bevy_ecs/world/

mod.rs

//! Defines the [`World`] and APIs for accessing it directly.

pub(crate) mod command_queue;
mod deferred_world;
mod entity_access;
mod entity_fetch;
mod filtered_resource;
mod identifier;
mod spawn_batch;

pub mod error;
#[cfg(feature = "bevy_reflect")]
pub mod reflect;
pub mod unsafe_world_cell;

pub use crate::{
    change_detection::{Mut, Ref, CHECK_TICK_THRESHOLD},
    world::command_queue::CommandQueue,
};
pub use bevy_ecs_macros::FromWorld;
pub use deferred_world::DeferredWorld;
pub use entity_access::{
    ComponentEntry, DynamicComponentFetch, EntityMut, EntityMutExcept, EntityRef, EntityRefExcept,
    EntityWorldMut, FilteredEntityMut, FilteredEntityRef, OccupiedComponentEntry,
    TryFromFilteredError, UnsafeFilteredEntityMut, VacantComponentEntry,
};
pub use entity_fetch::{EntityFetcher, WorldEntityFetch};
pub use filtered_resource::*;
pub use identifier::WorldId;
pub use spawn_batch::*;

use crate::{
    archetype::{ArchetypeCreated, ArchetypeId, Archetypes, ARCHETYPE_CREATED},
    bundle::{
        Bundle, BundleId, BundleInfo, BundleInserter, BundleSpawner, Bundles, DynamicBundle,
        InsertMode, NoBundleEffect,
    },
    change_detection::{
        CheckChangeTicks, ComponentTicks, ComponentTicksMut, MaybeLocation, MutUntyped, Tick,
    },
    component::{
        Component, ComponentDescriptor, ComponentId, ComponentIds, ComponentInfo, Components,
        ComponentsQueuedRegistrator, ComponentsRegistrator, Mutable, RequiredComponents,
        RequiredComponentsError,
    },
    entity::{Entities, Entity, EntityAllocator, EntityNotSpawnedError, SpawnError},
    entity_disabling::DefaultQueryFilters,
    error::{ErrorHandler, FallbackErrorHandler},
    lifecycle::{
        AddEvent, ComponentHooks, DespawnEvent, DiscardEvent, InsertEvent, RemoveEvent,
        RemovedComponentMessages, ADD, DESPAWN, DISCARD, INSERT, REMOVE,
    },
    message::{Message, MessageId, Messages, WriteBatchIds},
    observer::Observers,
    query::{DebugCheckedUnwrap, QueryData, QueryFilter, QueryState},
    relationship::RelationshipHookMode,
    resource::{IsResource, Resource, ResourceEntities, IS_RESOURCE},
    schedule::{Schedule, ScheduleLabel, Schedules},
    storage::{NonSendData, Storages},
    system::Commands,
    world::{
        command_queue::CommandQueueRunner,
        error::{
            EntityDespawnError, EntityMutableFetchError, TryInsertBatchError, TryRunScheduleError,
        },
    },
};
use alloc::{collections::VecDeque, vec::Vec};
use bevy_platform::{
    cell::SyncUnsafeCell,
    sync::atomic::{AtomicU32, Ordering},
};
use bevy_ptr::{move_as_ptr, MovingPtr, OwningPtr, Ptr};
use bevy_utils::prelude::DebugName;
use core::{any::TypeId, fmt, mem::ManuallyDrop};
use log::warn;
use unsafe_world_cell::{UnsafeEntityCell, UnsafeWorldCell};

/// Stores and exposes operations on [entities](Entity), [components](Component), resources,
/// and their associated metadata.
///
/// Each [`Entity`] has a set of unique components, based on their type.
/// Entity components can be created, updated, removed, and queried using a given [`World`].
///
/// For complex access patterns involving [`SystemParam`](crate::system::SystemParam),
/// consider using [`SystemState`](crate::system::SystemState).
///
/// To mutate different parts of the world simultaneously,
/// use [`World::resource_scope`] or [`SystemState`](crate::system::SystemState).
///
/// ## Resources
///
/// Worlds can also store [`Resource`]s,
/// which are unique instances of a given type that belong to a specific unique Entity.
/// There are also *non send resources*, which can only be accessed on the main thread.
/// These are stored outside of the ECS.
/// See [`Resource`] for usage.
pub struct World {
    id: WorldId,
    pub(crate) entities: Entities,
    pub(crate) entity_allocator: EntityAllocator,
    pub(crate) components: Components,
    pub(crate) component_ids: ComponentIds,
    pub(crate) resource_entities: ResourceEntities,
    pub(crate) archetypes: Archetypes,
    pub(crate) storages: Storages,
    pub(crate) bundles: Bundles,
    pub(crate) observers: Observers,
    pub(crate) removed_components: RemovedComponentMessages,
    pub(crate) change_tick: AtomicU32,
    pub(crate) last_change_tick: Tick,
    pub(crate) last_check_tick: Tick,
    pub(crate) last_trigger_id: u32,
    /// The byte index in [`Self::command_queue`] at which unapplied command start.
    ///
    /// This is nonzero while running commands to allow the same buffer to be shared by nested commands.
    command_queue_start: usize,
    /// The world's command queue.
    ///
    /// This is stored inside a [`SyncUnsafeCell`] to allow mutable access to
    /// commands from an [`UnsafeWorldCell`] without being invalidated by `&World`
    /// references used for metadata.
    ///
    /// This must not be exposed as a `&mut` to untrusted code,
    /// as calling `apply()` on it could execute commands before [`Self::command_queue_start`].
    command_queue: SyncUnsafeCell<CommandQueue>,
}

impl Default for World {
    fn default() -> Self {
        let mut world = Self {
            id: WorldId::new().expect("More `bevy` `World`s have been created than is supported"),
            entities: Entities::new(),
            entity_allocator: EntityAllocator::default(),
            components: Default::default(),
            resource_entities: Default::default(),
            archetypes: Archetypes::new(),
            storages: Default::default(),
            bundles: Default::default(),
            observers: Observers::default(),
            removed_components: Default::default(),
            // Default value is `1`, and `last_change_tick`s default to `0`, such that changes
            // are detected on first system runs and for direct world queries.
            change_tick: AtomicU32::new(1),
            last_change_tick: Tick::new(0),
            last_check_tick: Tick::new(0),
            last_trigger_id: 0,
            command_queue_start: 0,
            command_queue: SyncUnsafeCell::new(CommandQueue::silent()),
            component_ids: ComponentIds::default(),
        };
        world.bootstrap();
        world
    }
}

impl World {
    /// This performs initialization that _must_ happen for every [`World`] immediately upon creation (such as claiming specific component ids).
    /// This _must_ be run as part of constructing a [`World`], before it is returned to the caller.
    #[inline]
    fn bootstrap(&mut self) {
        // The order that we register these events is vital to ensure that the constants are correct!
        let on_add = self.register_event_key::<AddEvent>();
        assert_eq!(ADD, on_add);

        let on_insert = self.register_event_key::<InsertEvent>();
        assert_eq!(INSERT, on_insert);

        let on_discard = self.register_event_key::<DiscardEvent>();
        assert_eq!(DISCARD, on_discard);

        let on_remove = self.register_event_key::<RemoveEvent>();
        assert_eq!(REMOVE, on_remove);

        let on_despawn = self.register_event_key::<DespawnEvent>();
        assert_eq!(DESPAWN, on_despawn);

        let is_resource = self.register_component::<IsResource>();
        assert_eq!(IS_RESOURCE, is_resource);

        let archetype_created = self.register_event_key::<ArchetypeCreated>();
        assert_eq!(ARCHETYPE_CREATED, archetype_created);

        // This sets up `Disabled` as a disabling component, via the FromWorld impl
        self.init_resource::<DefaultQueryFilters>();
    }
    /// Creates a new empty [`World`].
    ///
    /// # Panics
    ///
    /// If [`usize::MAX`] [`World`]s have been created.
    /// This guarantee allows System Parameters to safely uniquely identify a [`World`],
    /// since its [`WorldId`] is unique
    #[inline]
    pub fn new() -> World {
        World::default()
    }

    /// Retrieves this [`World`]'s unique ID
    #[inline]
    pub fn id(&self) -> WorldId {
        self.id
    }

    /// Creates a new [`UnsafeWorldCell`] view with complete read+write access.
    #[inline]
    pub fn as_unsafe_world_cell(&mut self) -> UnsafeWorldCell<'_> {
        UnsafeWorldCell::new_mutable(self)
    }

    /// Creates a new [`UnsafeWorldCell`] view with only read access to everything.
    #[inline]
    pub fn as_unsafe_world_cell_readonly(&self) -> UnsafeWorldCell<'_> {
        UnsafeWorldCell::new_readonly(self)
    }

    /// Retrieves this world's [`Entities`] collection.
    #[inline]
    pub fn entities(&self) -> &Entities {
        &self.entities
    }

    /// Retrieves this world's [`EntityAllocator`] collection.
    #[inline]
    pub fn entity_allocator(&self) -> &EntityAllocator {
        &self.entity_allocator
    }

    /// Retrieves this world's [`EntityAllocator`] collection mutably.
    #[inline]
    pub fn entity_allocator_mut(&mut self) -> &mut EntityAllocator {
        &mut self.entity_allocator
    }

    /// Retrieves this world's [`Entities`] collection mutably.
    ///
    /// # Safety
    /// Mutable reference must not be used to put the [`Entities`] data
    /// in an invalid state for this [`World`]
    #[inline]
    pub unsafe fn entities_mut(&mut self) -> &mut Entities {
        &mut self.entities
    }

    /// Retrieves the number of [`Entities`] in the world.
    ///
    /// This is helpful as a diagnostic, but it can also be used effectively in tests.
    #[inline]
    pub fn entity_count(&self) -> u32 {
        self.entities.count_spawned()
    }

    /// Retrieves this world's [`Archetypes`] collection.
    #[inline]
    pub fn archetypes(&self) -> &Archetypes {
        &self.archetypes
    }

    /// Retrieves this world's [`Components`] collection.
    #[inline]
    pub fn components(&self) -> &Components {
        &self.components
    }

    /// Retrieves this world's [`ResourceEntities`].
    #[inline]
    pub fn resource_entities(&self) -> &ResourceEntities {
        &self.resource_entities
    }

    /// Prepares a [`ComponentsQueuedRegistrator`] for the world.
    /// **NOTE:** [`ComponentsQueuedRegistrator`] is easily misused.
    /// See its docs for important notes on when and how it should be used.
    #[inline]
    pub fn components_queue(&self) -> ComponentsQueuedRegistrator<'_> {
        // SAFETY: These are from the same world.
        unsafe { ComponentsQueuedRegistrator::new(&self.components, &self.component_ids) }
    }

    /// Prepares a [`ComponentsRegistrator`] for the world.
    #[inline]
    pub fn components_registrator(&mut self) -> ComponentsRegistrator<'_> {
        // SAFETY: These are from the same world.
        unsafe { ComponentsRegistrator::new(&mut self.components, &mut self.component_ids) }
    }

    /// Retrieves this world's [`Storages`] collection.
    #[inline]
    pub fn storages(&self) -> &Storages {
        &self.storages
    }

    /// Retrieves this world's [`Bundles`] collection.
    #[inline]
    pub fn bundles(&self) -> &Bundles {
        &self.bundles
    }

    /// Retrieves this world's [`RemovedComponentMessages`] collection
    #[inline]
    pub fn removed_components(&self) -> &RemovedComponentMessages {
        &self.removed_components
    }

    /// Retrieves this world's [`Observers`] list
    #[inline]
    pub fn observers(&self) -> &Observers {
        &self.observers
    }

    /// Creates a new [`Commands`] instance that writes to the world's command queue
    /// Use [`World::flush`] to apply all queued commands
    #[inline]
    pub fn commands(&mut self) -> Commands<'_, '_> {
        Commands::new_from_entities(
            self.command_queue.get_mut(),
            &self.entity_allocator,
            &self.entities,
        )
    }

    /// Registers a new [`Component`] type and returns the [`ComponentId`] created for it.
    ///
    /// # Usage Notes
    /// In most cases, you don't need to call this method directly since component registration
    /// happens automatically during system initialization.
    #[doc(alias = "register_resource")]
    pub fn register_component<T: Component>(&mut self) -> ComponentId {
        // This is a hot path, so return early to avoid the `Vec::new` in `ComponentsRegistrator`
        if let Some(id) = self.component_id::<T>() {
            return id;
        }

        self.components_registrator().register_component::<T>()
    }

    /// Registers a component type as "disabling",
    /// using [default query filters](DefaultQueryFilters) to exclude entities with the component from queries.
    pub fn register_disabling_component<C: Component>(&mut self) {
        let component_id = self.register_component::<C>();
        let mut dqf = self.resource_mut::<DefaultQueryFilters>();
        dqf.register_disabling_component(component_id);
    }

    /// Returns a mutable reference to the [`ComponentHooks`] for a [`Component`] type.
    ///
    /// Will panic if `T` exists in any archetypes.
    #[must_use]
    pub fn register_component_hooks<T: Component>(&mut self) -> &mut ComponentHooks {
        let index = self.register_component::<T>();
        assert!(!self.archetypes.archetypes.iter().any(|a| a.contains(index)), "Components hooks cannot be modified if the component already exists in an archetype, use register_component if {} may already be in use", core::any::type_name::<T>());
        // SAFETY: We just created this component
        unsafe { self.components.get_hooks_mut(index).debug_checked_unwrap() }
    }

    /// Returns a mutable reference to the [`ComponentHooks`] for a [`Component`] with the given id if it exists.
    ///
    /// Will panic if `id` exists in any archetypes.
    pub fn register_component_hooks_by_id(
        &mut self,
        id: ComponentId,
    ) -> Option<&mut ComponentHooks> {
        assert!(!self.archetypes.archetypes.iter().any(|a| a.contains(id)), "Components hooks cannot be modified if the component already exists in an archetype, use register_component if the component with id {id:?} may already be in use");
        self.components.get_hooks_mut(id)
    }

    /// Registers the given component `R` as a [required component] for `T`.
    ///
    /// When `T` is added to an entity, `R` and its own required components will also be added
    /// if `R` was not already provided. The [`Default`] `constructor` will be used for the creation of `R`.
    /// If a custom constructor is desired, use [`World::register_required_components_with`] instead.
    ///
    /// For the non-panicking version, see [`World::try_register_required_components`].
    ///
    /// Note that requirements must currently be registered before `T` is inserted into the world
    /// for the first time. This limitation may be fixed in the future.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Panics
    ///
    /// Panics if `R` is already a directly required component for `T`, or if `T` has ever been added
    /// on an entity before the registration.
    ///
    /// Indirect requirements through other components are allowed. In those cases, any existing requirements
    /// will only be overwritten if the new requirement is more specific.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct A;
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct B(usize);
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct C(u32);
    ///
    /// # let mut world = World::default();
    /// // Register B as required by A and C as required by B.
    /// world.register_required_components::<A, B>();
    /// world.register_required_components::<B, C>();
    ///
    /// // This will implicitly also insert B and C with their Default constructors.
    /// let id = world.spawn(A).id();
    /// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
    /// assert_eq!(&C(0), world.entity(id).get::<C>().unwrap());
    /// ```
    pub fn register_required_components<T: Component, R: Component + Default>(&mut self) {
        self.try_register_required_components::<T, R>().unwrap();
    }

    /// Registers the given component `R` as a [required component] for `T`.
    ///
    /// When `T` is added to an entity, `R` and its own required components will also be added
    /// if `R` was not already provided. The given `constructor` will be used for the creation of `R`.
    /// If a [`Default`] constructor is desired, use [`World::register_required_components`] instead.
    ///
    /// For the non-panicking version, see [`World::try_register_required_components_with`].
    ///
    /// Note that requirements must currently be registered before `T` is inserted into the world
    /// for the first time. This limitation may be fixed in the future.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Panics
    ///
    /// Panics if `R` is already a directly required component for `T`, or if `T` has ever been added
    /// on an entity before the registration.
    ///
    /// Indirect requirements through other components are allowed. In those cases, any existing requirements
    /// will only be overwritten if the new requirement is more specific.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct A;
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct B(usize);
    ///
    /// #[derive(Component, PartialEq, Eq, Debug)]
    /// struct C(u32);
    ///
    /// # let mut world = World::default();
    /// // Register B and C as required by A and C as required by B.
    /// // A requiring C directly will overwrite the indirect requirement through B.
    /// world.register_required_components::<A, B>();
    /// world.register_required_components_with::<B, C>(|| C(1));
    /// world.register_required_components_with::<A, C>(|| C(2));
    ///
    /// // This will implicitly also insert B with its Default constructor and C
    /// // with the custom constructor defined by A.
    /// let id = world.spawn(A).id();
    /// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
    /// assert_eq!(&C(2), world.entity(id).get::<C>().unwrap());
    /// ```
    pub fn register_required_components_with<T: Component, R: Component>(
        &mut self,
        constructor: impl Fn() -> R + 'static,
    ) {
        self.try_register_required_components_with::<T, R>(constructor)
            .unwrap();
    }

    /// Tries to register the given component `R` as a [required component] for `T`.
    ///
    /// When `T` is added to an entity, `R` and its own required components will also be added
    /// if `R` was not already provided. The [`Default`] `constructor` will be used for the creation of `R`.
    /// If a custom constructor is desired, use [`World::register_required_components_with`] instead.
    ///
    /// For the panicking version, see [`World::register_required_components`].
    ///
    /// Note that requirements must currently be registered before `T` is inserted into the world
    /// for the first time. This limitation may be fixed in the future.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Errors
    ///
    /// Returns a [`RequiredComponentsError`] if `R` is already a directly required component for `T`, or if `T` has ever been added
    /// on an entity before the registration.
    ///
    /// Indirect requirements through other components are allowed. In those cases, any existing requirements
    /// will only be overwritten if the new requirement is more specific.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct A;
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct B(usize);
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct C(u32);
    ///
    /// # let mut world = World::default();
    /// // Register B as required by A and C as required by B.
    /// world.register_required_components::<A, B>();
    /// world.register_required_components::<B, C>();
    ///
    /// // Duplicate registration! This will fail.
    /// assert!(world.try_register_required_components::<A, B>().is_err());
    ///
    /// // This will implicitly also insert B and C with their Default constructors.
    /// let id = world.spawn(A).id();
    /// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
    /// assert_eq!(&C(0), world.entity(id).get::<C>().unwrap());
    /// ```
    pub fn try_register_required_components<T: Component, R: Component + Default>(
        &mut self,
    ) -> Result<(), RequiredComponentsError> {
        self.try_register_required_components_with::<T, R>(R::default)
    }

