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

# Objective
- Fixes bevyengine#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 <[email protected]>

crates/bevy_ui/src/focus.rs

@@ -31,32 +31,47 @@ 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
     Clicked,
     /// 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 {