ pub enum CapsuleUvProfile {
     Fixed,
 }
 
-impl Default for CapsuleUvProfile {
-    fn default() -> Self {
-        CapsuleUvProfile::Aspect
-    }
-}
-
 impl From<Capsule> for Mesh {
     #[allow(clippy::needless_range_loop)]
     fn from(capsule: Capsule) -> Self {

crates/bevy_render/src/render_asset.rs

@@ -39,19 +39,14 @@ pub trait RenderAsset: Asset {
     ) -> Result<Self::PreparedAsset, PrepareAssetError<Self::ExtractedAsset>>;
 }
 
-#[derive(Clone, Hash, Debug, PartialEq, Eq, SystemLabel)]
+#[derive(Clone, Hash, Debug, Default, PartialEq, Eq, SystemLabel)]
 pub enum PrepareAssetLabel {
     PreAssetPrepare,
+    #[default]
     AssetPrepare,
     PostAssetPrepare,
 }
 
-impl Default for PrepareAssetLabel {
-    fn default() -> Self {
-        Self::AssetPrepare
-    }
-}
-
 /// This plugin extracts the changed assets from the "app world" into the "render world"
 /// and prepares them for the GPU. They can then be accessed from the [`RenderAssets`] resource.
 ///

crates/bevy_render/src/texture/image.rs

@@ -115,18 +115,14 @@ pub struct Image {
 /// Used in [`Image`], this determines what image sampler to use when rendering. The default setting,
 /// [`ImageSampler::Default`], will read the sampler from the [`ImageSettings`] resource at runtime.
 /// Setting this to [`ImageSampler::Descriptor`] will override the global default descriptor for this [`Image`].
-#[derive(Debug, Clone)]
+#[derive(Debug, Default, Clone)]
 pub enum ImageSampler {
     /// Default image sampler, derived from the [`ImageSettings`] resource.
+    #[default]
     Default,
     /// Custom sampler for this image which will override global default.
     Descriptor(wgpu::SamplerDescriptor<'static>),
 }
-impl Default for ImageSampler {
-    fn default() -> Self {
-        Self::Default
-    }
-}
 
 impl ImageSampler {
     /// Returns a sampler descriptor with `Linear` min and mag filters

crates/bevy_sprite/src/sprite.rs

@@ -21,9 +21,10 @@ pub struct Sprite {
 
 /// How a sprite is positioned relative to its [`Transform`](bevy_transform::components::Transform).
 /// It defaults to `Anchor::Center`.
-#[derive(Debug, Clone, Reflect)]
+#[derive(Debug, Clone, Default, Reflect)]
 #[doc(alias = "pivot")]
 pub enum Anchor {
+    #[default]
     Center,
     BottomLeft,
     BottomCenter,
@@ -38,12 +39,6 @@ pub enum Anchor {
     Custom(Vec2),
 }
 
-impl Default for Anchor {
-    fn default() -> Self {
-        Anchor::Center
-    }
-}
-
 impl Anchor {
     pub fn as_vec(&self) -> Vec2 {
         match self {

crates/bevy_ui/src/focus.rs

@@ -17,39 +17,32 @@ use smallvec::SmallVec;
 /// Describes what type of input interaction has occurred for a UI node.
 ///
 /// This is commonly queried with a `Changed<Interaction>` filter.
-#[derive(Component, Copy, Clone, Eq, PartialEq, Debug, Reflect, Serialize, Deserialize)]
+#[derive(
+    Component, Copy, Clone, Default, Eq, PartialEq, Debug, Reflect, Serialize, Deserialize,
+)]
 #[reflect_value(Component, Serialize, Deserialize, PartialEq)]
 pub enum Interaction {
     /// The node has been clicked
     Clicked,
     /// The node has been hovered over
     Hovered,
     /// Nothing has happened
+    #[default]
     None,
 }
 
-impl Default for Interaction {
-    fn default() -> Self {
-        Interaction::None
-    }
-}
-
 /// Describes whether the node should block interactions with lower nodes
-#[derive(Component, Copy, Clone, Eq, PartialEq, Debug, Reflect, Serialize, Deserialize)]
+#[derive(
+    Component, Copy, Clone, Default, Eq, PartialEq, Debug, Reflect, Serialize, Deserialize,
+)]
 #[reflect_value(Component, Serialize, Deserialize, PartialEq)]
 pub enum FocusPolicy {
     /// Blocks interaction
+    #[default]
     Block,
     /// Lets interaction pass through
     Pass,
 }
-
-impl Default for FocusPolicy {
-    fn default() -> Self {
-        FocusPolicy::Block
-    }
-}
-
 /// Contains entities whose Interaction should be set to None
 #[derive(Default)]
 pub struct State {