    /// Tries to register the given component `R` as a [required component] for `T`.
    ///
    /// When `T` is added to an entity, `R` and its own required components will also be added
    /// if `R` was not already provided. The given `constructor` will be used for the creation of `R`.
    /// If a [`Default`] constructor is desired, use [`World::register_required_components`] instead.
    ///
    /// For the panicking version, see [`World::register_required_components_with`].
    ///
    /// Note that requirements must currently be registered before `T` is inserted into the world
    /// for the first time. This limitation may be fixed in the future.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Errors
    ///
    /// Returns a [`RequiredComponentsError`] if `R` is already a directly required component for `T`, or if `T` has ever been added
    /// on an entity before the registration.
    ///
    /// Indirect requirements through other components are allowed. In those cases, any existing requirements
    /// will only be overwritten if the new requirement is more specific.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct A;
    ///
    /// #[derive(Component, Default, PartialEq, Eq, Debug)]
    /// struct B(usize);
    ///
    /// #[derive(Component, PartialEq, Eq, Debug)]
    /// struct C(u32);
    ///
    /// # let mut world = World::default();
    /// // Register B and C as required by A and C as required by B.
    /// // A requiring C directly will overwrite the indirect requirement through B.
    /// world.register_required_components::<A, B>();
    /// world.register_required_components_with::<B, C>(|| C(1));
    /// world.register_required_components_with::<A, C>(|| C(2));
    ///
    /// // Duplicate registration! Even if the constructors were different, this would fail.
    /// assert!(world.try_register_required_components_with::<B, C>(|| C(1)).is_err());
    ///
    /// // This will implicitly also insert B with its Default constructor and C
    /// // with the custom constructor defined by A.
    /// let id = world.spawn(A).id();
    /// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
    /// assert_eq!(&C(2), world.entity(id).get::<C>().unwrap());
    /// ```
    pub fn try_register_required_components_with<T: Component, R: Component>(
        &mut self,
        constructor: impl Fn() -> R + 'static,
    ) -> Result<(), RequiredComponentsError> {
        let requiree = self.register_component::<T>();

        // TODO: Remove this panic and update archetype edges accordingly when required components are added
        if self.archetypes().component_index().contains_key(&requiree) {
            return Err(RequiredComponentsError::ArchetypeExists(requiree));
        }

        let required = self.register_component::<R>();

        // SAFETY: We just created the `required` and `requiree` components.
        unsafe {
            self.components
                .register_required_components::<R>(requiree, required, constructor)
        }
    }

    /// Retrieves the [required components](RequiredComponents) for the given component type, if it exists.
    pub fn get_required_components<C: Component>(&self) -> Option<&RequiredComponents> {
        let id = self.components().valid_component_id::<C>()?;
        let component_info = self.components().get_info(id)?;
        Some(component_info.required_components())
    }

    /// Retrieves the [required components](RequiredComponents) for the component of the given [`ComponentId`], if it exists.
    pub fn get_required_components_by_id(&self, id: ComponentId) -> Option<&RequiredComponents> {
        let component_info = self.components().get_info(id)?;
        Some(component_info.required_components())
    }

    /// Registers a new [`Component`] type and returns the [`ComponentId`] created for it.
    ///
    /// This method differs from [`World::register_component`] in that it uses a [`ComponentDescriptor`]
    /// to register the new component type instead of statically available type information. This
    /// enables the dynamic registration of new component definitions at runtime for advanced use cases.
    ///
    /// While the option to register a component from a descriptor is useful in type-erased
    /// contexts, the standard [`World::register_component`] function should always be used instead
    /// when type information is available at compile time.
    pub fn register_component_with_descriptor(
        &mut self,
        descriptor: ComponentDescriptor,
    ) -> ComponentId {
        self.components_registrator()
            .register_component_with_descriptor(descriptor)
    }

    /// Returns the [`ComponentId`] of the given [`Component`] type `T`.
    ///
    /// The returned `ComponentId` is specific to the `World` instance
    /// it was retrieved from and should not be used with another `World` instance.
    ///
    /// Returns [`None`] if the `Component` type has not yet been initialized within
    /// the `World` using [`World::register_component`].
    ///
    /// ```
    /// use bevy_ecs::prelude::*;
    ///
    /// let mut world = World::new();
    ///
    /// #[derive(Component)]
    /// struct ComponentA;
    ///
    /// let component_a_id = world.register_component::<ComponentA>();
    ///
    /// assert_eq!(component_a_id, world.component_id::<ComponentA>().unwrap())
    /// ```
    ///
    /// # See also
    ///
    /// * [`ComponentIdFor`](crate::component::ComponentIdFor)
    /// * [`Components::component_id()`]
    /// * [`Components::get_id()`]
    #[inline]
    pub fn component_id<T: Component>(&self) -> Option<ComponentId> {
        self.components.component_id::<T>()
    }

    /// Returns [`EntityRef`]s that expose read-only operations for the given
    /// `entities`. This will panic if any of the given entities do not exist. Use
    /// [`World::get_entity`] if you want to check for entity existence instead
    /// of implicitly panicking.
    ///
    /// This function supports fetching a single entity or multiple entities:
    /// - Pass an [`Entity`] to receive a single [`EntityRef`].
    /// - Pass a slice of [`Entity`]s to receive a [`Vec<EntityRef>`].
    /// - Pass an array of [`Entity`]s to receive an equally-sized array of [`EntityRef`]s.
    ///
    /// # Panics
    ///
    /// If any of the given `entities` do not exist in the world.
    ///
    /// # Examples
    ///
    /// ## Single [`Entity`]
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    ///
    /// let position = world.entity(entity).get::<Position>().unwrap();
    /// assert_eq!(position.x, 0.0);
    /// ```
    ///
    /// ## Array of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// let e2 = world.spawn(Position { x: 1.0, y: 1.0 }).id();
    ///
    /// let [e1_ref, e2_ref] = world.entity([e1, e2]);
    /// let e1_position = e1_ref.get::<Position>().unwrap();
    /// assert_eq!(e1_position.x, 0.0);
    /// let e2_position = e2_ref.get::<Position>().unwrap();
    /// assert_eq!(e2_position.x, 1.0);
    /// ```
    ///
    /// ## Slice of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e2 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e3 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    ///
    /// let ids = vec![e1, e2, e3];
    /// for eref in world.entity(&ids[..]) {
    ///     assert_eq!(eref.get::<Position>().unwrap().y, 1.0);
    /// }
    /// ```
    ///
    /// ## [`EntityHashSet`](crate::entity::EntityHashSet)
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, entity::EntityHashSet};
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e2 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e3 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    ///
    /// let ids = EntityHashSet::from_iter([e1, e2, e3]);
    /// for (_id, eref) in world.entity(&ids) {
    ///     assert_eq!(eref.get::<Position>().unwrap().y, 1.0);
    /// }
    /// ```
    ///
    /// [`EntityHashSet`]: crate::entity::EntityHashSet
    #[inline]
    #[track_caller]
    pub fn entity<F: WorldEntityFetch>(&self, entities: F) -> F::Ref<'_> {
        match self.get_entity(entities) {
            Ok(res) => res,
            Err(err) => panic!("{err}"),
        }
    }

    /// Returns [`EntityMut`]s that expose read and write operations for the
    /// given `entities`. This will panic if any of the given entities do not
    /// exist. Use [`World::get_entity_mut`] if you want to check for entity
    /// existence instead of implicitly panicking.
    ///
    /// This function supports fetching a single entity or multiple entities:
    /// - Pass an [`Entity`] to receive a single [`EntityWorldMut`].
    ///    - This reference type allows for structural changes to the entity,
    ///      such as adding or removing components, or despawning the entity.
    /// - Pass a slice of [`Entity`]s to receive a [`Vec<EntityMut>`].
    /// - Pass an array of [`Entity`]s to receive an equally-sized array of [`EntityMut`]s.
    /// - Pass a reference to a [`EntityHashSet`](crate::entity::EntityHashMap) to receive an
    ///   [`EntityHashMap<EntityMut>`](crate::entity::EntityHashMap).
    ///
    /// In order to perform structural changes on the returned entity reference,
    /// such as adding or removing components, or despawning the entity, only a
    /// single [`Entity`] can be passed to this function. Allowing multiple
    /// entities at the same time with structural access would lead to undefined
    /// behavior, so [`EntityMut`] is returned when requesting multiple entities.
    ///
    /// # Panics
    ///
    /// If any of the given `entities` do not exist in the world.
    ///
    /// # Examples
    ///
    /// ## Single [`Entity`]
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    ///
    /// let mut entity_mut = world.entity_mut(entity);
    /// let mut position = entity_mut.get_mut::<Position>().unwrap();
    /// position.y = 1.0;
    /// assert_eq!(position.x, 0.0);
    /// entity_mut.despawn();
    /// # assert!(world.get_entity_mut(entity).is_err());
    /// ```
    ///
    /// ## Array of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// let e2 = world.spawn(Position { x: 1.0, y: 1.0 }).id();
    ///
    /// let [mut e1_ref, mut e2_ref] = world.entity_mut([e1, e2]);
    /// let mut e1_position = e1_ref.get_mut::<Position>().unwrap();
    /// e1_position.x = 1.0;
    /// assert_eq!(e1_position.x, 1.0);
    /// let mut e2_position = e2_ref.get_mut::<Position>().unwrap();
    /// e2_position.x = 2.0;
    /// assert_eq!(e2_position.x, 2.0);
    /// ```
    ///
    /// ## Slice of [`Entity`]s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e2 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e3 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    ///
    /// let ids = vec![e1, e2, e3];
    /// for mut eref in world.entity_mut(&ids[..]) {
    ///     let mut pos = eref.get_mut::<Position>().unwrap();
    ///     pos.y = 2.0;
    ///     assert_eq!(pos.y, 2.0);
    /// }
    /// ```
    ///
    /// ## [`EntityHashSet`](crate::entity::EntityHashSet)
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, entity::EntityHashSet};
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e2 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    /// let e3 = world.spawn(Position { x: 0.0, y: 1.0 }).id();
    ///
    /// let ids = EntityHashSet::from_iter([e1, e2, e3]);
    /// for (_id, mut eref) in world.entity_mut(&ids) {
    ///     let mut pos = eref.get_mut::<Position>().unwrap();
    ///     pos.y = 2.0;
    ///     assert_eq!(pos.y, 2.0);
    /// }
    /// ```
    ///
    /// [`EntityHashSet`]: crate::entity::EntityHashSet
    #[inline]
    #[track_caller]
    pub fn entity_mut<F: WorldEntityFetch>(&mut self, entities: F) -> F::Mut<'_> {
        #[inline(never)]
        #[cold]
        #[track_caller]
        fn panic_on_err(e: EntityMutableFetchError) -> ! {
            panic!("{e}");
        }

        match self.get_entity_mut(entities) {
            Ok(fetched) => fetched,
            Err(e) => panic_on_err(e),
        }
    }

    /// Returns the components of an [`Entity`] through [`ComponentInfo`].
    #[inline]
    pub fn inspect_entity(
        &self,
        entity: Entity,
    ) -> Result<impl Iterator<Item = (ComponentId, &ComponentInfo)>, EntityNotSpawnedError> {
        let entity_location = self.entities().get_spawned(entity)?;

        let archetype = self
            .archetypes()
            .get(entity_location.archetype_id)
            .expect("ArchetypeId was retrieved from an EntityLocation and should correspond to an Archetype");

        Ok(archetype
            .iter_components()
            .filter_map(|id| self.components().get_info(id).map(|info| (id, info))))
    }

    /// Returns [`EntityRef`]s that expose read-only operations for the given
    /// `entities`, returning [`Err`] if any of the given entities do not exist.
    /// Instead of immediately unwrapping the value returned from this function,
    /// prefer [`World::entity`].
    ///
    /// This function supports fetching a single entity or multiple entities:
    /// - Pass an [`Entity`] to receive a single [`EntityRef`].
    /// - Pass a slice of [`Entity`]s to receive a [`Vec<EntityRef>`].
    /// - Pass an array of [`Entity`]s to receive an equally-sized array of [`EntityRef`]s.
    /// - Pass a reference to a [`EntityHashSet`](crate::entity::EntityHashMap) to receive an
    ///   [`EntityHashMap<EntityRef>`](crate::entity::EntityHashMap).
    ///
    /// # Errors
    ///
    /// If any of the given `entities` do not exist in the world, the first
    /// [`Entity`] found to be missing will return an [`EntityNotSpawnedError`].
    ///
    /// # Examples
    ///
    /// For examples, see [`World::entity`].
    ///
    /// [`EntityHashSet`]: crate::entity::EntityHashSet
    #[inline]
    pub fn get_entity<F: WorldEntityFetch>(
        &self,
        entities: F,
    ) -> Result<F::Ref<'_>, EntityNotSpawnedError> {
        let cell = self.as_unsafe_world_cell_readonly();
        // SAFETY: `&self` gives read access to the entire world, and prevents mutable access.
        unsafe { entities.fetch_ref(cell) }
    }

    /// Returns [`EntityMut`]s that expose read and write operations for the
    /// given `entities`, returning [`Err`] if any of the given entities do not
    /// exist. Instead of immediately unwrapping the value returned from this
    /// function, prefer [`World::entity_mut`].
    ///
    /// This function supports fetching a single entity or multiple entities:
    /// - Pass an [`Entity`] to receive a single [`EntityWorldMut`].
    ///    - This reference type allows for structural changes to the entity,
    ///      such as adding or removing components, or despawning the entity.
    /// - Pass a slice of [`Entity`]s to receive a [`Vec<EntityMut>`].
    /// - Pass an array of [`Entity`]s to receive an equally-sized array of [`EntityMut`]s.
    /// - Pass a reference to a [`EntityHashSet`](crate::entity::EntityHashMap) to receive an
    ///   [`EntityHashMap<EntityMut>`](crate::entity::EntityHashMap).
    ///
    /// In order to perform structural changes on the returned entity reference,
    /// such as adding or removing components, or despawning the entity, only a
    /// single [`Entity`] can be passed to this function. Allowing multiple
    /// entities at the same time with structural access would lead to undefined
    /// behavior, so [`EntityMut`] is returned when requesting multiple entities.
    ///
    /// # Errors
    ///
    /// - Returns [`EntityMutableFetchError::NotSpawned`] if any of the given `entities` do not exist in the world.
    ///     - Only the first entity found to be missing will be returned.
    /// - Returns [`EntityMutableFetchError::AliasedMutability`] if the same entity is requested multiple times.
    ///
    /// # Examples
    ///
    /// For examples, see [`World::entity_mut`].
    ///
    /// [`EntityHashSet`]: crate::entity::EntityHashSet
    #[inline]
    pub fn get_entity_mut<F: WorldEntityFetch>(
        &mut self,
        entities: F,
    ) -> Result<F::Mut<'_>, EntityMutableFetchError> {
        let cell = self.as_unsafe_world_cell();
        // SAFETY: `&mut self` gives mutable access to the entire world,
        // and prevents any other access to the world.
        unsafe { entities.fetch_mut(cell) }
    }

    /// Returns an [`Entity`] iterator of current entities.
    ///
    /// This is useful in contexts where you only have immutable access to the [`World`].
    /// If you have mutable access to the [`World`], use
    /// [`query()::<EntityRef>().iter(&world)`](World::query()) instead.
    ///
    /// Note that this does iterate through *all* entities, including resource entities.
    #[inline]
    pub fn iter_entities(&self) -> impl Iterator<Item = EntityRef<'_>> + '_ {
        self.archetypes.iter().flat_map(|archetype| {
            archetype
                .entities_with_location()
                .map(|(entity, location)| {
                    // SAFETY: entity exists and location accurately specifies the archetype where the entity is stored.
                    let cell = UnsafeEntityCell::new(
                        self.as_unsafe_world_cell_readonly(),
                        entity,
                        location,
                        self.last_change_tick,
                        self.read_change_tick(),
                    );
                    // SAFETY: `&self` gives read access to the entire world.
                    unsafe { EntityRef::new(cell) }
                })
        })
    }

    /// Simultaneously provides access to entity data and a command queue, which
    /// will be applied when the world is next flushed.
    ///
    /// This allows using borrowed entity data to construct commands where the
    /// borrow checker would otherwise prevent it.
    ///
    /// See [`DeferredWorld::entities_and_commands`] for the deferred version.
    ///
    /// # Example
    ///
    /// ```rust
    /// # use bevy_ecs::{prelude::*, world::DeferredWorld};
    /// #[derive(Component)]
    /// struct Targets(Vec<Entity>);
    /// #[derive(Component)]
    /// struct TargetedBy(Entity);
    ///
    /// let mut world: World = // ...
    /// #    World::new();
    /// # let e1 = world.spawn_empty().id();
    /// # let e2 = world.spawn_empty().id();
    /// # let eid = world.spawn(Targets(vec![e1, e2])).id();
    /// let (entities, mut commands) = world.entities_and_commands();
    ///
    /// let entity = entities.get(eid).unwrap();
    /// for &target in entity.get::<Targets>().unwrap().0.iter() {
    ///     commands.entity(target).insert(TargetedBy(eid));
    /// }
    /// # world.flush();
    /// # assert_eq!(world.get::<TargetedBy>(e1).unwrap().0, eid);
    /// # assert_eq!(world.get::<TargetedBy>(e2).unwrap().0, eid);
    /// ```
    pub fn entities_and_commands(&mut self) -> (EntityFetcher<'_>, Commands<'_, '_>) {
        let cell = self.as_unsafe_world_cell();
        // SAFETY: `&mut self` gives mutable access to the entire world, and prevents simultaneous access.
        let fetcher = unsafe { EntityFetcher::new(cell) };
        // SAFETY:
        // - `&mut self` gives mutable access to the entire world, and prevents simultaneous access.
        // - Command queue access does not conflict with entity access.
        let commands = unsafe { cell.commands() };

