Add const to methods and const defaults to bevy_ui (#5542)

# Objective
- Fixes #5529 

## Solution
- Add assosciated constants named DEFAULT to as many types as possible
- Add const to as many methods in bevy_ui as possible

I have not applied the same treatment to the bundles in bevy_ui as it would require going into other bevy crates to implement const defaults for structs in bevy_text or relies on UiImage which calls HandleUntyped.typed() which isn't const safe.

Alternatively the defaults could relatively easily be turned into a macro to regain some of the readability and conciseness at the cost of explicitness.
Such a macro that partially implements this exists as a crate here: [const-default](https://docs.rs/const-default/latest/const_default/derive.ConstDefault.html) but does not support enums.

Let me know if there's anything I've missed or if I should push further into other crates.

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_ui/src/focus.rs

@@ -31,9 +31,7 @@ use smallvec::SmallVec;
 ///
 /// Note that you can also control the visibility of a node using the [`Display`](crate::ui_node::Display) property,
 /// which fully collapses it during layout calculations.
-#[derive(
-    Component, Copy, Clone, Default, Eq, PartialEq, Debug, Reflect, Serialize, Deserialize,
-)]
+#[derive(Component, Copy, Clone, Eq, PartialEq, Debug, Reflect, Serialize, Deserialize)]
 #[reflect(Component, Serialize, Deserialize, PartialEq)]
 pub enum Interaction {
     /// The node has been clicked
@@ -41,22 +39,39 @@ pub enum Interaction {
     /// The node has been hovered over
     Hovered,
     /// Nothing has happened
-    #[default]
     None,
 }
 
+impl Interaction {
+    const DEFAULT: Self = Self::None;
+}
+
+impl Default for Interaction {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Describes whether the node should block interactions with lower nodes
-#[derive(
-    Component, Copy, Clone, Default, Eq, PartialEq, Debug, Reflect, Serialize, Deserialize,
-)]
+#[derive(Component, Copy, Clone, Eq, PartialEq, Debug, Reflect, Serialize, Deserialize)]
 #[reflect(Component, Serialize, Deserialize, PartialEq)]
 pub enum FocusPolicy {
     /// Blocks interaction
-    #[default]
     Block,
     /// Lets interaction pass through
     Pass,
 }
+
+impl FocusPolicy {
+    const DEFAULT: Self = Self::Block;
+}
+
+impl Default for FocusPolicy {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Contains entities whose Interaction should be set to None
 #[derive(Default)]
 pub struct State {

crates/bevy_ui/src/geometry.rs

@@ -119,7 +119,7 @@ use std::ops::{Div, DivAssign, Mul, MulAssign};
 ///     bottom: Val::Px(40.0),
 /// };
 /// ```
-#[derive(Copy, Clone, PartialEq, Debug, Default, Reflect)]
+#[derive(Copy, Clone, PartialEq, Debug, Reflect)]
 #[reflect(PartialEq)]
 pub struct UiRect {
     /// The value corresponding to the left side of the UI rect.
@@ -133,6 +133,13 @@ pub struct UiRect {
 }
 
 impl UiRect {
+    pub const DEFAULT: Self = Self {
+        left: Val::DEFAULT,
+        right: Val::DEFAULT,
+        top: Val::DEFAULT,
+        bottom: Val::DEFAULT,
+    };
+
     /// Creates a new [`UiRect`] from the values specified.
     ///
     /// # Example
@@ -152,7 +159,7 @@ impl UiRect {
     /// assert_eq!(ui_rect.top, Val::Px(30.0));
     /// assert_eq!(ui_rect.bottom, Val::Px(40.0));
     /// ```
-    pub fn new(left: Val, right: Val, top: Val, bottom: Val) -> Self {
+    pub const fn new(left: Val, right: Val, top: Val, bottom: Val) -> Self {
         UiRect {
             left,
             right,
@@ -175,7 +182,7 @@ impl UiRect {
     /// assert_eq!(ui_rect.top, Val::Px(10.0));
     /// assert_eq!(ui_rect.bottom, Val::Px(10.0));
     /// ```
-    pub fn all(value: Val) -> Self {
+    pub const fn all(value: Val) -> Self {
         UiRect {
             left: value,
             right: value,
@@ -313,10 +320,16 @@ impl UiRect {
     }
 }
 
+impl Default for UiRect {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// A 2-dimensional area defined by a width and height.
 ///
 /// It is commonly used to define the size of a text or UI element.
-#[derive(Copy, Clone, PartialEq, Debug, Default, Reflect)]
+#[derive(Copy, Clone, PartialEq, Debug, Reflect)]
 #[reflect(PartialEq)]
 pub struct Size {
     /// The width of the 2-dimensional area.
@@ -326,6 +339,11 @@ pub struct Size {
 }
 
 impl Size {
+    pub const DEFAULT: Self = Self {
+        width: Val::DEFAULT,
+        height: Val::DEFAULT,
+    };
+
     /// Creates a new [`Size`] from a width and a height.
     ///
     /// # Example
@@ -355,6 +373,12 @@ impl Size {
     };
 }
 
