Renamed Entity::new to Entity::from_raw (#3465)

# Objective

- Rename `Entity::new(id: u32)` to `Entity::from_raw(id: u32)`.
- Add further documentation.
- fixes #3108

## Solution

- Renamed `Entity::new(id: u32)` to `Entity::from_raw(id: u32)`.
- Docs extended.

I derived the examples from the discussion of issue #3108 .

The [first case](https://github.com/bevyengine/bevy/issues/3108#issuecomment-966669781) mentioned in the linked issue is quite obvious but the [second one](https://github.com/bevyengine/bevy/issues/3108#issuecomment-967093902) probably needs further explanation.


Co-authored-by: r4gus <david@thesugar.de>

benches/benches/bevy_ecs/commands.rs

@@ -250,7 +250,7 @@ fn get_or_spawn(criterion: &mut Criterion) {
             let mut commands = Commands::new(&mut command_queue, &world);
             for i in 0..10_000 {
                 commands
-                    .get_or_spawn(Entity::new(i))
+                    .get_or_spawn(Entity::from_raw(i))
                     .insert_bundle((Matrix::default(), Vec3::default()));
             }
             command_queue.apply(&mut world);
@@ -265,7 +265,7 @@ fn get_or_spawn(criterion: &mut Criterion) {
             let mut commands = Commands::new(&mut command_queue, &world);
             let mut values = Vec::with_capacity(10_000);
             for i in 0..10_000 {
-                values.push((Entity::new(i), (Matrix::default(), Vec3::default())));
+                values.push((Entity::from_raw(i), (Matrix::default(), Vec3::default())));
             }
             commands.insert_or_spawn_batch(values);
             command_queue.apply(&mut world);

benches/benches/bevy_ecs/world_get.rs

@@ -37,7 +37,7 @@ fn world_entity(criterion: &mut Criterion) {
 
             bencher.iter(|| {
                 for i in 0..entity_count {
-                    let entity = Entity::new(i);
+                    let entity = Entity::from_raw(i);
                     black_box(world.entity(entity));
                 }
             });
@@ -58,7 +58,7 @@ fn world_get(criterion: &mut Criterion) {
 
             bencher.iter(|| {
                 for i in 0..entity_count {
-                    let entity = Entity::new(i);
+                    let entity = Entity::from_raw(i);
                     assert!(world.get::<Table>(entity).is_some());
                 }
             });
@@ -68,7 +68,7 @@ fn world_get(criterion: &mut Criterion) {
 
             bencher.iter(|| {
                 for i in 0..entity_count {
-                    let entity = Entity::new(i);
+                    let entity = Entity::from_raw(i);
                     assert!(world.get::<Sparse>(entity).is_some());
                 }
             });
@@ -90,7 +90,7 @@ fn world_query_get(criterion: &mut Criterion) {
 
             bencher.iter(|| {
                 for i in 0..entity_count {
-                    let entity = Entity::new(i);
+                    let entity = Entity::from_raw(i);
                     assert!(query.get(&world, entity).is_ok());
                 }
             });
@@ -101,7 +101,7 @@ fn world_query_get(criterion: &mut Criterion) {
 
             bencher.iter(|| {
                 for i in 0..entity_count {
-                    let entity = Entity::new(i);
+                    let entity = Entity::from_raw(i);
                     assert!(query.get(&world, entity).is_ok());
                 }
             });

crates/bevy_ecs/src/entity/mod.rs

@@ -57,15 +57,54 @@ pub enum AllocAtWithoutReplacement {
 }
 
 impl Entity {
-    /// Creates a new entity reference with a generation of 0.
+    /// Creates a new entity reference with the specified `id` and a generation of 0.
     ///
     /// # Note
     ///
-    /// Spawning a specific `entity` value is rarely the right choice. Most apps should favor
+    /// Spawning a specific `entity` value is __rarely the right choice__. Most apps should favor
     /// [`Commands::spawn`](crate::system::Commands::spawn). This method should generally
     /// only be used for sharing entities across apps, and only when they have a scheme
     /// worked out to share an ID space (which doesn't happen by default).
-    pub fn new(id: u32) -> Entity {
+    ///
+    /// In general, one should not try to synchronize the ECS by attempting to ensure that
+    /// `Entity` lines up between instances, but instead insert a secondary identifier as
+    /// a component.
+    ///
+    /// There are still some use cases where it might be appropriate to use this function
+    /// externally.
+    ///
+    /// ## Examples
+    ///
+    /// Initializing a collection (e.g. `array` or `Vec`) with a known size:
+    ///
+    /// ```no_run
+    /// # use bevy_ecs::prelude::*;
+    /// // Create a new array of size 10 and initialize it with (invalid) entities.
+    /// let mut entities: [Entity; 10] = [Entity::from_raw(0); 10];
+    ///
+    /// // ... replace the entities with valid ones.
+    /// ```
+    ///
+    /// Deriving `Reflect` for a component that has an `Entity` field:
+    ///
+    /// ```no_run
+    /// # use bevy_ecs::{prelude::*, component::*};
+    /// # use bevy_reflect::Reflect;
+    /// #[derive(Reflect, Component)]
+    /// #[reflect(Component)]
+    /// pub struct MyStruct {
+    ///     pub entity: Entity,
+    /// }
+    ///
+    /// impl FromWorld for MyStruct {
+    ///     fn from_world(_world: &mut World) -> Self {
+    ///         Self {
+    ///             entity: Entity::from_raw(u32::MAX),
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    pub fn from_raw(id: u32) -> Entity {
         Entity { id, generation: 0 }
     }
 