        (fetcher, commands)
    }

    /// Spawns the bundle on the valid but not spawned entity.
    /// If the entity can not be spawned for any reason, returns an error.
    ///
    /// If it succeeds, this declares the entity to have this bundle.
    ///
    /// In general, you should prefer [`spawn`](Self::spawn).
    /// Spawn internally calls this method, but it takes care of finding a suitable [`Entity`] for you.
    /// This is made available for advanced use, which you can see at [`EntityAllocator::alloc`].
    ///
    /// # Risk
    ///
    /// It is possible to spawn an `entity` that has not been allocated yet;
    /// however, doing so is currently a bad idea as the allocator may hand out this entity index in the future, assuming it to be not spawned.
    /// This would cause a panic.
    ///
    /// Manual spawning is a powerful tool, but must be used carefully.
    ///
    /// # Example
    ///
    /// Currently, this is primarily used to spawn entities that come from [`EntityAllocator::alloc`].
    /// See that for an example.
    #[track_caller]
    pub fn spawn_at<B: Bundle>(
        &mut self,
        entity: Entity,
        bundle: B,
    ) -> Result<EntityWorldMut<'_>, SpawnError> {
        move_as_ptr!(bundle);
        self.spawn_at_with_caller(entity, bundle, MaybeLocation::caller())
    }

    pub(crate) fn spawn_at_with_caller<B: Bundle>(
        &mut self,
        entity: Entity,
        bundle: MovingPtr<'_, B>,
        caller: MaybeLocation,
    ) -> Result<EntityWorldMut<'_>, SpawnError> {
        self.entities.check_can_spawn_at(entity)?;
        Ok(self.spawn_at_unchecked(entity, bundle, caller))
    }

    /// Spawns `bundle` on `entity`.
    ///
    /// # Panics
    ///
    /// Panics if the entity index is already constructed
    pub(crate) fn spawn_at_unchecked<B: Bundle>(
        &mut self,
        entity: Entity,
        bundle: MovingPtr<'_, B>,
        caller: MaybeLocation,
    ) -> EntityWorldMut<'_> {
        let change_tick = self.change_tick();
        let mut bundle_spawner = BundleSpawner::new::<B>(self, change_tick);
        let (bundle, entity_location) = bundle.partial_move(|bundle| {
            // SAFETY:
            // - `B` matches `bundle_spawner`'s type
            // -  `entity` is allocated but non-existent
            // - `B::Effect` is unconstrained, and `B::apply_effect` is called exactly once on the bundle after this call.
            // - This function ensures that the value pointed to by `bundle` must not be accessed for anything afterwards by consuming
            //   the `MovingPtr`. The value is otherwise only used to call `apply_effect` within this function, and the safety invariants
            //   of `DynamicBundle` ensure that only the elements that have not been moved out of by this call are accessed.
            unsafe { bundle_spawner.spawn_at::<B>(entity, bundle, caller) }
        });

        let mut entity_location = Some(entity_location);

        if !self.command_queue_is_empty() {
            self.flush();
            entity_location = self.entities().get_spawned(entity).ok();
        }

        // SAFETY: The entity and location started as valid.
        // If they were changed by commands, the location was updated to match.
        let mut entity = unsafe { EntityWorldMut::new(self, entity, entity_location) };
        // SAFETY:
        // - This is called exactly once after `get_components` has been called in `spawn_non_existent`.
        // - `bundle` had it's `get_components` function called exactly once inside `spawn_non_existent`.
        unsafe { B::apply_effect(bundle, &mut entity) };
        entity
    }

    /// A faster version of [`spawn_at`](Self::spawn_at) for the empty bundle.
    #[track_caller]
    pub fn spawn_empty_at(&mut self, entity: Entity) -> Result<EntityWorldMut<'_>, SpawnError> {
        self.spawn_empty_at_with_caller(entity, MaybeLocation::caller())
    }

    pub(crate) fn spawn_empty_at_with_caller(
        &mut self,
        entity: Entity,
        caller: MaybeLocation,
    ) -> Result<EntityWorldMut<'_>, SpawnError> {
        self.entities.check_can_spawn_at(entity)?;
        Ok(self.spawn_empty_at_unchecked(entity, caller))
    }

    /// A faster version of [`spawn_at_unchecked`](Self::spawn_at_unchecked) for the empty bundle.
    ///
    /// # Panics
    ///
    /// Panics if the entity index is already spawned
    pub(crate) fn spawn_empty_at_unchecked(
        &mut self,
        entity: Entity,
        caller: MaybeLocation,
    ) -> EntityWorldMut<'_> {
        // SAFETY: Locations are immediately made valid
        unsafe {
            let archetype = self.archetypes.empty_mut();
            // PERF: consider avoiding allocating entities in the empty archetype unless needed
            let table_row = self.storages.tables[archetype.table_id()].allocate(entity);
            // SAFETY: no components are allocated by archetype.allocate() because the archetype is
            // empty
            let location = archetype.allocate(entity, table_row);
            let change_tick = self.change_tick();
            let was_at = self.entities.set_location(entity.index(), Some(location));
            assert!(
                was_at.is_none(),
                "Attempting to construct an empty entity, but it was already constructed."
            );
            self.entities
                .mark_spawned_or_despawned(entity.index(), caller, change_tick);

            EntityWorldMut::new(self, entity, Some(location))
        }
    }

    /// Spawns a new [`Entity`] with a given [`Bundle`] of [components](`Component`) and returns
    /// a corresponding [`EntityWorldMut`], which can be used to add components to the entity or
    /// retrieve its id. In case large batches of entities need to be spawned, consider using
    /// [`World::spawn_batch`] instead.
    ///
    /// ```
    /// use bevy_ecs::{bundle::Bundle, component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// #[derive(Component)]
    /// struct Velocity {
    ///     x: f32,
    ///     y: f32,
    /// };
    ///
    /// #[derive(Component)]
    /// struct Name(&'static str);
    ///
    /// #[derive(Bundle)]
    /// struct PhysicsBundle {
    ///     position: Position,
    ///     velocity: Velocity,
    /// }
    ///
    /// let mut world = World::new();
    ///
    /// // `spawn` can accept a single component:
    /// world.spawn(Position { x: 0.0, y: 0.0 });
    ///
    /// // It can also accept a tuple of components:
    /// world.spawn((
    ///     Position { x: 0.0, y: 0.0 },
    ///     Velocity { x: 1.0, y: 1.0 },
    /// ));
    ///
    /// // Or it can accept a pre-defined Bundle of components:
    /// world.spawn(PhysicsBundle {
    ///     position: Position { x: 2.0, y: 2.0 },
    ///     velocity: Velocity { x: 0.0, y: 4.0 },
    /// });
    ///
    /// let entity = world
    ///     // Tuples can also mix Bundles and Components
    ///     .spawn((
    ///         PhysicsBundle {
    ///             position: Position { x: 2.0, y: 2.0 },
    ///             velocity: Velocity { x: 0.0, y: 4.0 },
    ///         },
    ///         Name("Elaina Proctor"),
    ///     ))
    ///     // Calling id() will return the unique identifier for the spawned entity
    ///     .id();
    /// let position = world.entity(entity).get::<Position>().unwrap();
    /// assert_eq!(position.x, 2.0);
    /// ```
    #[track_caller]
    pub fn spawn<B: Bundle>(&mut self, bundle: B) -> EntityWorldMut<'_> {
        move_as_ptr!(bundle);
        self.spawn_with_caller(bundle, MaybeLocation::caller())
    }

    pub(crate) fn spawn_with_caller<B: Bundle>(
        &mut self,
        bundle: MovingPtr<'_, B>,
        caller: MaybeLocation,
    ) -> EntityWorldMut<'_> {
        let entity = self.entity_allocator.alloc();
        // This was just spawned from null, so it shouldn't panic.
        self.spawn_at_unchecked(entity, bundle, caller)
    }

    /// Spawns a new [`Entity`] and returns a corresponding [`EntityWorldMut`], which can be used
    /// to add components to the entity or retrieve its id.
    ///
    /// ```
    /// use bevy_ecs::{component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    /// #[derive(Component)]
    /// struct Label(&'static str);
    /// #[derive(Component)]
    /// struct Num(u32);
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn_empty()
    ///     .insert(Position { x: 0.0, y: 0.0 }) // add a single component
    ///     .insert((Num(1), Label("hello"))) // add a bundle of components
    ///     .id();
    ///
    /// let position = world.entity(entity).get::<Position>().unwrap();
    /// assert_eq!(position.x, 0.0);
    /// ```
    #[track_caller]
    pub fn spawn_empty(&mut self) -> EntityWorldMut<'_> {
        self.spawn_empty_with_caller(MaybeLocation::caller())
    }

    pub(crate) fn spawn_empty_with_caller(&mut self, caller: MaybeLocation) -> EntityWorldMut<'_> {
        let entity = self.entity_allocator.alloc();
        // This was just spawned from null, so it shouldn't panic.
        self.spawn_empty_at_unchecked(entity, caller)
    }

    /// Spawns a batch of entities with the same component [`Bundle`] type. Takes a given
    /// [`Bundle`] iterator and returns a corresponding [`Entity`] iterator.
    /// This is more efficient than spawning entities and adding components to them individually
    /// using [`World::spawn`], but it is limited to spawning entities with the same [`Bundle`]
    /// type, whereas spawning individually is more flexible.
    ///
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component)]
    /// struct Str(&'static str);
    /// #[derive(Component)]
    /// struct Num(u32);
    ///
    /// let mut world = World::new();
    /// let entities = world.spawn_batch(vec![
    ///   (Str("a"), Num(0)), // the first entity
    ///   (Str("b"), Num(1)), // the second entity
    /// ]).collect::<Vec<Entity>>();
    ///
    /// assert_eq!(entities.len(), 2);
    /// ```
    #[track_caller]
    pub fn spawn_batch<I>(&mut self, iter: I) -> SpawnBatchIter<'_, I::IntoIter>
    where
        I: IntoIterator,
        I::Item: Bundle<Effect: NoBundleEffect>,
    {
        SpawnBatchIter::new(self, iter.into_iter(), MaybeLocation::caller())
    }

    /// Retrieves a reference to the given `entity`'s [`Component`] of the given type.
    /// Returns `None` if the `entity` does not have a [`Component`] of the given type.
    /// ```
    /// use bevy_ecs::{component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// let position = world.get::<Position>(entity).unwrap();
    /// assert_eq!(position.x, 0.0);
    /// ```
    #[inline]
    pub fn get<T: Component>(&self, entity: Entity) -> Option<&T> {
        self.get_entity(entity).ok()?.get()
    }

    /// Retrieves a mutable reference to the given `entity`'s [`Component`] of the given type.
    /// Returns `None` if the `entity` does not have a [`Component`] of the given type.
    /// ```
    /// use bevy_ecs::{component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// let mut position = world.get_mut::<Position>(entity).unwrap();
    /// position.x = 1.0;
    /// ```
    #[inline]
    pub fn get_mut<T: Component<Mutability = Mutable>>(
        &mut self,
        entity: Entity,
    ) -> Option<Mut<'_, T>> {
        self.get_entity_mut(entity).ok()?.into_mut()
    }

    /// Temporarily removes a [`Component`] `T` from the provided [`Entity`] and
    /// runs the provided closure on it, returning the result if `T` was available.
    /// This will trigger the `Remove` and `Discard` component hooks without
    /// causing an archetype move.
    ///
    /// This is most useful with immutable components, where removal and reinsertion
    /// is the only way to modify a value.
    ///
    /// If you do not need to ensure the above hooks are triggered, and your component
    /// is mutable, prefer using [`get_mut`](World::get_mut).
    ///
    /// # Examples
    ///
    /// ```rust
    /// # use bevy_ecs::prelude::*;
    /// #
    /// #[derive(Component, PartialEq, Eq, Debug)]
    /// #[component(immutable)]
    /// struct Foo(bool);
    ///
    /// # let mut world = World::default();
    /// # world.register_component::<Foo>();
    /// #
    /// # let entity = world.spawn(Foo(false)).id();
    /// #
    /// world.modify_component(entity, |foo: &mut Foo| {
    ///     foo.0 = true;
    /// });
    /// #
    /// # assert_eq!(world.get::<Foo>(entity), Some(&Foo(true)));
    /// ```
    #[inline]
    #[track_caller]
    pub fn modify_component<T: Component, R>(
        &mut self,
        entity: Entity,
        f: impl FnOnce(&mut T) -> R,
    ) -> Result<Option<R>, EntityMutableFetchError> {
        let mut world = DeferredWorld::from(&mut *self);

        let result = world.modify_component_with_relationship_hook_mode(
            entity,
            RelationshipHookMode::Run,
            f,
        )?;

        self.flush();
        Ok(result)
    }

    /// Temporarily removes a [`Component`] identified by the provided
    /// [`ComponentId`] from the provided [`Entity`] and runs the provided
    /// closure on it, returning the result if the component was available.
    /// This will trigger the `Remove` and `Discard` component hooks without
    /// causing an archetype move.
    ///
    /// This is most useful with immutable components, where removal and reinsertion
    /// is the only way to modify a value.
    ///
    /// If you do not need to ensure the above hooks are triggered, and your component
    /// is mutable, prefer using [`get_mut_by_id`](World::get_mut_by_id).
    ///
    /// You should prefer the typed [`modify_component`](World::modify_component)
    /// whenever possible.
    #[inline]
    #[track_caller]
    pub fn modify_component_by_id<R>(
        &mut self,
        entity: Entity,
        component_id: ComponentId,
        f: impl for<'a> FnOnce(MutUntyped<'a>) -> R,
    ) -> Result<Option<R>, EntityMutableFetchError> {
        let mut world = DeferredWorld::from(&mut *self);

        let result = world.modify_component_by_id_with_relationship_hook_mode(
            entity,
            component_id,
            RelationshipHookMode::Run,
            f,
        )?;

        self.flush();
        Ok(result)
    }

    /// Temporarily removes a [`Resource`] `R` and
    /// runs the provided closure on it, returning the result if `R` was available.
    /// This will trigger the `Remove` and `Discard` component hooks without
    /// causing an archetype move.
    ///
    /// This is most useful with immutable resources, where removal and reinsertion
    /// is the only way to modify a value.
    ///
    /// If you do not need to ensure the above hooks are triggered, and your resource
    /// is mutable, prefer using [`get_resource_mut`](World::get_resource_mut).
    ///
    /// # Examples
    ///
    /// ```rust
    /// # use bevy_ecs::prelude::*;
    /// #
    /// #[derive(Resource, PartialEq, Eq, Debug)]
    /// #[component(immutable)]
    /// struct Bar(bool);
    ///
    /// # let mut world = World::default();
    /// # world.insert_resource(Bar(false));
    /// #
    /// world.modify_resource(|bar: &mut Bar| {
    ///     bar.0 = true;
    /// });
    /// #
    /// # assert_eq!(world.get_resource::<Bar>(), Some(&Bar(true)));
    /// ```
    #[inline]
    #[track_caller]
    pub fn modify_resource<R: Resource, S>(
        &mut self,
        f: impl FnOnce(&mut R) -> S,
    ) -> Result<Option<S>, EntityMutableFetchError> {
        let component_id = self.register_component::<R>();
        if let Some(entity) = self.resource_entities.get(component_id) {
            let mut world = DeferredWorld::from(&mut *self);
            let result = world.modify_component_with_relationship_hook_mode(
                entity,
                RelationshipHookMode::Run,
                f,
            )?;

            self.flush();
            Ok(result)
        } else {
            Ok(None)
        }
    }

    /// Temporarily removes a [`Resource`] identified by the provided
    /// [`ComponentId`] and runs the provided
    /// closure on it, returning the result if the component was available.
    /// This will trigger the `Remove` and `Discard` component hooks without
    /// causing an archetype move.
    ///
    /// This is most useful with immutable resources, where removal and reinsertion
    /// is the only way to modify a value.
    ///
    /// If you do not need to ensure the above hooks are triggered, and your resource
    /// is mutable, prefer using [`get_resource_mut_by_id`](World::get_resource_mut_by_id).
    ///
    /// You should prefer the typed [`modify_resource`](World::modify_resource)
    /// whenever possible.
    #[inline]
    #[track_caller]
    pub fn modify_resource_by_id<S>(
        &mut self,
        component_id: ComponentId,
        f: impl for<'a> FnOnce(MutUntyped<'a>) -> S,
    ) -> Result<Option<S>, EntityMutableFetchError> {
        if let Some(entity) = self.resource_entities.get(component_id) {
            let mut world = DeferredWorld::from(&mut *self);

            let result = world.modify_component_by_id_with_relationship_hook_mode(
                entity,
                component_id,
                RelationshipHookMode::Run,
                f,
            )?;

            self.flush();
            Ok(result)
        } else {
            Ok(None)
        }
    }

    /// Despawns the given [`Entity`], if it exists.
    /// This will also remove all of the entity's [`Components`](Component).
    ///
    /// Returns `true` if the entity is successfully despawned and `false` if
    /// the entity does not exist.
    /// This counts despawning a not constructed entity as a success, and frees it to the allocator.
    /// See [entity](crate::entity) module docs for more about construction.
    ///
    /// # Note
    ///
    /// This will also despawn the entities in any [`RelationshipTarget`](crate::relationship::RelationshipTarget) that is configured
    /// to despawn descendants. For example, this will recursively despawn [`Children`](crate::hierarchy::Children).
    ///
    /// ```
    /// use bevy_ecs::{component::Component, world::World};
    ///
    /// #[derive(Component)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entity = world.spawn(Position { x: 0.0, y: 0.0 }).id();
    /// assert!(world.despawn(entity));
    /// assert!(world.get_entity(entity).is_err());
    /// assert!(world.get::<Position>(entity).is_none());
    /// ```
    #[track_caller]
    #[inline]
    pub fn despawn(&mut self, entity: Entity) -> bool {
        if let Err(error) = self.despawn_with_caller(entity, MaybeLocation::caller()) {
            warn!("{error}");
            false
        } else {
            true
        }
    }