+impl Default for Size {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 impl From<(Val, Val)> for Size {
     fn from(vals: (Val, Val)) -> Self {
         Self {

crates/bevy_ui/src/ui_node.rs

@@ -12,7 +12,7 @@ use std::ops::{Div, DivAssign, Mul, MulAssign};
 use thiserror::Error;
 
 /// Describes the size of a UI node
-#[derive(Component, Debug, Clone, Default, Reflect)]
+#[derive(Component, Debug, Clone, Reflect)]
 #[reflect(Component, Default)]
 pub struct Node {
     /// The size of the node as width and height in pixels
@@ -28,12 +28,23 @@ impl Node {
     }
 }
 
+impl Node {
+    pub const DEFAULT: Self = Self {
+        calculated_size: Vec2::ZERO,
+    };
+}
+
+impl Default for Node {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// An enum that describes possible types of value in flexbox layout options
-#[derive(Copy, Clone, PartialEq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum Val {
     /// No value defined
-    #[default]
     Undefined,
     /// Automatically determine this value
     Auto,
@@ -43,6 +54,16 @@ pub enum Val {
     Percent(f32),
 }
 
+impl Val {
+    pub const DEFAULT: Self = Self::Undefined;
+}
+
+impl Default for Val {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 impl Mul<f32> for Val {
     type Output = Val;
 
@@ -243,37 +264,41 @@ pub struct Style {
     pub gap: Size,
 }
 
+impl Style {
+    pub const DEFAULT: Self = Self {
+        display: Display::DEFAULT,
+        position_type: PositionType::DEFAULT,
+        direction: Direction::DEFAULT,
+        flex_direction: FlexDirection::DEFAULT,
+        flex_wrap: FlexWrap::DEFAULT,
+        align_items: AlignItems::DEFAULT,
+        align_self: AlignSelf::DEFAULT,
+        align_content: AlignContent::DEFAULT,
+        justify_content: JustifyContent::DEFAULT,
+        position: UiRect::DEFAULT,
+        margin: UiRect::DEFAULT,
+        padding: UiRect::DEFAULT,
+        border: UiRect::DEFAULT,
+        flex_grow: 0.0,
+        flex_shrink: 1.0,
+        flex_basis: Val::Auto,
+        size: Size::AUTO,
+        min_size: Size::AUTO,
+        max_size: Size::AUTO,
+        aspect_ratio: None,
+        overflow: Overflow::DEFAULT,
+        gap: Size::UNDEFINED,
+    };
+}
+
 impl Default for Style {
     fn default() -> Self {
-        Self {
-            display: Default::default(),
-            position_type: Default::default(),
-            direction: Default::default(),
-            flex_direction: Default::default(),
-            flex_wrap: Default::default(),
-            align_items: Default::default(),
-            align_self: Default::default(),
-            align_content: Default::default(),
-            justify_content: Default::default(),
-            position: Default::default(),
-            margin: Default::default(),
-            padding: Default::default(),
-            border: Default::default(),
-            flex_grow: 0.0,
-            flex_shrink: 1.0,
-            flex_basis: Val::Auto,
-            size: Size::AUTO,
-            min_size: Size::AUTO,
-            max_size: Size::AUTO,
-            aspect_ratio: Default::default(),
-            overflow: Default::default(),
-            gap: Size::UNDEFINED,
-        }
+        Self::DEFAULT
     }
 }
 
 /// How items are aligned according to the cross axis
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum AlignItems {
     /// Items are aligned at the start
@@ -285,16 +310,24 @@ pub enum AlignItems {
     /// Items are aligned at the baseline
     Baseline,
     /// Items are stretched across the whole cross axis
-    #[default]
     Stretch,
 }
 
+impl AlignItems {
+    pub const DEFAULT: Self = Self::Stretch;
+}
+
+impl Default for AlignItems {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Works like [`AlignItems`] but applies only to a single item
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum AlignSelf {
     /// Use the value of [`AlignItems`]
-    #[default]
     Auto,
     /// If the parent has [`AlignItems::Center`] only this item will be at the start
     FlexStart,
@@ -308,10 +341,20 @@ pub enum AlignSelf {
     Stretch,
 }
 
+impl AlignSelf {
+    pub const DEFAULT: Self = Self::Auto;
+}
+
+impl Default for AlignSelf {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Defines how each line is aligned within the flexbox.
 ///
 /// It only applies if [`FlexWrap::Wrap`] is present and if there are multiple lines of items.
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum AlignContent {
     /// Each line moves towards the start of the cross axis
@@ -321,7 +364,6 @@ pub enum AlignContent {
     /// Each line moves towards the center of the cross axis
     Center,
     /// Each line will stretch to fill the remaining space
-    #[default]
     Stretch,
     /// Each line fills the space it needs, putting the remaining space, if any
     /// inbetween the lines
@@ -331,14 +373,23 @@ pub enum AlignContent {
     SpaceAround,
 }
 
