Mark ghost nodes as experimental and partially feature flag them (#15961)

# Objective

As discussed in #15341, ghost nodes are a contentious and experimental
feature. In the interest of enabling ecosystem experimentation, we've
decided to keep them in Bevy 0.15.

That said, we don't use them internally, and don't expect third-party
crates to support them. If the experimentation returns a negative result
(they aren't very useful, an alternative design is preferred etc) they
will be removed.

We should clearly communicate this status to users, and make sure that
users don't use ghost nodes in their projects without a very clear
understanding of what they're getting themselves into.

## Solution

To make life easy for users (and Bevy), `GhostNode` and all associated
helpers remain public and are always available.

However, actually constructing these requires enabling a feature flag
that's clearly marked as experimental. To do so, I've added a
meaningless private field.

When the feature flag is enabled, our constructs (`new` and `default`)
can be used. I've added a `new` constructor, which should be preferred
over `Default::default` as that can be readily deprecated, allowing us
to prompt users to swap over to the much nicer `GhostNode` syntax once
this is a unit struct again.

Full credit: this was mostly @cart's design: I'm just implementing it!

## Testing

I've run the ghost_nodes example and it fails to compile without the
feature flag. With the feature flag, it works fine :)

---------

Co-authored-by: Zachary Harrold <[email protected]>

Author: alice-i-cecile

Cargo.toml

@@ -444,6 +444,9 @@ reflect_functions = ["bevy_internal/reflect_functions"]
 # Enable winit custom cursor support
 custom_cursor = ["bevy_internal/custom_cursor"]
 
+# Experimental support for nodes that are ignored for UI layouting
+ghost_nodes = ["bevy_internal/ghost_nodes"]
+
 [dependencies]
 bevy_internal = { path = "crates/bevy_internal", version = "0.15.0-dev", default-features = false }
 
@@ -3081,6 +3084,7 @@ wasm = true
 name = "ghost_nodes"
 path = "examples/ui/ghost_nodes.rs"
 doc-scrape-examples = true
+required-features = ["ghost_nodes"]
 
 [package.metadata.example.ghost_nodes]
 name = "Ghost Nodes"

crates/bevy_internal/Cargo.toml

@@ -241,6 +241,9 @@ reflect_functions = [
 # Enable winit custom cursor support
 custom_cursor = ["bevy_winit/custom_cursor"]
 
+# Experimental support for nodes that are ignored for UI layouting
+ghost_nodes = ["bevy_ui/ghost_nodes"]
+
 [dependencies]
 # bevy
 bevy_a11y = { path = "../bevy_a11y", version = "0.15.0-dev" }

crates/bevy_ui/Cargo.toml

@@ -49,6 +49,9 @@ default = ["bevy_ui_picking_backend"]
 serialize = ["serde", "smallvec/serde", "bevy_math/serialize"]
 bevy_ui_picking_backend = ["bevy_picking"]
 
+# Experimental features
+ghost_nodes = []
+
 [lints]
 workspace = true
 

crates/bevy_ui/src/accessibility.rs

@@ -1,7 +1,8 @@
 use crate::{
+    experimental::UiChildren,
     prelude::{Button, Label},
     widget::TextUiReader,
-    Node, UiChildren, UiImage,
+    Node, UiImage,
 };
 use bevy_a11y::{
     accesskit::{NodeBuilder, Rect, Role},

crates/bevy_ui/src/ghost_hierarchy.rs renamed to crates/bevy_ui/src/experimental/ghost_hierarchy.rs

@@ -5,6 +5,7 @@ use bevy_hierarchy::{Children, HierarchyQueryExt, Parent};
 use bevy_reflect::prelude::*;
 use bevy_render::view::Visibility;
 use bevy_transform::prelude::Transform;
+use core::marker::PhantomData;
 use smallvec::SmallVec;
 
 use crate::Node;
@@ -14,10 +15,30 @@ use crate::Node;
 /// 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.
-#[derive(Component, Default, Debug, Copy, Clone, Reflect)]
+///
+/// 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,
+        }
+    }
+}
 
 /// System param that allows iteration of all UI root nodes.
 ///
@@ -140,7 +161,7 @@ impl<'w, 's> Iterator for UiChildrenIter<'w, 's> {
     }
 }
 
-#[cfg(test)]
+#[cfg(all(test, feature = "ghost_nodes"))]
 mod tests {
     use bevy_ecs::{
         prelude::Component,
@@ -165,18 +186,20 @@ mod tests {
             .with_children(|parent| {
                 parent.spawn((A(2), NodeBundle::default()));
                 parent
-                    .spawn((A(3), GhostNode))
+                    .spawn((A(3), GhostNode::new()))
                     .with_child((A(4), NodeBundle::default()));
             });
 