    /// Despawns the given `entity`, if it exists. This will also remove all of the entity's
    /// [`Components`](Component).
    ///
    /// Returns an [`EntityDespawnError`] if the entity is not spawned to be despawned.
    ///
    /// # Note
    ///
    /// This will also despawn the entities in any [`RelationshipTarget`](crate::relationship::RelationshipTarget) that is configured
    /// to despawn descendants. For example, this will recursively despawn [`Children`](crate::hierarchy::Children).
    #[track_caller]
    #[inline]
    pub fn try_despawn(&mut self, entity: Entity) -> Result<(), EntityDespawnError> {
        self.despawn_with_caller(entity, MaybeLocation::caller())
    }

    #[inline]
    pub(crate) fn despawn_with_caller(
        &mut self,
        entity: Entity,
        caller: MaybeLocation,
    ) -> Result<(), EntityDespawnError> {
        match self.get_entity_mut(entity) {
            Ok(entity) => {
                entity.despawn_with_caller(caller);
                Ok(())
            }
            // Only one entity.
            Err(EntityMutableFetchError::AliasedMutability(_)) => unreachable!(),
            Err(EntityMutableFetchError::NotSpawned(err)) => Err(EntityDespawnError(err)),
        }
    }

    /// Performs [`try_despawn_no_free`](Self::try_despawn_no_free), warning on errors.
    /// See that method for more information.
    #[track_caller]
    #[inline]
    pub fn despawn_no_free(&mut self, entity: Entity) -> Option<Entity> {
        match self.despawn_no_free_with_caller(entity, MaybeLocation::caller()) {
            Ok(entity) => Some(entity),
            Err(error) => {
                warn!("{error}");
                None
            }
        }
    }

    /// Despawns the given `entity`, if it exists.
    /// This will also remove all of the entity's [`Component`]s.
    ///
    /// The *only* difference between this and [despawning](Self::despawn) an entity is that this does not release the `entity` to be reused.
    /// It is up to the caller to either re-spawn or free the `entity`; otherwise, the [`EntityIndex`](crate::entity::EntityIndex) will not be able to be reused.
    /// In general, [`despawn`](Self::despawn) should be used instead, which automatically allows the row to be reused.
    ///
    /// Returns the new [`Entity`] if of the despawned [`EntityIndex`](crate::entity::EntityIndex), which should eventually either be re-spawned or freed to the allocator.
    /// Returns an [`EntityDespawnError`] if the entity is not spawned.
    ///
    /// # Note
    ///
    /// This will also *despawn* the entities in any [`RelationshipTarget`](crate::relationship::RelationshipTarget) that is configured
    /// to despawn descendants. For example, this will recursively despawn [`Children`](crate::hierarchy::Children).
    ///
    /// # Example
    ///
    /// There is no simple example in which this would be practical, but one use for this is a custom entity allocator.
    /// Despawning internally calls this and frees the entity id to Bevy's default entity allocator.
    /// The same principal can be used to create custom allocators with additional properties.
    /// For example, this could be used to make an allocator that yields groups of consecutive [`EntityIndex`](crate::entity::EntityIndex)s, etc.
    /// See [`EntityAllocator::alloc`] for more on this.
    #[track_caller]
    #[inline]
    pub fn try_despawn_no_free(&mut self, entity: Entity) -> Result<Entity, EntityDespawnError> {
        self.despawn_no_free_with_caller(entity, MaybeLocation::caller())
    }

    #[inline]
    pub(crate) fn despawn_no_free_with_caller(
        &mut self,
        entity: Entity,
        caller: MaybeLocation,
    ) -> Result<Entity, EntityDespawnError> {
        let mut entity = self.get_entity_mut(entity).map_err(|err| match err {
            EntityMutableFetchError::NotSpawned(err) => err,
            // Only one entity.
            EntityMutableFetchError::AliasedMutability(_) => unreachable!(),
        })?;
        entity.despawn_no_free_with_caller(caller);
        Ok(entity.id())
    }

    pub(crate) fn despawn_no_free_no_flush_with_caller(
        &mut self,
        entity: Entity,
        caller: MaybeLocation,
    ) -> Result<Entity, EntityDespawnError> {
        let mut entity = self.get_entity_mut(entity).map_err(|err| match err {
            EntityMutableFetchError::NotSpawned(err) => err,
            // Only one entity.
            EntityMutableFetchError::AliasedMutability(_) => unreachable!(),
        })?;
        entity.despawn_no_free_no_flush_with_caller(caller);
        Ok(entity.id())
    }

    /// [`Despawns`](Self::despawn) all entities matching the [`QueryFilter`].
    #[track_caller]
    #[inline]
    pub fn despawn_all<F: QueryFilter>(&mut self) {
        self.despawn_all_with_caller::<F>(MaybeLocation::caller());
    }

    /// [`Despawns`](Self::despawn) all entities matching a specific [`QueryFilter`] and condition.
    #[track_caller]
    #[inline]
    pub fn despawn_all_where<D: QueryData, F: QueryFilter>(
        &mut self,
        cond: impl FnMut(D::Item<'_, '_>) -> bool,
    ) {
        self.despawn_all_where_with_caller::<D, F>(cond, MaybeLocation::caller());
    }

    /// [`despawn_all`](Self::despawn_all) that takes a caller explicitly.
    #[inline]
    pub(crate) fn despawn_all_with_caller<F: QueryFilter>(&mut self, caller: MaybeLocation) {
        self.despawn_all_where_with_caller::<(), F>(|_| true, caller);
    }

    /// [`despawn_all_where`](Self::despawn_all_where) that takes a caller explicitly.
    pub(crate) fn despawn_all_where_with_caller<D: QueryData, F: QueryFilter>(
        &mut self,
        mut cond: impl FnMut(D::Item<'_, '_>) -> bool,
        caller: MaybeLocation,
    ) {
        let mut query = self.query_filtered::<(Entity, D), F>();
        let mut query = query.iter_mut(self);

        let mut entities_to_despawn = VecDeque::new();

        while let Some((entity, data)) = query.fetch_next() {
            if cond(data) {
                // We want to despawn the entities backwards since we're
                // less likely to leave holes.
                entities_to_despawn.push_front(entity);
            }
        }
        // We have to explicitly drop the query to release the world borrow.
        drop(query);

        // This part of the closure does not need to be generic.
        // Compiling it once saves a bit of compile time.
        fn despawn_entities(
            world: &mut World,
            mut entities_to_despawn: VecDeque<Entity>,
            caller: MaybeLocation,
        ) {
            entities_to_despawn.retain(|entity| {
                let _ = world.despawn_no_free_no_flush_with_caller(*entity, caller);

                // Check if the entity wasn't already freed or reconstructed.
                matches!(world.entities.get(*entity), Ok(None))
            });

            let (head, tail) = entities_to_despawn.as_slices();

            world.entity_allocator.free_many(head);
            world.entity_allocator.free_many(tail);

            world.flush();
        }

        despawn_entities(self, entities_to_despawn, caller);
    }

    /// Clears the internal component tracker state.
    ///
    /// The world maintains some internal state about changed and removed components. This state
    /// is used by [`RemovedComponents`] to provide access to the entities that had a specific type
    /// of component removed since last tick.
    ///
    /// The state is also used for change detection when accessing components and resources outside
    /// of a system, for example via [`World::get_mut()`] or [`World::get_resource_mut()`].
    ///
    /// By clearing this internal state, the world "forgets" about those changes, allowing a new round
    /// of detection to be recorded.
    ///
    /// When using `bevy_ecs` as part of the full Bevy engine, this method is called automatically
    /// by `bevy_app::App::update` and `bevy_app::SubApp::update`, so you don't need to call it manually.
    /// When using `bevy_ecs` as a separate standalone crate however, you do need to call this manually.
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # #[derive(Component, Default)]
    /// # struct Transform;
    /// // a whole new world
    /// let mut world = World::new();
    ///
    /// // you changed it
    /// let entity = world.spawn(Transform::default()).id();
    ///
    /// // change is detected
    /// let transform = world.get_mut::<Transform>(entity).unwrap();
    /// assert!(transform.is_changed());
    ///
    /// // update the last change tick
    /// world.clear_trackers();
    ///
    /// // change is no longer detected
    /// let transform = world.get_mut::<Transform>(entity).unwrap();
    /// assert!(!transform.is_changed());
    /// ```
    ///
    /// [`RemovedComponents`]: crate::lifecycle::RemovedComponents
    pub fn clear_trackers(&mut self) {
        self.removed_components.update();
        self.last_change_tick = self.increment_change_tick();
    }

    /// Returns [`QueryState`] for the given [`QueryData`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component, Debug, PartialEq)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// #[derive(Component)]
    /// struct Velocity {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// let entities = world.spawn_batch(vec![
    ///     (Position { x: 0.0, y: 0.0}, Velocity { x: 1.0, y: 0.0 }),
    ///     (Position { x: 0.0, y: 0.0}, Velocity { x: 0.0, y: 1.0 }),
    /// ]).collect::<Vec<Entity>>();
    ///
    /// let mut query = world.query::<(&mut Position, &Velocity)>();
    /// for (mut position, velocity) in query.iter_mut(&mut world) {
    ///    position.x += velocity.x;
    ///    position.y += velocity.y;
    /// }
    ///
    /// assert_eq!(world.get::<Position>(entities[0]).unwrap(), &Position { x: 1.0, y: 0.0 });
    /// assert_eq!(world.get::<Position>(entities[1]).unwrap(), &Position { x: 0.0, y: 1.0 });
    /// ```
    ///
    /// To iterate over entities in a deterministic order,
    /// sort the results of the query using the desired component as a key.
    /// Note that this requires fetching the whole result set from the query
    /// and allocation of a [`Vec`] to store it.
    ///
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component, PartialEq, Eq, PartialOrd, Ord, Debug)]
    /// struct Order(i32);
    /// #[derive(Component, PartialEq, Debug)]
    /// struct Label(&'static str);
    ///
    /// let mut world = World::new();
    /// let a = world.spawn((Order(2), Label("second"))).id();
    /// let b = world.spawn((Order(3), Label("third"))).id();
    /// let c = world.spawn((Order(1), Label("first"))).id();
    /// let mut entities = world.query::<(Entity, &Order, &Label)>()
    ///     .iter(&world)
    ///     .collect::<Vec<_>>();
    /// // Sort the query results by their `Order` component before comparing
    /// // to expected results. Query iteration order should not be relied on.
    /// entities.sort_by_key(|e| e.1);
    /// assert_eq!(entities, vec![
    ///     (c, &Order(1), &Label("first")),
    ///     (a, &Order(2), &Label("second")),
    ///     (b, &Order(3), &Label("third")),
    /// ]);
    /// ```
    #[inline]
    pub fn query<D: QueryData>(&mut self) -> QueryState<D, ()> {
        self.query_filtered::<D, ()>()
    }

    /// Returns [`QueryState`] for the given filtered [`QueryData`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World, query::With};
    ///
    /// #[derive(Component)]
    /// struct A;
    /// #[derive(Component)]
    /// struct B;
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(A).id();
    /// let e2 = world.spawn((A, B)).id();
    ///
    /// let mut query = world.query_filtered::<Entity, With<B>>();
    /// let matching_entities = query.iter(&world).collect::<Vec<Entity>>();
    ///
    /// assert_eq!(matching_entities, vec![e2]);
    /// ```
    #[inline]
    pub fn query_filtered<D: QueryData, F: QueryFilter>(&mut self) -> QueryState<D, F> {
        QueryState::new(self)
    }

    /// Returns [`QueryState`] for the given [`QueryData`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component, Debug, PartialEq)]
    /// struct Position {
    ///   x: f32,
    ///   y: f32,
    /// }
    ///
    /// let mut world = World::new();
    /// world.spawn_batch(vec![
    ///     Position { x: 0.0, y: 0.0 },
    ///     Position { x: 1.0, y: 1.0 },
    /// ]);
    ///
    /// fn get_positions(world: &World) -> Vec<(Entity, &Position)> {
    ///     let mut query = world.try_query::<(Entity, &Position)>().unwrap();
    ///     query.iter(world).collect()
    /// }
    ///
    /// let positions = get_positions(&world);
    ///
    /// assert_eq!(world.get::<Position>(positions[0].0).unwrap(), positions[0].1);
    /// assert_eq!(world.get::<Position>(positions[1].0).unwrap(), positions[1].1);
    /// ```
    ///
    /// Requires only an immutable world reference, but may fail if, for example,
    /// the components that make up this query have not been registered into the world.
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World};
    ///
    /// #[derive(Component)]
    /// struct A;
    ///
    /// let mut world = World::new();
    ///
    /// let none_query = world.try_query::<&A>();
    /// assert!(none_query.is_none());
    ///
    /// world.register_component::<A>();
    ///
    /// let some_query = world.try_query::<&A>();
    /// assert!(some_query.is_some());
    /// ```
    #[inline]
    pub fn try_query<D: QueryData>(&self) -> Option<QueryState<D, ()>> {
        self.try_query_filtered::<D, ()>()
    }

    /// Returns [`QueryState`] for the given filtered [`QueryData`], which is used to efficiently
    /// run queries on the [`World`] by storing and reusing the [`QueryState`].
    /// ```
    /// use bevy_ecs::{component::Component, entity::Entity, world::World, query::With};
    ///
    /// #[derive(Component)]
    /// struct A;
    /// #[derive(Component)]
    /// struct B;
    ///
    /// let mut world = World::new();
    /// let e1 = world.spawn(A).id();
    /// let e2 = world.spawn((A, B)).id();
    ///
    /// let mut query = world.try_query_filtered::<Entity, With<B>>().unwrap();
    /// let matching_entities = query.iter(&world).collect::<Vec<Entity>>();
    ///
    /// assert_eq!(matching_entities, vec![e2]);
    /// ```
    ///
    /// Requires only an immutable world reference, but may fail if, for example,
    /// the components that make up this query have not been registered into the world.
    #[inline]
    pub fn try_query_filtered<D: QueryData, F: QueryFilter>(&self) -> Option<QueryState<D, F>> {
        QueryState::try_new(self)
    }

    /// Returns an iterator of entities that had components of type `T` removed
    /// since the last call to [`World::clear_trackers`].
    pub fn removed<T: Component>(&self) -> impl Iterator<Item = Entity> + '_ {
        self.components
            .get_valid_id(TypeId::of::<T>())
            .map(|component_id| self.removed_with_id(component_id))
            .into_iter()
            .flatten()
    }

    /// Returns an iterator of entities that had components with the given `component_id` removed
    /// since the last call to [`World::clear_trackers`].
    pub fn removed_with_id(&self, component_id: ComponentId) -> impl Iterator<Item = Entity> + '_ {
        self.removed_components
            .get(component_id)
            .map(|removed| removed.iter_current_update_messages().cloned())
            .into_iter()
            .flatten()
            .map(Into::into)
    }

    /// Registers a new non-send resource type and returns the [`ComponentId`] created for it.
    ///
    /// This enables the dynamic registration of new non-send resources definitions at runtime for
    /// advanced use cases.
    ///
    /// # Note
    ///
    /// Registering a non-send resource does not insert it into [`World`]. For insertion, you could use
    /// [`World::insert_non_send_by_id`].
    pub fn register_non_send_with_descriptor(
        &mut self,
        descriptor: ComponentDescriptor,
    ) -> ComponentId {
        self.components_registrator()
            .register_component_with_descriptor(descriptor)
    }

    fn insert_resource_if_not_exists_with_caller<R: Resource>(
        &mut self,
        func: impl FnOnce(&mut World) -> R,
        caller: MaybeLocation,
    ) -> (ComponentId, EntityWorldMut<'_>) {
        let resource_id = self.register_component::<R>();

        if let Some(entity) = self.resource_entities.get(resource_id) {
            let entity_ref = self.get_entity(entity).expect("ResourceCache is in sync");
            if !entity_ref.contains_id(resource_id) {
                let resource = func(self);
                move_as_ptr!(resource);
                self.entity_mut(entity).insert_with_caller(
                    resource,
                    InsertMode::Replace,
                    caller,
                    RelationshipHookMode::Run,
                );
            }
            return (resource_id, self.entity_mut(entity));
        }

        let resource = func(self);
        move_as_ptr!(resource);
        let entity_mut = self.spawn_with_caller(resource, caller); // ResourceCache is updated automatically
        (resource_id, entity_mut)
    }

    /// Initializes a new resource and returns the [`ComponentId`] created for it.
    ///
    /// If the resource already exists, nothing happens.
    ///
    /// The value given by the [`FromWorld::from_world`] method will be used.
    /// Note that any resource with the [`Default`] trait automatically implements [`FromWorld`],
    /// and those default values will be here instead.
    #[inline]
    #[track_caller]
    pub fn init_resource<R: Resource + FromWorld>(&mut self) -> ComponentId {
        let caller = MaybeLocation::caller();
        self.insert_resource_if_not_exists_with_caller(R::from_world, caller)
            .0
    }

    /// Inserts a new resource with the given `value`.
    ///
    /// Resources are "unique" data of a given type.
    /// If you insert a resource of a type that already exists,
    /// you will overwrite any existing data.
    #[inline]
    #[track_caller]
    pub fn insert_resource<R: Resource>(&mut self, value: R) {
        self.insert_resource_with_caller(value, MaybeLocation::caller());
    }