+impl AlignContent {
+    pub const DEFAULT: Self = Self::Stretch;
+}
+
+impl Default for AlignContent {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Defines the text direction
 ///
 /// For example English is written LTR (left-to-right) while Arabic is written RTL (right-to-left).
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum Direction {
     /// Inherit from parent node
-    #[default]
     Inherit,
     /// Text is written left to right
     LeftToRight,
@@ -346,14 +397,23 @@ pub enum Direction {
     RightToLeft,
 }
 
+impl Direction {
+    pub const DEFAULT: Self = Self::Inherit;
+}
+
+impl Default for Direction {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Whether to use a Flexbox layout model.
 ///
 /// Part of the [`Style`] component.
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum Display {
     /// Use Flexbox layout model to determine the position of this [`Node`].
-    #[default]
     Flex,
     /// Use no layout, don't render this node and its children.
     ///
@@ -362,12 +422,21 @@ pub enum Display {
     None,
 }
 
+impl Display {
+    pub const DEFAULT: Self = Self::Flex;
+}
+
+impl Default for Display {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Defines how flexbox items are ordered within a flexbox
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum FlexDirection {
     /// Same way as text direction along the main axis
-    #[default]
     Row,
     /// Flex from top to bottom
     Column,
@@ -377,12 +446,21 @@ pub enum FlexDirection {
     ColumnReverse,
 }
 
+impl FlexDirection {
+    pub const DEFAULT: Self = Self::Row;
+}
+
+impl Default for FlexDirection {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Defines how items are aligned according to the main axis
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum JustifyContent {
     /// Pushed towards the start
-    #[default]
     FlexStart,
     /// Pushed towards the end
     FlexEnd,
@@ -396,23 +474,41 @@ pub enum JustifyContent {
     SpaceEvenly,
 }
 
+impl JustifyContent {
+    pub const DEFAULT: Self = Self::FlexStart;
+}
+
+impl Default for JustifyContent {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Whether to show or hide overflowing items
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Reflect, Serialize, Deserialize)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Reflect, Serialize, Deserialize)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum Overflow {
     /// Show overflowing items
-    #[default]
     Visible,
     /// Hide overflowing items
     Hidden,
 }
 
+impl Overflow {
+    pub const DEFAULT: Self = Self::Visible;
+}
+
+impl Default for Overflow {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// The strategy used to position this node
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum PositionType {
     /// Relative to all other nodes with the [`PositionType::Relative`] value
-    #[default]
     Relative,
     /// Independent of all other nodes
     ///
@@ -420,12 +516,21 @@ pub enum PositionType {
     Absolute,
 }
 
+impl PositionType {
+    const DEFAULT: Self = Self::Relative;
+}
+
+impl Default for PositionType {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// Defines if flexbox items appear on a single line or on multiple lines
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Default, Serialize, Deserialize, Reflect)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Reflect)]
 #[reflect(PartialEq, Serialize, Deserialize)]
 pub enum FlexWrap {
     /// Single line, will overflow if needed
-    #[default]
     NoWrap,
     /// Multiple lines, if needed
     Wrap,
@@ -433,8 +538,18 @@ pub enum FlexWrap {
     WrapReverse,
 }
 
+impl FlexWrap {
+    const DEFAULT: Self = Self::NoWrap;
+}
+
+impl Default for FlexWrap {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// The calculated size of the node
-#[derive(Component, Default, Copy, Clone, Debug, Reflect)]
+#[derive(Component, Copy, Clone, Debug, Reflect)]
 #[reflect(Component)]
 pub struct CalculatedSize {
     /// The size of the node
@@ -443,14 +558,37 @@ pub struct CalculatedSize {
     pub preserve_aspect_ratio: bool,
 }
 
+impl CalculatedSize {
+    const DEFAULT: Self = Self {
+        size: Size::DEFAULT,
+        preserve_aspect_ratio: false,
+    };
+}
+
+impl Default for CalculatedSize {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 /// The background color of the node
 ///
 /// This serves as the "fill" color.
 /// When combined with [`UiImage`], tints the provided texture.
-#[derive(Component, Default, Copy, Clone, Debug, Reflect)]
+#[derive(Component, Copy, Clone, Debug, Reflect)]
 #[reflect(Component, Default)]
 pub struct BackgroundColor(pub Color);
 
+impl BackgroundColor {
+    pub const DEFAULT: Self = Self(Color::WHITE);
+}
+
+impl Default for BackgroundColor {
+    fn default() -> Self {
+        Self::DEFAULT
+    }
+}
+
 impl From<Color> for BackgroundColor {
     fn from(color: Color) -> Self {
         Self(color)