@@ -120,7 +159,7 @@ impl SparseSetIndex for Entity {
     }
 
     fn get_sparse_set_index(value: usize) -> Self {
-        Entity::new(value as u32)
+        Entity::from_raw(value as u32)
     }
 }
 

crates/bevy_ecs/src/entity/serde.rs

@@ -32,6 +32,6 @@ impl<'de> Visitor<'de> for EntityVisitor {
     where
         E: serde::de::Error,
     {
-        Ok(Entity::new(v))
+        Ok(Entity::from_raw(v))
     }
 }

crates/bevy_ecs/src/lib.rs

@@ -1307,8 +1307,8 @@ mod tests {
         let mut world_a = World::new();
         let world_b = World::new();
         let mut query = world_a.query::<&A>();
-        let _ = query.get(&world_a, Entity::new(0));
-        let _ = query.get(&world_b, Entity::new(0));
+        let _ = query.get(&world_a, Entity::from_raw(0));
+        let _ = query.get(&world_b, Entity::from_raw(0));
     }
 
     #[test]
@@ -1531,7 +1531,7 @@ mod tests {
     fn insert_or_spawn_batch() {
         let mut world = World::default();
         let e0 = world.spawn().insert(A(0)).id();
-        let e1 = Entity::new(1);
+        let e1 = Entity::from_raw(1);
 
         let values = vec![(e0, (B(0), C)), (e1, (B(1), C))];
 
@@ -1568,7 +1568,7 @@ mod tests {
     fn insert_or_spawn_batch_invalid() {
         let mut world = World::default();
         let e0 = world.spawn().insert(A(0)).id();
-        let e1 = Entity::new(1);
+        let e1 = Entity::from_raw(1);
         let e2 = world.spawn().id();
         let invalid_e2 = Entity {
             generation: 1,

crates/bevy_ecs/src/storage/sparse_set.rs

@@ -448,11 +448,11 @@ mod tests {
     #[test]
     fn sparse_set() {
         let mut set = SparseSet::<Entity, Foo>::default();
-        let e0 = Entity::new(0);
-        let e1 = Entity::new(1);
-        let e2 = Entity::new(2);
-        let e3 = Entity::new(3);
-        let e4 = Entity::new(4);
+        let e0 = Entity::from_raw(0);
+        let e1 = Entity::from_raw(1);
+        let e2 = Entity::from_raw(2);
+        let e3 = Entity::from_raw(3);
+        let e4 = Entity::from_raw(4);
 
         set.insert(e1, Foo(1));
         set.insert(e2, Foo(2));

crates/bevy_ecs/src/storage/table.rs

@@ -540,7 +540,7 @@ mod tests {
         let columns = &[component_id];
         let mut table = Table::with_capacity(0, columns.len());
         table.add_column(components.get_info(component_id).unwrap());
-        let entities = (0..200).map(Entity::new).collect::<Vec<_>>();
+        let entities = (0..200).map(Entity::from_raw).collect::<Vec<_>>();
         for entity in entities.iter() {
             // SAFE: we allocate and immediately set data afterwards
             unsafe {

crates/bevy_scene/src/dynamic_scene.rs

@@ -87,7 +87,7 @@ impl DynamicScene {
             // or spawn a new entity with a transiently unique id if there is
             // no corresponding entry.
             let entity = *entity_map
-                .entry(bevy_ecs::entity::Entity::new(scene_entity.entity))
+                .entry(bevy_ecs::entity::Entity::from_raw(scene_entity.entity))
                 .or_insert_with(|| world.spawn().id());
 
             // Apply/ add each component to the given entity.

crates/bevy_transform/src/components/parent.rs

@@ -17,7 +17,7 @@ pub struct Parent(pub Entity);
 // better ways to handle cases like this.
 impl FromWorld for Parent {
     fn from_world(_world: &mut World) -> Self {
-        Parent(Entity::new(u32::MAX))
+        Parent(Entity::from_raw(u32::MAX))
     }
 }
 
@@ -56,6 +56,6 @@ impl MapEntities for PreviousParent {
 // TODO: Better handle this case see `impl FromWorld for Parent`
 impl FromWorld for PreviousParent {
     fn from_world(_world: &mut World) -> Self {
-        PreviousParent(Entity::new(u32::MAX))
+        PreviousParent(Entity::from_raw(u32::MAX))
     }
 }