    /// Split into a new function so we can pass the calling location into the function when using
    /// as a command.
    #[inline]
    pub(crate) fn insert_resource_with_caller<R: Resource>(
        &mut self,
        value: R,
        caller: MaybeLocation,
    ) {
        let component_id = self.components_registrator().register_component::<R>();
        OwningPtr::make(value, |ptr| {
            // SAFETY: component_id was just initialized and corresponds to resource of type R.
            unsafe {
                self.insert_resource_by_id(component_id, ptr, caller);
            }
        });
    }

    /// Initializes new non-send data and returns the [`ComponentId`] created for it.
    ///
    /// If the data already exists, nothing happens.
    ///
    /// The value given by the [`FromWorld::from_world`] method will be used.
    /// Note that any non-send data with the `Default` trait automatically implements
    /// `FromWorld`, and those default values will be here instead.
    ///
    /// # Panics
    ///
    /// Panics if called from a thread other than the main thread.
    #[inline]
    #[track_caller]
    pub fn init_non_send<R: 'static + FromWorld>(&mut self) -> ComponentId {
        let caller = MaybeLocation::caller();
        let component_id = self.components_registrator().register_non_send::<R>();
        if self
            .storages
            .non_sends
            .get(component_id)
            .is_none_or(|data| !data.is_present())
        {
            let value = R::from_world(self);
            OwningPtr::make(value, |ptr| {
                // SAFETY: component_id was just initialized and corresponds to resource of type R.
                unsafe {
                    self.insert_non_send_by_id(component_id, ptr, caller);
                }
            });
        }
        component_id
    }

    /// Inserts new non-send data with the given `value`.
    ///
    /// `NonSend` data cannot be sent across threads,
    /// and do not need the `Send + Sync` bounds.
    /// Systems with `NonSend` resources are always scheduled on the main thread.
    ///
    /// # Panics
    /// If a value is already present, this function will panic if called
    /// from a different thread than where the original value was inserted from.
    #[inline]
    #[track_caller]
    pub fn insert_non_send<R: 'static>(&mut self, value: R) {
        let caller = MaybeLocation::caller();
        let component_id = self.components_registrator().register_non_send::<R>();
        OwningPtr::make(value, |ptr| {
            // SAFETY: component_id was just initialized and corresponds to the data of type R.
            unsafe {
                self.insert_non_send_by_id(component_id, ptr, caller);
            }
        });
    }

    /// Removes the resource of a given type and returns it, if it exists. Otherwise returns `None`.
    #[inline]
    pub fn remove_resource<R: Resource>(&mut self) -> Option<R> {
        let resource_id = self.component_id::<R>()?;
        let entity = self.resource_entities.get(resource_id)?;
        let value = self
            .get_entity_mut(entity)
            .expect("ResourceCache is in sync")
            .take::<R>()?;
        Some(value)
    }

    /// Removes `!Send` data from the world and returns it, if present.
    ///
    /// `NonSend` resources cannot be sent across threads,
    /// and do not need the `Send + Sync` bounds.
    /// Systems with `NonSend` data are always scheduled on the main thread.
    ///
    /// Returns `None` if a value was not previously present.
    ///
    /// # Panics
    /// If a value is present, this function will panic if called from a different
    /// thread than where the value was inserted from.
    #[inline]
    pub fn remove_non_send<R: 'static>(&mut self) -> Option<R> {
        let component_id = self.components.get_valid_id(TypeId::of::<R>())?;
        let (ptr, _, _) = self.storages.non_sends.get_mut(component_id)?.remove()?;
        // SAFETY: `component_id` was gotten via looking up the `R` type
        unsafe { Some(ptr.read::<R>()) }
    }

    /// Returns `true` if a resource of type `R` exists. Otherwise returns `false`.
    #[inline]
    pub fn contains_resource<R: Resource>(&self) -> bool {
        self.components
            .get_valid_id(TypeId::of::<R>())
            .is_some_and(|component_id| self.contains_resource_by_id(component_id))
    }

    /// Returns `true` if a resource with provided `component_id` exists. Otherwise returns `false`.
    #[inline]
    pub fn contains_resource_by_id(&self, component_id: ComponentId) -> bool {
        if let Some(entity) = self.resource_entities.get(component_id)
            && let Ok(entity_ref) = self.get_entity(entity)
        {
            return entity_ref.contains_id(component_id);
        }
        false
    }

    /// Returns `true` if `!Send` data of type `R` exists. Otherwise returns `false`.
    #[inline]
    pub fn contains_non_send<R: 'static>(&self) -> bool {
        self.components
            .get_valid_id(TypeId::of::<R>())
            .and_then(|component_id| self.storages.non_sends.get(component_id))
            .is_some_and(NonSendData::is_present)
    }

    /// Returns `true` if `!Send` data with `component_id` exists. Otherwise returns `false`.
    #[inline]
    pub fn contains_non_send_by_id(&self, component_id: ComponentId) -> bool {
        self.storages
            .non_sends
            .get(component_id)
            .is_some_and(NonSendData::is_present)
    }

    /// Returns `true` if a resource of type `R` exists and was added since the world's
    /// [`last_change_tick`](World::last_change_tick()). Otherwise, this returns `false`.
    ///
    /// This means that:
    /// - When called from an exclusive system, this will check for additions since the system last ran.
    /// - When called elsewhere, this will check for additions since the last time that [`World::clear_trackers`]
    ///   was called.
    pub fn is_resource_added<R: Resource>(&self) -> bool {
        self.components
            .get_valid_id(TypeId::of::<R>())
            .is_some_and(|component_id| self.is_resource_added_by_id(component_id))
    }

    /// Returns `true` if a resource with id `component_id` exists and was added since the world's
    /// [`last_change_tick`](World::last_change_tick()). Otherwise, this returns `false`.
    ///
    /// This means that:
    /// - When called from an exclusive system, this will check for additions since the system last ran.
    /// - When called elsewhere, this will check for additions since the last time that [`World::clear_trackers`]
    ///   was called.
    pub fn is_resource_added_by_id(&self, component_id: ComponentId) -> bool {
        self.get_resource_change_ticks_by_id(component_id)
            .is_some_and(|ticks| ticks.is_added(self.last_change_tick(), self.read_change_tick()))
    }

    /// Returns `true` if a resource of type `R` exists and was modified since the world's
    /// [`last_change_tick`](World::last_change_tick()). Otherwise, this returns `false`.
    ///
    /// This means that:
    /// - When called from an exclusive system, this will check for changes since the system last ran.
    /// - When called elsewhere, this will check for changes since the last time that [`World::clear_trackers`]
    ///   was called.
    pub fn is_resource_changed<R: Resource>(&self) -> bool {
        self.components
            .get_valid_id(TypeId::of::<R>())
            .is_some_and(|component_id| self.is_resource_changed_by_id(component_id))
    }

    /// Returns `true` if a resource with id `component_id` exists and was modified since the world's
    /// [`last_change_tick`](World::last_change_tick()). Otherwise, this returns `false`.
    ///
    /// This means that:
    /// - When called from an exclusive system, this will check for changes since the system last ran.
    /// - When called elsewhere, this will check for changes since the last time that [`World::clear_trackers`]
    ///   was called.
    pub fn is_resource_changed_by_id(&self, component_id: ComponentId) -> bool {
        self.get_resource_change_ticks_by_id(component_id)
            .is_some_and(|ticks| ticks.is_changed(self.last_change_tick(), self.read_change_tick()))
    }

    /// Retrieves the change ticks for the given resource.
    pub fn get_resource_change_ticks<R: Resource>(&self) -> Option<ComponentTicks> {
        self.components
            .get_valid_id(TypeId::of::<R>())
            .and_then(|component_id| self.get_resource_change_ticks_by_id(component_id))
    }

    /// Retrieves the change ticks for the given [`ComponentId`].
    ///
    /// **You should prefer to use the typed API [`World::get_resource_change_ticks`] where possible.**
    pub fn get_resource_change_ticks_by_id(
        &self,
        component_id: ComponentId,
    ) -> Option<ComponentTicks> {
        let entity = self.resource_entities.get(component_id)?;
        let entity_ref = self.get_entity(entity).ok()?;
        entity_ref.get_change_ticks_by_id(component_id)
    }

    /// Gets a reference to the resource of the given type
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_resource`](World::get_resource) instead if you want to handle this case.
    ///
    /// If you want to instead insert a value if the resource does not exist,
    /// use [`get_resource_or_insert_with`](World::get_resource_or_insert_with).
    #[inline]
    #[track_caller]
    pub fn resource<R: Resource>(&self) -> &R {
        match self.get_resource() {
            Some(x) => x,
            None => panic!(
                "Requested resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
                Resources are also implicitly added via `app.add_message`,
                and can be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a reference to the resource of the given type
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_resource_ref`](World::get_resource_ref) instead if you want to handle this case.
    ///
    /// If you want to instead insert a value if the resource does not exist,
    /// use [`get_resource_or_insert_with`](World::get_resource_or_insert_with).
    #[inline]
    #[track_caller]
    pub fn resource_ref<R: Resource>(&self) -> Ref<'_, R> {
        match self.get_resource_ref() {
            Some(x) => x,
            None => panic!(
                "Requested resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
                Resources are also implicitly added via `app.add_message`,
                and can be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a mutable reference to the resource of the given type
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`get_resource_mut`](World::get_resource_mut) instead if you want to handle this case.
    ///
    /// If you want to instead insert a value if the resource does not exist,
    /// use [`get_resource_or_insert_with`](World::get_resource_or_insert_with).
    #[inline]
    #[track_caller]
    pub fn resource_mut<R: Resource<Mutability = Mutable>>(&mut self) -> Mut<'_, R> {
        match self.get_resource_mut() {
            Some(x) => x,
            None => panic!(
                "Requested resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
                Resources are also implicitly added via `app.add_message`,
                and can be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a reference to the resource of the given type if it exists
    #[inline]
    pub fn get_resource<R: Resource>(&self) -> Option<&R> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access everything immutably
        // - `&self` ensures nothing in world is borrowed mutably
        unsafe { self.as_unsafe_world_cell_readonly().get_resource() }
    }

    /// Gets a reference including change detection to the resource of the given type if it exists.
    #[inline]
    pub fn get_resource_ref<R: Resource>(&self) -> Option<Ref<'_, R>> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access everything immutably
        // - `&self` ensures nothing in world is borrowed mutably
        unsafe { self.as_unsafe_world_cell_readonly().get_resource_ref() }
    }

    /// Gets a mutable reference to the resource of the given type if it exists
    #[inline]
    pub fn get_resource_mut<R: Resource<Mutability = Mutable>>(&mut self) -> Option<Mut<'_, R>> {
        // SAFETY:
        // - `as_unsafe_world_cell` gives permission to access everything mutably
        // - `&mut self` ensures nothing in world is borrowed
        unsafe { self.as_unsafe_world_cell().get_resource_mut() }
    }

    /// Gets a mutable reference to the resource of type `T` if it exists,
    /// otherwise inserts the resource using the result of calling `func`.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #
    /// #[derive(Resource)]
    /// struct MyResource(i32);
    ///
    /// # let mut world = World::new();
    /// let my_res = world.get_resource_or_insert_with(|| MyResource(10));
    /// assert_eq!(my_res.0, 10);
    /// ```
    #[inline]
    #[track_caller]
    pub fn get_resource_or_insert_with<R: Resource<Mutability = Mutable>>(
        &mut self,
        func: impl FnOnce() -> R,
    ) -> Mut<'_, R> {
        let caller = MaybeLocation::caller();
        let (resource_id, entity) =
            self.insert_resource_if_not_exists_with_caller(|_world: &mut World| func(), caller);
        let untyped = entity
            .into_mut_by_id(resource_id)
            .expect("Resource must exist");
        // SAFETY: resource is of type R
        unsafe { untyped.with_type() }
    }

    /// Gets a mutable reference to the resource of type `T` if it exists,
    /// otherwise initializes the resource by calling its [`FromWorld`]
    /// implementation.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// #
    /// #[derive(Resource)]
    /// struct Foo(i32);
    ///
    /// impl Default for Foo {
    ///     fn default() -> Self {
    ///         Self(15)
    ///     }
    /// }
    ///
    /// #[derive(Resource)]
    /// struct MyResource(i32);
    ///
    /// impl FromWorld for MyResource {
    ///     fn from_world(world: &mut World) -> Self {
    ///         let foo = world.get_resource_or_init::<Foo>();
    ///         Self(foo.0 * 2)
    ///     }
    /// }
    ///
    /// # let mut world = World::new();
    /// let my_res = world.get_resource_or_init::<MyResource>();
    /// assert_eq!(my_res.0, 30);
    /// ```
    #[track_caller]
    pub fn get_resource_or_init<R: Resource<Mutability = Mutable> + FromWorld>(
        &mut self,
    ) -> Mut<'_, R> {
        let caller = MaybeLocation::caller();
        let (resource_id, entity) =
            self.insert_resource_if_not_exists_with_caller(R::from_world, caller);
        let untyped = entity
            .into_mut_by_id(resource_id)
            .expect("Resource must exist");
        // SAFETY: resource is of type R
        unsafe { untyped.with_type() }
    }

    /// Retrieves the [`Entity`] associated with the resource of type `R`, if it exists.
    #[inline]
    #[track_caller]
    pub fn resource_entity<R: Resource>(&self) -> Option<Entity> {
        let component_id = self.component_id::<R>()?;
        self.resource_entities().get(component_id)
    }

    /// Gets an immutable reference to the non-send data of the given type, if it exists.
    ///
    /// # Panics
    ///
    /// Panics if the data does not exist.
    /// Use [`get_non_send`](World::get_non_send) instead if you want to handle this case.
    ///
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    #[track_caller]
    pub fn non_send<R: 'static>(&self) -> &R {
        match self.get_non_send() {
            Some(x) => x,
            None => panic!(
                "Requested non-send resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_non_send` / `app.init_non_send`?
                Non-send resources can also be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a mutable reference to the non-send data of the given type, if it exists.
    ///
    /// # Panics
    ///
    /// Panics if the data does not exist.
    /// Use [`get_non_send_mut`](World::get_non_send_mut) instead if you want to handle this case.
    ///
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    #[track_caller]
    pub fn non_send_mut<R: 'static>(&mut self) -> Mut<'_, R> {
        match self.get_non_send_mut() {
            Some(x) => x,
            None => panic!(
                "Requested non-send resource {} does not exist in the `World`.
                Did you forget to add it using `app.insert_non_send` / `app.init_non_send`?
                Non-send resources can also be added by plugins.",
                DebugName::type_name::<R>()
            ),
        }
    }

    /// Gets a reference to the non-send data of the given type, if it exists.
    /// Otherwise returns `None`.
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    pub fn get_non_send<R: 'static>(&self) -> Option<&R> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access the entire world immutably
        // - `&self` ensures that there are no mutable borrows of world data
        unsafe { self.as_unsafe_world_cell_readonly().get_non_send() }
    }

    /// Gets a mutable reference to the non-send data of the given type, if it exists.
    /// Otherwise returns `None`.
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    pub fn get_non_send_mut<R: 'static>(&mut self) -> Option<Mut<'_, R>> {
        // SAFETY:
        // - `as_unsafe_world_cell` gives permission to access the entire world mutably
        // - `&mut self` ensures that there are no borrows of world data
        unsafe { self.as_unsafe_world_cell().get_non_send_mut() }
    }

    /// For a given batch of ([`Entity`], [`Bundle`]) pairs,
    /// adds the `Bundle` of components to each `Entity`.
    /// This is faster than doing equivalent operations one-by-one.
    ///
    /// A batch can be any type that implements [`IntoIterator`] containing `(Entity, Bundle)` tuples,
    /// such as a [`Vec<(Entity, Bundle)>`] or an array `[(Entity, Bundle); N]`.
    ///
    /// This will overwrite any previous values of components shared by the `Bundle`.
    /// See [`World::insert_batch_if_new`] to keep the old values instead.
    ///
    /// # Panics
    ///
    /// This function will panic if any of the associated entities do not exist.
    ///
    /// For the fallible version, see [`World::try_insert_batch`].
    #[track_caller]
    pub fn insert_batch<I, B>(&mut self, batch: I)
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        self.insert_batch_with_caller(batch, InsertMode::Replace, MaybeLocation::caller());
    }

    /// For a given batch of ([`Entity`], [`Bundle`]) pairs,
    /// adds the `Bundle` of components to each `Entity` without overwriting.
    /// This is faster than doing equivalent operations one-by-one.
    ///
    /// A batch can be any type that implements [`IntoIterator`] containing `(Entity, Bundle)` tuples,
    /// such as a [`Vec<(Entity, Bundle)>`] or an array `[(Entity, Bundle); N]`.
    ///
    /// This is the same as [`World::insert_batch`], but in case of duplicate
    /// components it will leave the old values instead of replacing them with new ones.
    ///
    /// # Panics
    ///
    /// This function will panic if any of the associated entities do not exist.
    ///
    /// For the fallible version, see [`World::try_insert_batch_if_new`].
    #[track_caller]
    pub fn insert_batch_if_new<I, B>(&mut self, batch: I)
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        self.insert_batch_with_caller(batch, InsertMode::Keep, MaybeLocation::caller());
    }