         // Ghost root
-        world.spawn((A(5), GhostNode)).with_children(|parent| {
-            parent.spawn((A(6), NodeBundle::default()));
-            parent
-                .spawn((A(7), GhostNode))
-                .with_child((A(8), NodeBundle::default()))
-                .with_child(A(9));
-        });
+        world
+            .spawn((A(5), GhostNode::new()))
+            .with_children(|parent| {
+                parent.spawn((A(6), NodeBundle::default()));
+                parent
+                    .spawn((A(7), GhostNode::new()))
+                    .with_child((A(8), NodeBundle::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);
@@ -191,15 +214,15 @@ mod tests {
         let world = &mut World::new();
 
         let n1 = world.spawn((A(1), NodeBundle::default())).id();
-        let n2 = world.spawn((A(2), GhostNode)).id();
-        let n3 = world.spawn((A(3), GhostNode)).id();
+        let n2 = world.spawn((A(2), GhostNode::new())).id();
+        let n3 = world.spawn((A(3), GhostNode::new())).id();
         let n4 = world.spawn((A(4), NodeBundle::default())).id();
         let n5 = world.spawn((A(5), NodeBundle::default())).id();
 
-        let n6 = world.spawn((A(6), GhostNode)).id();
-        let n7 = world.spawn((A(7), GhostNode)).id();
+        let n6 = world.spawn((A(6), GhostNode::new())).id();
+        let n7 = world.spawn((A(7), GhostNode::new())).id();
         let n8 = world.spawn((A(8), NodeBundle::default())).id();
-        let n9 = world.spawn((A(9), GhostNode)).id();
+        let n9 = world.spawn((A(9), GhostNode::new())).id();
         let n10 = world.spawn((A(10), NodeBundle::default())).id();
 
         let no_ui = world.spawn_empty().id();

crates/bevy_ui/src/experimental/mod.rs

@@ -0,0 +1,17 @@
+//! Experimental features are not yet stable and may change or be removed in the future.
+//!
+//! These features are not recommended for production use, but are available to ease experimentation
+//! within Bevy's ecosystem. Please let us know how you are using these features and what you would
+//! like to see improved!
+//!
+//! These may be feature-flagged: check the `Cargo.toml` for `bevy_ui` to see what options
+//! are available.
+//!
+//! # Warning
+//!
+//! Be careful when using these features, especially in concert with third-party crates,
+//! as they may not be fully supported, functional or stable.
+
+mod ghost_hierarchy;
+
+pub use ghost_hierarchy::*;

crates/bevy_ui/src/layout/mod.rs

@@ -1,6 +1,7 @@
 use crate::{
+    experimental::{UiChildren, UiRootNodes},
     BorderRadius, ContentSize, DefaultUiCamera, Display, Node, Outline, OverflowAxis,
-    ScrollPosition, Style, TargetCamera, UiChildren, UiRootNodes, UiScale,
+    ScrollPosition, Style, TargetCamera, UiScale,
 };
 use bevy_ecs::{
     change_detection::{DetectChanges, DetectChangesMut},

crates/bevy_ui/src/lib.rs

@@ -23,17 +23,18 @@ pub mod picking_backend;
 use bevy_derive::{Deref, DerefMut};
 use bevy_reflect::{std_traits::ReflectDefault, Reflect};
 mod accessibility;
+// This module is not re-exported, but is instead made public.
+// This is intended to discourage accidental use of the experimental API.
+pub mod experimental;
 mod focus;
 mod geometry;
-mod ghost_hierarchy;
 mod layout;
 mod render;
 mod stack;
 mod ui_node;
 
 pub use focus::*;
 pub use geometry::*;
-pub use ghost_hierarchy::*;
 pub use layout::*;
 pub use measurement::*;
 pub use render::*;

crates/bevy_ui/src/stack.rs

@@ -3,7 +3,10 @@
 use bevy_ecs::prelude::*;
 use bevy_utils::HashSet;
 
-use crate::{GlobalZIndex, Node, UiChildren, UiRootNodes, ZIndex};
+use crate::{
+    experimental::{UiChildren, UiRootNodes},
+    GlobalZIndex, Node, ZIndex,
+};
 
 /// The current UI stack, which contains all UI nodes ordered by their depth (back-to-front).
 ///

crates/bevy_ui/src/update.rs

@@ -1,6 +1,9 @@
 //! This module contains systems that update the UI when something changes
 
-use crate::{CalculatedClip, Display, OverflowAxis, Style, TargetCamera, UiChildren, UiRootNodes};
+use crate::{
+    experimental::{UiChildren, UiRootNodes},
+    CalculatedClip, Display, OverflowAxis, Style, TargetCamera,
+};
 
 use super::Node;
 use bevy_ecs::{