Change GhostNode into a unit type (#17692)

# Objective

The feature gates for the `UiChildren` and `UiRootNodes` system params
make the unconstructable `GhostNode` `PhantomData` trick redundant.


## Solution

Remove the `GhostNode::new` method and change `GhostNode` into a unit
struct.

## Testing

```cargo run --example ghost_nodes```

still works

crates/bevy_ui/src/experimental/ghost_hierarchy.rs

@@ -2,11 +2,13 @@
 
 use crate::Node;
 use bevy_ecs::{prelude::*, system::SystemParam};
-use bevy_reflect::prelude::*;
-use bevy_render::view::Visibility;
-use bevy_transform::prelude::Transform;
-use core::marker::PhantomData;
 
+#[cfg(feature = "ghost_nodes")]
+use bevy_reflect::prelude::*;
+#[cfg(feature = "ghost_nodes")]
+use bevy_render::view::Visibility;
+#[cfg(feature = "ghost_nodes")]
+use bevy_transform::prelude::Transform;
 #[cfg(feature = "ghost_nodes")]
 use smallvec::SmallVec;
 
@@ -15,30 +17,12 @@ use smallvec::SmallVec;
 /// The UI systems will traverse past these and treat their first non-ghost descendants as direct children of their first non-ghost ancestor.
 ///
 /// Any components necessary for transform and visibility propagation will be added automatically.
-///
+#[cfg(feature = "ghost_nodes")]
-/// Instances of this type cannot be constructed unless the `ghost_nodes` feature is enabled.
 #[derive(Component, Debug, Copy, Clone, Reflect)]
 #[cfg_attr(feature = "ghost_nodes", derive(Default))]
 #[reflect(Component, Debug)]
 #[require(Visibility, Transform)]
-pub struct GhostNode {
+pub struct GhostNode;
-    // This is a workaround to ensure that GhostNode is only constructable when the appropriate feature flag is enabled
-    #[reflect(ignore)]
-    unconstructable: PhantomData<()>, // Spooky!
-}
-
-#[cfg(feature = "ghost_nodes")]
-impl GhostNode {
-    /// Creates a new ghost node.
-    ///
-    /// This method is only available when the `ghost_node` feature is enabled,
-    /// and will eventually be deprecated then removed in favor of simply using `GhostNode` as no meaningful data is stored.
-    pub const fn new() -> Self {
-        GhostNode {
-            unconstructable: PhantomData,
-        }
-    }
-}
 
 #[cfg(feature = "ghost_nodes")]
 /// System param that allows iteration of all UI root nodes.
@@ -232,20 +216,18 @@ mod tests {
             .with_children(|parent| {
                 parent.spawn((A(2), Node::default()));
                 parent
-                    .spawn((A(3), GhostNode::new()))
+                    .spawn((A(3), GhostNode))
                     .with_child((A(4), Node::default()));
             });
 
         // Ghost root
-        world
+        world.spawn((A(5), GhostNode)).with_children(|parent| {
-            .spawn((A(5), GhostNode::new()))
+            parent.spawn((A(6), Node::default()));
-            .with_children(|parent| {
+            parent
-                parent.spawn((A(6), Node::default()));
+                .spawn((A(7), GhostNode))
-                parent
+                .with_child((A(8), Node::default()))
-                    .spawn((A(7), GhostNode::new()))
+                .with_child(A(9));
-                    .with_child((A(8), Node::default()))
+        });
-                    .with_child(A(9));
-            });
 
         let mut system_state = SystemState::<(UiRootNodes, Query<&A>)>::new(world);
         let (ui_root_nodes, a_query) = system_state.get(world);
@@ -260,15 +242,15 @@ mod tests {
         let world = &mut World::new();
 
         let n1 = world.spawn((A(1), Node::default())).id();
-        let n2 = world.spawn((A(2), GhostNode::new())).id();
+        let n2 = world.spawn((A(2), GhostNode)).id();
-        let n3 = world.spawn((A(3), GhostNode::new())).id();
+        let n3 = world.spawn((A(3), GhostNode)).id();
         let n4 = world.spawn((A(4), Node::default())).id();
         let n5 = world.spawn((A(5), Node::default())).id();
 
-        let n6 = world.spawn((A(6), GhostNode::new())).id();
+        let n6 = world.spawn((A(6), GhostNode)).id();
-        let n7 = world.spawn((A(7), GhostNode::new())).id();
+        let n7 = world.spawn((A(7), GhostNode)).id();
         let n8 = world.spawn((A(8), Node::default())).id();
-        let n9 = world.spawn((A(9), GhostNode::new())).id();
+        let n9 = world.spawn((A(9), GhostNode)).id();
         let n10 = world.spawn((A(10), Node::default())).id();
 
         let no_ui = world.spawn_empty().id();

examples/ui/ghost_nodes.rs

@@ -7,8 +7,7 @@
 //! This is an experimental feature, and should be used with caution,
 //! especially in concert with 3rd party plugins or systems that may not be aware of ghost nodes.
 //!
-//! To add [`GhostNode`] components to entities, you must enable the `ghost_nodes` feature flag,
+//! In order to use [`GhostNode`]s you must enable the `ghost_nodes` feature flag.
-//! as they are otherwise unconstructable even though the type is defined.
 
 use bevy::{prelude::*, ui::experimental::GhostNode, winit::WinitSettings};
 
@@ -30,14 +29,12 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
     commands.spawn(Camera2d);
 
     // Ghost UI root
-    commands
+    commands.spawn(GhostNode).with_children(|ghost_root| {
-        .spawn(GhostNode::new())
+        ghost_root.spawn(Node::default()).with_child(create_label(
-        .with_children(|ghost_root| {
+            "This text node is rendered under a ghost root",
-            ghost_root.spawn(Node::default()).with_child(create_label(
+            font_handle.clone(),
-                "This text node is rendered under a ghost root",
+        ));
-                font_handle.clone(),
+    });
-            ));
-        });
 
     // Normal UI root
     commands
@@ -53,7 +50,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
                 .spawn((Node::default(), Counter(0)))
                 .with_children(|layout_parent| {
                     layout_parent
-                        .spawn((GhostNode::new(), Counter(0)))
+                        .spawn((GhostNode, Counter(0)))
                         .with_children(|ghost_parent| {
                             // Ghost children using a separate counter state
                             // These buttons are being treated as children of layout_parent in the context of UI