    /// Split into a new function so we can differentiate the calling location.
    ///
    /// This can be called by:
    /// - [`World::insert_batch`]
    /// - [`World::insert_batch_if_new`]
    #[inline]
    pub(crate) fn insert_batch_with_caller<I, B>(
        &mut self,
        batch: I,
        insert_mode: InsertMode,
        caller: MaybeLocation,
    ) where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        struct InserterArchetypeCache<'w> {
            inserter: BundleInserter<'w>,
            archetype_id: ArchetypeId,
        }

        let change_tick = self.change_tick();
        let bundle_id = self.register_bundle_info::<B>();

        let mut batch_iter = batch.into_iter();

        if let Some((first_entity, first_bundle)) = batch_iter.next() {
            match self.entities().get_spawned(first_entity) {
                Err(err) => {
                    panic!("error[B0003]: Could not insert a bundle (of type `{}`) for entity {first_entity} because: {err}. See: https://bevyengine.org/learn/errors/b0003", core::any::type_name::<B>());
                }
                Ok(first_location) => {
                    let mut cache = InserterArchetypeCache {
                        // SAFETY: we initialized this bundle_id in `register_info`
                        inserter: unsafe {
                            BundleInserter::new_with_id(
                                self,
                                first_location.archetype_id,
                                bundle_id,
                                change_tick,
                            )
                        },
                        archetype_id: first_location.archetype_id,
                    };
                    move_as_ptr!(first_bundle);
                    // SAFETY: `entity` is valid, `location` matches entity, bundle matches inserter, B::Effect: NoBundleEffect
                    unsafe {
                        cache.inserter.insert(
                            first_entity,
                            first_location,
                            first_bundle,
                            insert_mode,
                            caller,
                            RelationshipHookMode::Run,
                        )
                    };

                    for (entity, bundle) in batch_iter {
                        match cache.inserter.entities().get_spawned(entity) {
                            Ok(location) => {
                                if location.archetype_id != cache.archetype_id {
                                    cache = InserterArchetypeCache {
                                        // SAFETY: we initialized this bundle_id in `register_info`
                                        inserter: unsafe {
                                            BundleInserter::new_with_id(
                                                self,
                                                location.archetype_id,
                                                bundle_id,
                                                change_tick,
                                            )
                                        },
                                        archetype_id: location.archetype_id,
                                    }
                                }
                                move_as_ptr!(bundle);
                                // SAFETY: `entity` is valid, `location` matches entity, bundle matches inserter, B::Effect: NoBundleEffect
                                unsafe {
                                    cache.inserter.insert(
                                        entity,
                                        location,
                                        bundle,
                                        insert_mode,
                                        caller,
                                        RelationshipHookMode::Run,
                                    )
                                };
                            }
                            Err(err) => {
                                panic!("error[B0003]: Could not insert a bundle (of type `{}`) for entity {entity} because: {err}. See: https://bevyengine.org/learn/errors/b0003", core::any::type_name::<B>());
                            }
                        }
                    }
                }
            }
        }
    }

    /// For a given batch of ([`Entity`], [`Bundle`]) pairs,
    /// adds the `Bundle` of components to each `Entity`.
    /// This is faster than doing equivalent operations one-by-one.
    ///
    /// A batch can be any type that implements [`IntoIterator`] containing `(Entity, Bundle)` tuples,
    /// such as a [`Vec<(Entity, Bundle)>`] or an array `[(Entity, Bundle); N]`.
    ///
    /// This will overwrite any previous values of components shared by the `Bundle`.
    /// See [`World::try_insert_batch_if_new`] to keep the old values instead.
    ///
    /// Returns a [`TryInsertBatchError`] if any of the provided entities do not exist.
    ///
    /// For the panicking version, see [`World::insert_batch`].
    #[track_caller]
    pub fn try_insert_batch<I, B>(&mut self, batch: I) -> Result<(), TryInsertBatchError>
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        self.try_insert_batch_with_caller(batch, InsertMode::Replace, MaybeLocation::caller())
    }
    /// For a given batch of ([`Entity`], [`Bundle`]) pairs,
    /// adds the `Bundle` of components to each `Entity` without overwriting.
    /// This is faster than doing equivalent operations one-by-one.
    ///
    /// A batch can be any type that implements [`IntoIterator`] containing `(Entity, Bundle)` tuples,
    /// such as a [`Vec<(Entity, Bundle)>`] or an array `[(Entity, Bundle); N]`.
    ///
    /// This is the same as [`World::try_insert_batch`], but in case of duplicate
    /// components it will leave the old values instead of replacing them with new ones.
    ///
    /// Returns a [`TryInsertBatchError`] if any of the provided entities do not exist.
    ///
    /// For the panicking version, see [`World::insert_batch_if_new`].
    #[track_caller]
    pub fn try_insert_batch_if_new<I, B>(&mut self, batch: I) -> Result<(), TryInsertBatchError>
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        self.try_insert_batch_with_caller(batch, InsertMode::Keep, MaybeLocation::caller())
    }

    /// Split into a new function so we can differentiate the calling location.
    ///
    /// This can be called by:
    /// - [`World::try_insert_batch`]
    /// - [`World::try_insert_batch_if_new`]
    /// - [`Commands::insert_batch`]
    /// - [`Commands::insert_batch_if_new`]
    /// - [`Commands::try_insert_batch`]
    /// - [`Commands::try_insert_batch_if_new`]
    #[inline]
    pub(crate) fn try_insert_batch_with_caller<I, B>(
        &mut self,
        batch: I,
        insert_mode: InsertMode,
        caller: MaybeLocation,
    ) -> Result<(), TryInsertBatchError>
    where
        I: IntoIterator,
        I::IntoIter: Iterator<Item = (Entity, B)>,
        B: Bundle<Effect: NoBundleEffect>,
    {
        struct InserterArchetypeCache<'w> {
            inserter: BundleInserter<'w>,
            archetype_id: ArchetypeId,
        }

        let change_tick = self.change_tick();
        let bundle_id = self.register_bundle_info::<B>();

        let mut invalid_entities = Vec::<Entity>::new();
        let mut batch_iter = batch.into_iter();

        // We need to find the first valid entity so we can initialize the bundle inserter.
        // This differs from `insert_batch_with_caller` because that method can just panic
        // if the first entity is invalid, whereas this method needs to keep going.
        let cache = loop {
            if let Some((first_entity, first_bundle)) = batch_iter.next() {
                if let Ok(first_location) = self.entities().get_spawned(first_entity) {
                    let mut cache = InserterArchetypeCache {
                        // SAFETY: we initialized this bundle_id in `register_bundle_info`
                        inserter: unsafe {
                            BundleInserter::new_with_id(
                                self,
                                first_location.archetype_id,
                                bundle_id,
                                change_tick,
                            )
                        },
                        archetype_id: first_location.archetype_id,
                    };

                    move_as_ptr!(first_bundle);
                    // SAFETY:
                    // - `entity` is valid, `location` matches entity, bundle matches inserter
                    // - B::Effect: NoBundleEffect`
                    // - `first_bundle` is not be accessed or dropped after this.
                    unsafe {
                        cache.inserter.insert(
                            first_entity,
                            first_location,
                            first_bundle,
                            insert_mode,
                            caller,
                            RelationshipHookMode::Run,
                        )
                    };
                    break Some(cache);
                }
                invalid_entities.push(first_entity);
            } else {
                // We reached the end of the entities the caller provided and none were valid.
                break None;
            }
        };

        if let Some(mut cache) = cache {
            for (entity, bundle) in batch_iter {
                if let Ok(location) = cache.inserter.entities().get_spawned(entity) {
                    if location.archetype_id != cache.archetype_id {
                        cache = InserterArchetypeCache {
                            // SAFETY: we initialized this bundle_id in `register_info`
                            inserter: unsafe {
                                BundleInserter::new_with_id(
                                    self,
                                    location.archetype_id,
                                    bundle_id,
                                    change_tick,
                                )
                            },
                            archetype_id: location.archetype_id,
                        }
                    }

                    move_as_ptr!(bundle);
                    // SAFETY:
                    // - `entity` is valid, `location` matches entity, bundle matches inserter
                    // - `B::Effect: NoBundleEffect`
                    // - `bundle` is not be accessed or dropped after this.
                    unsafe {
                        cache.inserter.insert(
                            entity,
                            location,
                            bundle,
                            insert_mode,
                            caller,
                            RelationshipHookMode::Run,
                        )
                    };
                } else {
                    invalid_entities.push(entity);
                }
            }
        }

        if invalid_entities.is_empty() {
            Ok(())
        } else {
            Err(TryInsertBatchError {
                bundle_type: DebugName::type_name::<B>(),
                entities: invalid_entities,
            })
        }
    }

    /// Temporarily removes the requested resource from this [`World`], runs custom user code,
    /// then re-adds the resource before returning.
    ///
    /// This enables safe simultaneous mutable access to both a resource and the rest of the [`World`].
    /// For more complex access patterns, consider using [`SystemState`](crate::system::SystemState).
    ///
    /// # Panics
    ///
    /// Panics if the resource does not exist.
    /// Use [`try_resource_scope`](Self::try_resource_scope) instead if you want to handle this case.
    ///
    /// # Example
    /// ```
    /// use bevy_ecs::prelude::*;
    /// #[derive(Resource)]
    /// struct A(u32);
    /// #[derive(Component)]
    /// struct B(u32);
    /// let mut world = World::new();
    /// world.insert_resource(A(1));
    /// let entity = world.spawn(B(1)).id();
    ///
    /// world.resource_scope(|world, mut a: Mut<A>| {
    ///     let b = world.get_mut::<B>(entity).unwrap();
    ///     a.0 += b.0;
    /// });
    /// assert_eq!(world.get_resource::<A>().unwrap().0, 2);
    /// ```
    ///
    /// # Note
    ///
    /// If the world's resource metadata is cleared within the scope, such as by calling
    /// [`World::clear_resources`] or [`World::clear_all`], the resource will *not* be re-inserted
    /// at the end of the scope.
    #[track_caller]
    pub fn resource_scope<R: Resource, U>(&mut self, f: impl FnOnce(&mut World, Mut<R>) -> U) -> U {
        self.try_resource_scope(f)
            .unwrap_or_else(|| panic!("resource does not exist: {}", DebugName::type_name::<R>()))
    }

    /// Temporarily removes the requested resource from this [`World`] if it exists, runs custom user code,
    /// then re-adds the resource before returning. Returns `None` if the resource does not exist in this [`World`].
    ///
    /// This enables safe simultaneous mutable access to both a resource and the rest of the [`World`].
    /// For more complex access patterns, consider using [`SystemState`](crate::system::SystemState).
    ///
    /// See also [`resource_scope`](Self::resource_scope).
    ///
    /// # Note
    ///
    /// If the world's resource metadata is cleared within the scope, such as by calling
    /// [`World::clear_resources`] or [`World::clear_all`], the resource will *not* be re-inserted
    /// at the end of the scope.
    pub fn try_resource_scope<R: Resource, U>(
        &mut self,
        f: impl FnOnce(&mut World, Mut<R>) -> U,
    ) -> Option<U> {
        let last_change_tick = self.last_change_tick();
        let change_tick = self.change_tick();

        let component_id = self.components.valid_component_id::<R>()?;
        let entity = self.resource_entities.get(component_id)?;
        let mut entity_mut = self.get_entity_mut(entity).ok()?;

        let mut ticks = entity_mut.get_change_ticks::<R>()?;
        let changed_by = entity_mut.get_changed_by::<R>()?;
        let value = entity_mut.take::<R>()?;

        // type used to manage reinserting the resource at the end of the scope. use of a drop impl means that
        // the resource is inserted even if the user-provided closure unwinds.
        // this facilitates localized panic recovery and makes app shutdown in response to a panic more graceful
        // by avoiding knock-on errors.
        struct ReinsertGuard<'a, R: Resource> {
            world: &'a mut World,
            entity: Entity,
            component_id: ComponentId,
            value: ManuallyDrop<R>,
            caller: MaybeLocation,
        }
        impl<R: Resource> Drop for ReinsertGuard<'_, R> {
            fn drop(&mut self) {
                // take ownership of the value first so it'll get dropped if we return early
                // SAFETY: drop semantics ensure that `self.value` will never be accessed again after this call
                let value = unsafe { ManuallyDrop::take(&mut self.value) };

                let Ok(mut entity_mut) = self.world.get_entity_mut(self.entity) else {
                    return;
                };

                // in debug mode, raise a panic if user code re-inserted a resource of this type within the scope.
                // resource insertion usually indicates a logic error in user code, which is useful to catch at dev time,
                // however it does not inherently lead to corrupted state, so we avoid introducing an unnecessary crash
                // for production builds.
                if entity_mut.contains_id(self.component_id) {
                    #[cfg(debug_assertions)]
                    {
                        // if we're already panicking, log an error instead of panicking, as double-panics result in an abort
                        #[cfg(feature = "std")]
                        if std::thread::panicking() {
                            log::error!("Resource `{}` was inserted during a call to World::resource_scope, which may result in unexpected behavior.\n\
                                   In release builds, the value inserted will be overwritten at the end of the scope.",
                                   DebugName::type_name::<R>());
                            // return early to maintain consistent behavior with non-panicking calls in debug builds
                            return;
                        }

                        panic!("Resource `{}` was inserted during a call to World::resource_scope, which may result in unexpected behavior.\n\
                               In release builds, the value inserted will be overwritten at the end of the scope.",
                               DebugName::type_name::<R>());
                    }
                    #[cfg(not(debug_assertions))]
                    {
                        #[cold]
                        #[inline(never)]
                        fn warn_reinsert(resource_name: &str) {
                            warn!(
                                "Resource `{resource_name}` was inserted during a call to World::resource_scope: the inserted value will be overwritten.",
                            );
                        }

                        warn_reinsert(&DebugName::type_name::<R>());
                    }
                }

                move_as_ptr!(value);

                // See EntityWorldMut::insert_with_caller for the original code.
                // This is copied here to update the change ticks. This way we can ensure that the commands
                // ran during self.flush(), interact with the correct ticks on the resource component.
                {
                    let location = entity_mut.location();
                    // SAFETY:
                    // - We update the entity location like in `EntityWorldMut::insert_with_caller`.
                    let world = unsafe { entity_mut.world_mut() };
                    let tick = world.change_tick();
                    // SAFETY:
                    // - `location.archetype_id` is part of a valid `EntityLocation`.
                    let mut bundle_inserter =
                        unsafe { BundleInserter::new::<R>(world, location.archetype_id, tick) };
                    // SAFETY:
                    // - `location` matches current entity and thus must currently exist in the source
                    //   archetype for this inserter and its location within the archetype.
                    // - `T` matches the type used to create the `BundleInserter`.
                    // - `apply_effect` is called exactly once after this function.
                    // - The value pointed at by `bundle` is not accessed for anything other than `apply_effect`
                    //   and the caller ensures that the value is not accessed or dropped after this function
                    //   returns.
                    let (bundle, _) = value.partial_move(|bundle| unsafe {
                        bundle_inserter.insert(
                            self.entity,
                            location,
                            bundle,
                            InsertMode::Replace,
                            self.caller,
                            RelationshipHookMode::Run,
                        )
                    });
                    entity_mut.update_location();

                    // SAFETY: We update the entity location afterwards.
                    unsafe { entity_mut.world_mut() }.flush();

                    entity_mut.update_location();
                    // SAFETY:
                    // - This is called exactly once after the `BundleInsert::insert` call before returning to safe code.
                    // - `bundle` points to the same `B` that `BundleInsert::insert` was called on.
                    unsafe { R::apply_effect(bundle, &mut entity_mut) };
                }
            }
        }

        let mut guard = ReinsertGuard {
            world: self,
            entity,
            component_id,
            value: ManuallyDrop::new(value),
            caller: changed_by,
        };

        let value_mut = Mut {
            value: &mut *guard.value,
            ticks: ComponentTicksMut {
                added: &mut ticks.added,
                changed: &mut ticks.changed,
                changed_by: guard.caller.as_mut(),
                last_run: last_change_tick,
                this_run: change_tick,
                summary_tick: None,
            },
        };

        let result = f(guard.world, value_mut);

        Some(result)
    }

    /// Writes a [`Message`].
    /// This method returns the [`MessageId`] of the written `message`,
    /// or [`None`] if the `message` could not be written.
    #[inline]
    pub fn write_message<M: Message>(&mut self, message: M) -> Option<MessageId<M>> {
        self.write_message_batch(core::iter::once(message))?.next()
    }

    /// Writes the default value of the [`Message`] of type `M`.
    /// This method returns the [`MessageId`] of the written message,
    /// or [`None`] if the `event` could not be written.
    #[inline]
    pub fn write_message_default<M: Message + Default>(&mut self) -> Option<MessageId<M>> {
        self.write_message(M::default())
    }

    /// Writes a batch of [`Message`]s from an iterator.
    /// This method returns the [IDs](`MessageId`) of the written `messages`,
    /// or [`None`] if the `events` could not be written.
    #[inline]
    pub fn write_message_batch<M: Message>(
        &mut self,
        messages: impl IntoIterator<Item = M>,
    ) -> Option<WriteBatchIds<M>> {
        let Some(mut events_resource) = self.get_resource_mut::<Messages<M>>() else {
            log::error!(
                "Unable to send event `{}`\n\tEvent must be added to the app with `add_event()`\n\thttps://docs.rs/bevy/*/bevy/app/struct.App.html#method.add_message ",
                DebugName::type_name::<M>()
            );
            return None;
        };
        Some(events_resource.write_batch(messages))
    }

    /// Inserts a new resource with the given `value`. Will replace the value if it already existed.
    ///
    /// **You should prefer to use the typed API [`World::insert_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Safety
    /// The value referenced by `value` must be valid for the given [`ComponentId`] of this world.
    #[inline]
    #[track_caller]
    pub unsafe fn insert_resource_by_id(
        &mut self,
        component_id: ComponentId,
        value: OwningPtr<'_>,
        caller: MaybeLocation,
    ) {
        // if the resource already exists, we replace it on the same entity
        let mut entity_mut = if let Some(entity) = self.resource_entities.get(component_id) {
            self.get_entity_mut(entity)
                .expect("ResourceCache is in sync")
        } else {
            self.spawn_empty()
        };
        // SAFETY: pointer valid for this component id per precondition
        unsafe {
            entity_mut.insert_by_id_with_caller(
                component_id,
                value,
                InsertMode::Replace,
                caller,
                RelationshipHookMode::Run,
            )
        };
    }

    /// Inserts new `!Send` data with the given `value`. Will replace the value if it already
    /// existed.
    ///
    /// **You should prefer to use the typed API [`World::insert_non_send`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// If a value is already present, this function will panic if not called from the same
    /// thread that the original value was inserted from.
    ///
    /// # Safety
    /// The value referenced by `value` must be valid for the given [`ComponentId`] of this world.
    #[inline]
    #[track_caller]
    pub unsafe fn insert_non_send_by_id(
        &mut self,
        component_id: ComponentId,
        value: OwningPtr<'_>,
        caller: MaybeLocation,
    ) {
        let change_tick = self.change_tick();

        let resource = self.initialize_non_send_internal(component_id);
        // SAFETY: `value` is valid for `component_id`, ensured by caller
        unsafe {
            resource.insert(value, change_tick, caller);
        }
    }

    /// # Panics
    /// Panics if `component_id` is not registered in this world
    #[inline]
    pub(crate) fn initialize_non_send_internal(
        &mut self,
        component_id: ComponentId,
    ) -> &mut NonSendData {
        self.flush_components();
        self.storages
            .non_sends
            .initialize_with(component_id, &self.components)
    }

    /// Applies any commands in the world's internal [`CommandQueue`].
    /// This does not apply commands from any systems, only those stored in the world.
    ///
    /// # Panics
    /// This will panic if any of the queued commands are [`spawn`](Commands::spawn).
    /// If this is possible, you should instead use [`flush`](Self::flush).
    pub(crate) fn flush_commands(&mut self) {
        if self.command_queue_is_empty() {
            return;
        }

        // Prevent nested calls to `flush_commands()` from accessing the commands being run now.
        // Set `command_queue_start` to the end of the buffer,
        // and use a RAII type to set it back when done.
        struct Guard<'a> {
            world: &'a mut World,
            start: usize,
        }
        impl Drop for Guard<'_> {
            fn drop(&mut self) {
                // Return `command_queue_start` to its original value.
                // `CommandQueueRunner` will have set `len()` to `start`,
                // so this will result in a zero-length queue.
                debug_assert_eq!(self.world.command_queue.get_mut().len(), self.start);
                self.world.command_queue_start = self.start;
            }
        }

        let start = self.command_queue_start;
        let end = self.command_queue.get_mut().len();
        let guard = Guard { world: self, start };
        guard.world.command_queue_start = end;

        // SAFETY:
        // * The world's command queue is always returned
        // * `start` was set by a call to `flush_commands` to equal `end`,
        //   so any new commands started there
        // * `command_queue_start = end` prevents nested calls from accessing commands between `start` and `command_queue.len`
        let mut runner = unsafe {
            CommandQueueRunner::new(
                &mut *guard.world,
                |world| world.command_queue.get_mut(),
                start,
            )
        };
        runner.run(|world| Some(world));
    }

    /// Returns false if there are any commands in the queue.
    ///
    /// This must be used instead of [`CommandQueue::is_empty`]
    /// to ignore any commands earlier than [`Self::command_queue_start`].
    fn command_queue_is_empty(&mut self) -> bool {
        self.command_queue_start >= self.command_queue.get_mut().len()
    }

    /// Applies any queued component registration.
    /// For spawning vanilla rust component types and resources, this is not strictly necessary.
    /// However, flushing components can make information available more quickly, and can have performance benefits.
    /// Additionally, for components and resources registered dynamically through a raw descriptor or similar,
    /// this is the only way to complete their registration.
    pub(crate) fn flush_components(&mut self) {
        self.components_registrator().apply_queued_registrations();
    }

    /// Flushes queued entities and commands.
    ///
    /// Queued entities will be spawned, and then commands will be applied.
    #[inline]
    #[track_caller]
    pub fn flush(&mut self) {
        self.flush_components();
        self.flush_commands();
    }

    /// Increments the world's current change tick and returns the old value.
    ///
    /// If you need to call this method, but do not have `&mut` access to the world,
    /// consider using [`as_unsafe_world_cell_readonly`](Self::as_unsafe_world_cell_readonly)
    /// to obtain an [`UnsafeWorldCell`] and calling [`increment_change_tick`](UnsafeWorldCell::increment_change_tick) on that.
    /// Note that this *can* be done in safe code, despite the name of the type.
    #[inline]
    pub fn increment_change_tick(&mut self) -> Tick {
        let change_tick = self.change_tick.get_mut();
        let prev_tick = *change_tick;
        *change_tick = change_tick.wrapping_add(1);
        Tick::new(prev_tick)
    }

    /// Reads the current change tick of this world.
    ///
    /// If you have exclusive (`&mut`) access to the world, consider using [`change_tick()`](Self::change_tick),
    /// which is more efficient since it does not require atomic synchronization.
    #[inline]
    pub fn read_change_tick(&self) -> Tick {
        let tick = self.change_tick.load(Ordering::Acquire);
        Tick::new(tick)
    }

    /// Reads the current change tick of this world.
    ///
    /// This does the same thing as [`read_change_tick()`](Self::read_change_tick), only this method
    /// is more efficient since it does not require atomic synchronization.
    #[inline]
    pub fn change_tick(&mut self) -> Tick {
        let tick = *self.change_tick.get_mut();
        Tick::new(tick)
    }

    /// When called from within an exclusive system (a [`System`] that takes `&mut World` as its first
    /// parameter), this method returns the [`Tick`] indicating the last time the exclusive system was run.
    ///
    /// Otherwise, this returns the `Tick` indicating the last time that [`World::clear_trackers`] was called.
    ///
    /// [`System`]: crate::system::System
    #[inline]
    pub fn last_change_tick(&self) -> Tick {
        self.last_change_tick
    }

    /// Returns the id of the last ECS event that was fired.
    /// Used internally to ensure observers don't trigger multiple times for the same event.
    #[inline]
    pub(crate) fn last_trigger_id(&self) -> u32 {
        self.last_trigger_id
    }

    /// Sets [`World::last_change_tick()`] to the specified value during a scope.
    /// When the scope terminates, it will return to its old value.
    ///
    /// This is useful if you need a region of code to be able to react to earlier changes made in the same system.
    ///
    /// # Examples
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// // This function runs an update loop repeatedly, allowing each iteration of the loop
    /// // to react to changes made in the previous loop iteration.
    /// fn update_loop(
    ///     world: &mut World,
    ///     mut update_fn: impl FnMut(&mut World) -> std::ops::ControlFlow<()>,
    /// ) {
    ///     let mut last_change_tick = world.last_change_tick();
    ///
    ///     // Repeatedly run the update function until it requests a break.
    ///     loop {
    ///         let control_flow = world.last_change_tick_scope(last_change_tick, |world| {
    ///             // Increment the change tick so we can detect changes from the previous update.
    ///             last_change_tick = world.change_tick();
    ///             world.increment_change_tick();
    ///
    ///             // Update once.
    ///             update_fn(world)
    ///         });
    ///
    ///         // End the loop when the closure returns `ControlFlow::Break`.
    ///         if control_flow.is_break() {
    ///             break;
    ///         }
    ///     }
    /// }
    /// #
    /// # #[derive(Resource)] struct Count(u32);
    /// # let mut world = World::new();
    /// # world.insert_resource(Count(0));
    /// # let saved_last_tick = world.last_change_tick();
    /// # let mut num_updates = 0;
    /// # update_loop(&mut world, |world| {
    /// #     let mut c = world.resource_mut::<Count>();
    /// #     match c.0 {
    /// #         0 => {
    /// #             assert_eq!(num_updates, 0);
    /// #             assert!(c.is_added());
    /// #             c.0 = 1;
    /// #         }
    /// #         1 => {
    /// #             assert_eq!(num_updates, 1);
    /// #             assert!(!c.is_added());
    /// #             assert!(c.is_changed());
    /// #             c.0 = 2;
    /// #         }
    /// #         2 if c.is_changed() => {
    /// #             assert_eq!(num_updates, 2);
    /// #             assert!(!c.is_added());
    /// #         }
    /// #         2 => {
    /// #             assert_eq!(num_updates, 3);
    /// #             assert!(!c.is_changed());
    /// #             world.remove_resource::<Count>();
    /// #             world.insert_resource(Count(3));
    /// #         }
    /// #         3 if c.is_changed() => {
    /// #             assert_eq!(num_updates, 4);
    /// #             assert!(c.is_added());
    /// #         }
    /// #         3 => {
    /// #             assert_eq!(num_updates, 5);
    /// #             assert!(!c.is_added());
    /// #             c.0 = 4;
    /// #             return std::ops::ControlFlow::Break(());
    /// #         }
    /// #         _ => unreachable!(),
    /// #     }
    /// #     num_updates += 1;
    /// #     std::ops::ControlFlow::Continue(())
    /// # });
    /// # assert_eq!(num_updates, 5);
    /// # assert_eq!(world.resource::<Count>().0, 4);
    /// # assert_eq!(world.last_change_tick(), saved_last_tick);
    /// ```
    pub fn last_change_tick_scope<T>(
        &mut self,
        last_change_tick: Tick,
        f: impl FnOnce(&mut World) -> T,
    ) -> T {
        struct LastTickGuard<'a> {
            world: &'a mut World,
            last_tick: Tick,
        }

        // By setting the change tick in the drop impl, we ensure that
        // the change tick gets reset even if a panic occurs during the scope.
        impl Drop for LastTickGuard<'_> {
            fn drop(&mut self) {
                self.world.last_change_tick = self.last_tick;
            }
        }

        let guard = LastTickGuard {
            last_tick: self.last_change_tick,
            world: self,
        };

        guard.world.last_change_tick = last_change_tick;

        f(guard.world)
    }

    /// Iterates all component change ticks and clamps any older than [`MAX_CHANGE_AGE`](crate::change_detection::MAX_CHANGE_AGE).
    /// This also triggers [`CheckChangeTicks`] observers and returns the same event here.
    ///
    /// Calling this method prevents [`Tick`]s overflowing and thus prevents false positives when comparing them.
    ///
    /// **Note:** Does nothing and returns `None` if the [`World`] counter has not been incremented at least [`CHECK_TICK_THRESHOLD`]
    /// times since the previous pass.
    // TODO: benchmark and optimize
    pub fn check_change_ticks(&mut self) -> Option<CheckChangeTicks> {
        let change_tick = self.change_tick();
        if change_tick.relative_to(self.last_check_tick).get() < CHECK_TICK_THRESHOLD {
            return None;
        }

        let check = CheckChangeTicks(change_tick);

        let Storages {
            ref mut tables,
            ref mut sparse_sets,
            ref mut non_sends,
        } = self.storages;

        #[cfg(feature = "trace")]
        let _span = tracing::info_span!("check component ticks").entered();
        tables.check_change_ticks(check);
        sparse_sets.check_change_ticks(check);
        non_sends.check_change_ticks(check);
        self.entities.check_change_ticks(check);

        if let Some(mut schedules) = self.get_resource_mut::<Schedules>() {
            schedules.check_change_ticks(check);
        }

        self.trigger(check);
        self.flush();

        self.last_check_tick = change_tick;

        Some(check)
    }

    /// Clears all entities, resources, and non-send data.
    /// This invalidates all [`Entity`] and resource fetches such as [`Res`](crate::system::Res),
    /// [`ResMut`](crate::system::ResMut)
    pub fn clear_all(&mut self) {
        self.clear_entities();
        self.clear_non_send();
    }

    /// Despawns all entities in this [`World`].
    ///
    /// **Note:** This includes all resources, as they are stored as components.
    /// Any resource fetch to this [`World`] will fail unless they are re-initialized,
    /// including engine-internal resources that are only initialized on app/world construction.
    ///
    /// This can easily cause systems expecting certain resources to immediately start panicking.
    /// Use with caution.
    pub fn clear_entities(&mut self) {
        self.storages.tables.clear();
        self.storages.sparse_sets.clear_entities();
        self.archetypes.clear_entities();
        self.entities.clear();
        self.entity_allocator.restart();
    }

    /// Clears all resources in this [`World`].
    ///
    /// **Note:** Any resource fetch to this [`World`] will fail unless they are re-initialized,
    /// including engine-internal resources that are only initialized on app/world construction.
    ///
    /// This can easily cause systems expecting certain resources to immediately start panicking.
    /// Use with caution.
    pub fn clear_resources(&mut self) {
        let pairs: Vec<(ComponentId, Entity)> = self.resource_entities().iter().collect();
        for (component_id, entity) in pairs {
            self.entity_mut(entity).remove_by_id(component_id);
        }
    }

    /// Clears all non-send data in this [`World`].
    pub fn clear_non_send(&mut self) {
        self.storages.non_sends.clear();
    }

    /// Registers all of the components in the given [`Bundle`] and returns both the component
    /// ids and the bundle id.
    ///
    /// This is largely equivalent to calling [`register_component`](Self::register_component) on each
    /// component in the bundle.
    #[inline]
    pub fn register_bundle<B: Bundle>(&mut self) -> &BundleInfo {
        let id = self.register_bundle_info::<B>();

        // SAFETY: We just initialized the bundle so its id should definitely be valid.
        unsafe { self.bundles.get(id).debug_checked_unwrap() }
    }

    pub(crate) fn register_bundle_info<B: Bundle>(&mut self) -> BundleId {
        // This is a hot path, so return early to avoid the `Vec::new` in `ComponentsRegistrator`
        if let Some(bundle_id) = self.bundles.get_id(TypeId::of::<B>()) {
            return bundle_id;
        }

        // SAFETY: These come from the same world. `Self.components_registrator` can't be used since we borrow other fields too.
        let mut registrator =
            unsafe { ComponentsRegistrator::new(&mut self.components, &mut self.component_ids) };

        // SAFETY: `registrator`, `self.storages` and `self.bundles` all come from this world.
        unsafe {
            self.bundles
                .register_info::<B>(&mut registrator, &mut self.storages)
        }
    }

    pub(crate) fn register_contributed_bundle_info<B: Bundle>(&mut self) -> BundleId {
        // This is a hot path, so return early to avoid the `Vec::new` in `ComponentsRegistrator`
        if let Some(bundle_id) = self.bundles.get_contributed_bundle_id(TypeId::of::<B>()) {
            return bundle_id;
        }

        // SAFETY: These come from the same world. `Self.components_registrator` can't be used since we borrow other fields too.
        let mut registrator =
            unsafe { ComponentsRegistrator::new(&mut self.components, &mut self.component_ids) };

        // SAFETY: `registrator`, `self.bundles` and `self.storages` are all from this world.
        unsafe {
            self.bundles
                .register_contributed_bundle_info::<B>(&mut registrator, &mut self.storages)
        }
    }

    /// Registers the given [`ComponentId`]s as a dynamic bundle and returns both the required component ids and the bundle id.
    ///
    /// Note that the components need to be registered first, this function only creates a bundle combining them. Components
    /// can be registered with [`World::register_component`]/[`_with_descriptor`](World::register_component_with_descriptor).
    ///
    /// **You should prefer to use the typed API [`World::register_bundle`] where possible and only use this in cases where
    /// not all of the actual types are known at compile time.**
    ///
    /// # Panics
    /// This function will panic if any of the provided component ids do not belong to a component known to this [`World`].
    #[inline]
    pub fn register_dynamic_bundle(&mut self, component_ids: &[ComponentId]) -> &BundleInfo {
        let id =
            self.bundles
                .init_dynamic_info(&mut self.storages, &self.components, component_ids);
        // SAFETY: We just initialized the bundle so its id should definitely be valid.
        unsafe { self.bundles.get(id).debug_checked_unwrap() }
    }

    /// Convenience method for accessing the world's fallback error handler,
    /// which can be overwritten with [`FallbackErrorHandler`].
    #[inline]
    pub fn fallback_error_handler(&self) -> ErrorHandler {
        self.get_resource::<FallbackErrorHandler>()
            .copied()
            .unwrap_or_default()
            .0
    }
}

impl World {
    /// Gets a pointer to the resource with the id [`ComponentId`] if it exists.
    /// The returned pointer must not be used to modify the resource, and must not be
    /// dereferenced after the immutable borrow of the [`World`] ends.
    ///
    /// **You should prefer to use the typed API [`World::get_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    #[inline]
    pub fn get_resource_by_id(&self, component_id: ComponentId) -> Option<Ptr<'_>> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access the whole world immutably
        // - `&self` ensures there are no mutable borrows on world data
        unsafe {
            self.as_unsafe_world_cell_readonly()
                .get_resource_by_id(component_id)
        }
    }

    /// Gets a pointer to the resource with the id [`ComponentId`] if it exists and is mutable.
    /// The returned pointer may be used to modify the resource, as long as the mutable borrow
    /// of the [`World`] is still valid.
    ///
    /// **You should prefer to use the typed API [`World::get_resource_mut`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    #[inline]
    pub fn get_resource_mut_by_id(&mut self, component_id: ComponentId) -> Option<MutUntyped<'_>> {
        // SAFETY:
        // - `&mut self` ensures that all accessed data is unaliased
        // - `as_unsafe_world_cell` provides mutable permission to the whole world
        unsafe {
            self.as_unsafe_world_cell()
                .get_resource_mut_by_id(component_id)
        }
    }

    /// Iterates over all resources in the world.
    ///
    /// The returned iterator provides lifetimed, but type-unsafe pointers. Actually reading the contents
    /// of each resource will require the use of unsafe code.
    ///
    /// # Examples
    ///
    /// ## Printing the size of all resources
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # #[derive(Resource)]
    /// # struct A(u32);
    /// # #[derive(Resource)]
    /// # struct B(u32);
    /// #
    /// # let mut world = World::new();
    /// # world.remove_resource::<bevy_ecs::entity_disabling::DefaultQueryFilters>();
    /// # world.insert_resource(A(1));
    /// # world.insert_resource(B(2));
    /// let mut total = 0;
    /// for (_, info, _) in world.iter_resources() {
    ///    println!("Resource: {}", info.name());
    ///    println!("Size: {} bytes", info.layout().size());
    ///    total += info.layout().size();
    /// }
    /// println!("Total size: {} bytes", total);
    /// # assert_eq!(total, size_of::<A>() + size_of::<B>());
    /// ```
    ///
    /// ## Dynamically running closures for resources matching specific `TypeId`s
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # use std::collections::HashMap;
    /// # use std::any::TypeId;
    /// # use bevy_ptr::Ptr;
    /// # #[derive(Resource)]
    /// # struct A(u32);
    /// # #[derive(Resource)]
    /// # struct B(u32);
    /// #
    /// # let mut world = World::new();
    /// # world.insert_resource(A(1));
    /// # world.insert_resource(B(2));
    /// #
    /// // In this example, `A` and `B` are resources. We deliberately do not use the
    /// // `bevy_reflect` crate here to showcase the low-level [`Ptr`] usage. You should
    /// // probably use something like `ReflectFromPtr` in a real-world scenario.
    ///
    /// // Create the hash map that will store the closures for each resource type
    /// let mut closures: HashMap<TypeId, Box<dyn Fn(&Ptr<'_>)>> = HashMap::default();
    ///
    /// // Add closure for `A`
    /// closures.insert(TypeId::of::<A>(), Box::new(|ptr| {
    ///     // SAFETY: We assert ptr is the same type of A with TypeId of A
    ///     let a = unsafe { &ptr.deref::<A>() };
    /// #   assert_eq!(a.0, 1);
    ///     // ... do something with `a` here
    /// }));
    ///
    /// // Add closure for `B`
    /// closures.insert(TypeId::of::<B>(), Box::new(|ptr| {
    ///     // SAFETY: We assert ptr is the same type of B with TypeId of B
    ///     let b = unsafe { &ptr.deref::<B>() };
    /// #   assert_eq!(b.0, 2);
    ///     // ... do something with `b` here
    /// }));
    ///
    /// // Iterate all resources, in order to run the closures for each matching resource type
    /// for (_, info, ptr) in world.iter_resources() {
    ///     let Some(type_id) = info.type_id() else {
    ///        // It's possible for resources to not have a `TypeId` (e.g. non-Rust resources
    ///        // dynamically inserted via a scripting language) in which case we can't match them.
    ///        continue;
    ///     };
    ///
    ///     let Some(closure) = closures.get(&type_id) else {
    ///        // No closure for this resource type, skip it.
    ///        continue;
    ///     };
    ///
    ///     // Run the closure for the resource
    ///     closure(&ptr);
    /// }
    /// ```
    #[inline]
    pub fn iter_resources(&self) -> impl Iterator<Item = (ComponentId, &ComponentInfo, Ptr<'_>)> {
        self.resource_entities
            .iter()
            .filter_map(|(component_id, entity)| {
                let component_info = self.components().get_info(component_id)?;
                let entity_cell = self.get_entity(entity).ok()?;
                let resource = entity_cell.get_by_id(component_id).ok()?;
                Some((component_id, component_info, resource))
            })
    }

    /// Mutably iterates over all resources in the world.
    ///
    /// The returned iterator provides lifetimed, but type-unsafe pointers. Actually reading from or writing
    /// to the contents of each resource will require the use of unsafe code.
    ///
    /// # Example
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # use bevy_ecs::change_detection::MutUntyped;
    /// # use std::collections::HashMap;
    /// # use std::any::TypeId;
    /// # #[derive(Resource)]
    /// # struct A(u32);
    /// # #[derive(Resource)]
    /// # struct B(u32);
    /// #
    /// # let mut world = World::new();
    /// # world.insert_resource(A(1));
    /// # world.insert_resource(B(2));
    /// #
    /// // In this example, `A` and `B` are resources. We deliberately do not use the
    /// // `bevy_reflect` crate here to showcase the low-level `MutUntyped` usage. You should
    /// // probably use something like `ReflectFromPtr` in a real-world scenario.
    ///
    /// // Create the hash map that will store the mutator closures for each resource type
    /// let mut mutators: HashMap<TypeId, Box<dyn Fn(&mut MutUntyped<'_>)>> = HashMap::default();
    ///
    /// // Add mutator closure for `A`
    /// mutators.insert(TypeId::of::<A>(), Box::new(|mut_untyped| {
    ///     // Note: `MutUntyped::as_mut()` automatically marks the resource as changed
    ///     // for ECS change detection, and gives us a `PtrMut` we can use to mutate the resource.
    ///     // SAFETY: We assert ptr is the same type of A with TypeId of A
    ///     let a = unsafe { &mut mut_untyped.as_mut().deref_mut::<A>() };
    /// #   a.0 += 1;
    ///     // ... mutate `a` here
    /// }));
    ///
    /// // Add mutator closure for `B`
    /// mutators.insert(TypeId::of::<B>(), Box::new(|mut_untyped| {
    ///     // SAFETY: We assert ptr is the same type of B with TypeId of B
    ///     let b = unsafe { &mut mut_untyped.as_mut().deref_mut::<B>() };
    /// #   b.0 += 1;
    ///     // ... mutate `b` here
    /// }));
    ///
    /// // Iterate all resources, in order to run the mutator closures for each matching resource type
    /// for (_, info, mut mut_untyped) in world.iter_resources_mut() {
    ///     let Some(type_id) = info.type_id() else {
    ///        // It's possible for resources to not have a `TypeId` (e.g. non-Rust resources
    ///        // dynamically inserted via a scripting language) in which case we can't match them.
    ///        continue;
    ///     };
    ///
    ///     let Some(mutator) = mutators.get(&type_id) else {
    ///        // No mutator closure for this resource type, skip it.
    ///        continue;
    ///     };
    ///
    ///     // Run the mutator closure for the resource
    ///     mutator(&mut mut_untyped);
    /// }
    /// # assert_eq!(world.resource::<A>().0, 2);
    /// # assert_eq!(world.resource::<B>().0, 3);
    /// ```
    pub fn iter_resources_mut(
        &mut self,
    ) -> impl Iterator<Item = (ComponentId, &ComponentInfo, MutUntyped<'_>)> {
        let unsafe_world = self.as_unsafe_world_cell();
        // SAFETY: exclusive world access to all resources
        let resource_entities = unsafe { unsafe_world.resource_entities() };
        let components = unsafe_world.components();

        resource_entities
            .iter()
            .filter_map(move |(component_id, entity)| {
                // SAFETY: If a resource has been initialized, a corresponding ComponentInfo must exist with its ID.
                let component_info =
                    unsafe { components.get_info(component_id).debug_checked_unwrap() };

                let entity_cell = unsafe_world.get_entity(entity).ok()?;

                // SAFETY:
                // - We have exclusive world access
                // - `UnsafeEntityCell::get_mut_by_id` doesn't access components
                // or resource_entities mutably
                // - `resource_entities` doesn't contain duplicate entities, so
                // no duplicate references are created
                let mut_untyped = unsafe { entity_cell.get_mut_by_id(component_id).ok()? };

                Some((component_id, component_info, mut_untyped))
            })
    }

    /// Gets a pointer to `!Send` data with the id [`ComponentId`] if it exists.
    /// The returned pointer must not be used to modify the resource, and must not be
    /// dereferenced after the immutable borrow of the [`World`] ends.
    ///
    /// **You should prefer to use the typed API [`World::get_non_send`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the data was inserted from.
    #[inline]
    pub fn get_non_send_by_id(&self, component_id: ComponentId) -> Option<Ptr<'_>> {
        // SAFETY:
        // - `as_unsafe_world_cell_readonly` gives permission to access the whole world immutably
        // - `&self` ensures there are no mutable borrows on world data
        unsafe {
            self.as_unsafe_world_cell_readonly()
                .get_non_send_by_id(component_id)
        }
    }

    /// Gets mutable access to `!Send` data with the id [`ComponentId`] if it exists.
    /// The returned pointer may be used to modify the data, as long as the mutable borrow
    /// of the [`World`] is still valid.
    ///
    /// **You should prefer to use the typed API [`World::get_non_send_mut`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the data was inserted from.
    #[inline]
    pub fn get_non_send_mut_by_id(&mut self, component_id: ComponentId) -> Option<MutUntyped<'_>> {
        // SAFETY:
        // - `&mut self` ensures that all accessed data is unaliased
        // - `as_unsafe_world_cell` provides mutable permission to the whole world
        unsafe {
            self.as_unsafe_world_cell()
                .get_non_send_mut_by_id(component_id)
        }
    }

    /// Removes the resource of a given type, if it exists.
    /// Returns `true` if the resource is successfully removed and `false` if
    /// the entity does not exist.
    ///
    /// **You should prefer to use the typed API [`World::remove_resource`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    pub fn remove_resource_by_id(&mut self, component_id: ComponentId) -> bool {
        if let Some(entity) = self.resource_entities.get(component_id)
            && let Ok(mut entity_mut) = self.get_entity_mut(entity)
            && entity_mut.contains_id(component_id)
        {
            entity_mut.remove_by_id(component_id);
            true
        } else {
            false
        }
    }

    /// Removes the non-send data of a given type, if it exists. Otherwise returns `None`.
    ///
    /// **You should prefer to use the typed API [`World::remove_non_send`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the data was inserted from.
    pub fn remove_non_send_by_id(&mut self, component_id: ComponentId) -> Option<()> {
        self.storages
            .non_sends
            .get_mut(component_id)?
            .remove_and_drop();
        Some(())
    }

    /// Retrieves an immutable untyped reference to the given `entity`'s [`Component`] of the given [`ComponentId`].
    /// Returns `None` if the `entity` does not have a [`Component`] of the given type.
    ///
    /// **You should prefer to use the typed API [`World::get_mut`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    ///
    /// # Panics
    /// This function will panic if it isn't called from the same thread that the resource was inserted from.
    #[inline]
    pub fn get_by_id(&self, entity: Entity, component_id: ComponentId) -> Option<Ptr<'_>> {
        self.get_entity(entity).ok()?.get_by_id(component_id).ok()
    }

    /// Retrieves a mutable untyped reference to the given `entity`'s [`Component`] of the given [`ComponentId`].
    /// Returns `None` if the `entity` does not have a [`Component`] of the given type.
    ///
    /// **You should prefer to use the typed API [`World::get_mut`] where possible and only
    /// use this in cases where the actual types are not known at compile time.**
    #[inline]
    pub fn get_mut_by_id(
        &mut self,
        entity: Entity,
        component_id: ComponentId,
    ) -> Option<MutUntyped<'_>> {
        self.get_entity_mut(entity)
            .ok()?
            .into_mut_by_id(component_id)
            .ok()
    }
}

// Schedule-related methods
impl World {
    /// Adds the specified [`Schedule`] to the world.
    /// If a schedule already exists with the same [label](Schedule::label), it will be replaced.
    ///
    /// The schedule can later be run
    /// by calling [`.run_schedule(label)`](Self::run_schedule) or by directly
    /// accessing the [`Schedules`] resource.
    ///
    /// The `Schedules` resource will be initialized if it does not already exist.
    ///
    /// An alternative to this is to call [`Schedules::add_systems()`] with some
    /// [`ScheduleLabel`] and let the schedule for that label be created if it
    /// does not already exist.
    pub fn add_schedule(&mut self, schedule: Schedule) {
        let mut schedules = self.get_resource_or_init::<Schedules>();
        schedules.insert(schedule);
    }

    /// Temporarily removes the schedule associated with `label` from the world,
    /// runs user code, and finally re-adds the schedule.
    /// This returns a [`TryRunScheduleError`] if there is no schedule
    /// associated with `label`.
    ///
    /// The [`Schedule`] is fetched from the [`Schedules`] resource of the world by its label,
    /// and system state is cached.
    ///
    /// For simple cases where you just need to call the schedule once,
    /// consider using [`World::try_run_schedule`] instead.
    /// For other use cases, see the example on [`World::schedule_scope`].
    pub fn try_schedule_scope<R>(
        &mut self,
        label: impl ScheduleLabel,
        f: impl FnOnce(&mut World, &mut Schedule) -> R,
    ) -> Result<R, TryRunScheduleError> {
        let label = label.intern();
        let Some(mut schedule) = self
            .get_resource_mut::<Schedules>()
            .and_then(|mut s| s.remove_temporarily(label))
        else {
            return Err(TryRunScheduleError(label));
        };

        let value = f(self, &mut schedule);

        let old = self.resource_mut::<Schedules>().reinsert(schedule);
        if old.is_some() {
            warn!("Schedule `{label:?}` was inserted during a call to `World::schedule_scope`: its value has been overwritten");
        }

        Ok(value)
    }

    /// Temporarily removes the schedule associated with `label` from the world,
    /// runs user code, and finally re-adds the schedule.
    ///
    /// The [`Schedule`] is fetched from the [`Schedules`] resource of the world by its label,
    /// and system state is cached.
    ///
    /// # Examples
    ///
    /// ```
    /// # use bevy_ecs::{prelude::*, schedule::ScheduleLabel};
    /// # #[derive(ScheduleLabel, Debug, Clone, Copy, PartialEq, Eq, Hash)]
    /// # pub struct MySchedule;
    /// # #[derive(Resource)]
    /// # struct Counter(usize);
    /// #
    /// # let mut world = World::new();
    /// # world.insert_resource(Counter(0));
    /// # let mut schedule = Schedule::new(MySchedule);
    /// # schedule.add_systems(tick_counter);
    /// # world.init_resource::<Schedules>();
    /// # world.add_schedule(schedule);
    /// # fn tick_counter(mut counter: ResMut<Counter>) { counter.0 += 1; }
    /// // Run the schedule five times.
    /// world.schedule_scope(MySchedule, |world, schedule| {
    ///     for _ in 0..5 {
    ///         schedule.run(world);
    ///     }
    /// });
    /// # assert_eq!(world.resource::<Counter>().0, 5);
    /// ```
    ///
    /// For simple cases where you just need to call the schedule once,
    /// consider using [`World::run_schedule`] instead.
    ///
    /// # Panics
    ///
    /// If the requested schedule does not exist.
    pub fn schedule_scope<R>(
        &mut self,
        label: impl ScheduleLabel,
        f: impl FnOnce(&mut World, &mut Schedule) -> R,
    ) -> R {
        self.try_schedule_scope(label, f)
            .unwrap_or_else(|e| panic!("{e}"))
    }

    /// Attempts to run the [`Schedule`] associated with the `label` a single time,
    /// and returns a [`TryRunScheduleError`] if the schedule does not exist.
    ///
    /// The [`Schedule`] is fetched from the [`Schedules`] resource of the world by its label,
    /// and system state is cached.
    ///
    /// For simple testing use cases, call [`Schedule::run(&mut world)`](Schedule::run) instead.
    pub fn try_run_schedule(
        &mut self,
        label: impl ScheduleLabel,
    ) -> Result<(), TryRunScheduleError> {
        self.try_schedule_scope(label, |world, sched| sched.run(world))
    }

    /// Runs the [`Schedule`] associated with the `label` a single time.
    ///
    /// The [`Schedule`] is fetched from the [`Schedules`] resource of the world by its label,
    /// and system state is cached.
    ///
    /// For simple testing use cases, call [`Schedule::run(&mut world)`](Schedule::run) instead.
    /// This avoids the need to create a unique [`ScheduleLabel`].
    ///
    /// # Panics
    ///
    /// If the requested schedule does not exist.
    pub fn run_schedule(&mut self, label: impl ScheduleLabel) {
        self.schedule_scope(label, |world, sched| sched.run(world));
    }

    /// Ignore system order ambiguities caused by conflicts on [`Component`]s of type `T`.
    pub fn allow_ambiguous_component<T: Component>(&mut self) {
        let mut schedules = self.remove_resource::<Schedules>().unwrap_or_default();
        schedules.allow_ambiguous_component::<T>(self);
        self.insert_resource(schedules);
    }

    /// Ignore system order ambiguities caused by conflicts on [`Resource`]s of type `T`.
    pub fn allow_ambiguous_resource<T: Resource>(&mut self) {
        let mut schedules = self.remove_resource::<Schedules>().unwrap_or_default();
        schedules.allow_ambiguous_resource::<T>(self);
        self.insert_resource(schedules);
    }
}

impl fmt::Debug for World {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // SAFETY: `UnsafeWorldCell` requires that this must only access metadata.
        // Accessing any data stored in the world would be unsound.
        f.debug_struct("World")
            .field("id", &self.id)
            .field("entity_count", &self.entities.count_spawned())
            .field("archetype_count", &self.archetypes.len())
            .field("component_count", &self.components.len())
            .finish()
    }
}

// SAFETY: all methods on the world ensure that non-send resources are only accessible on the main thread
unsafe impl Send for World {}
// SAFETY: all methods on the world ensure that non-send resources are only accessible on the main thread
unsafe impl Sync for World {}

/// Creates an instance of the type this trait is implemented for
/// using data from the supplied [`World`].
///
/// This can be helpful for complex initialization or context-aware defaults.
///
/// [`FromWorld`] is automatically implemented for any type implementing [`Default`]
/// and may also be derived for:
/// - any struct whose fields all implement `FromWorld`
/// - any enum where one variant has the attribute `#[from_world]`
///
/// ```rs
///
/// #[derive(Default)]
/// struct A;
///
/// #[derive(Default)]
/// struct B(Option<u32>)
///
/// struct C;
///
/// impl FromWorld for C {
///     fn from_world(_world: &mut World) -> Self {
///         Self
///     }
/// }
///
/// #[derive(FromWorld)]
/// struct D(A, B, C);
///
/// #[derive(FromWorld)]
/// enum E {
///     #[from_world]
///     F,
///     G
/// }
/// ```
pub trait FromWorld {
    /// Creates `Self` using data from the given [`World`].
    fn from_world(world: &mut World) -> Self;
}

impl<T: Default> FromWorld for T {
    /// Creates `Self` using [`default()`](`Default::default`).
    #[track_caller]
    fn from_world(_world: &mut World) -> Self {
        T::default()
    }
}

#[cfg(test)]
#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
mod tests {
    use super::{FromWorld, World};
    use crate::{
        change_detection::{DetectChangesMut, MaybeLocation},
        component::{
            ComponentCloneBehavior, ComponentDescriptor, ComponentId, ComponentInfo, StorageType,
        },
        entity::EntityHashSet,
        entity_disabling::{DefaultQueryFilters, Disabled},
        prelude::{DetectChanges, Event, Mut, On, Res},
        ptr::OwningPtr,
        resource::Resource,
        world::{error::EntityMutableFetchError, DeferredWorld},
    };
    use alloc::{
        borrow::ToOwned,
        string::{String, ToString},
        sync::Arc,
        vec,
        vec::Vec,
    };
    use bevy_ecs_macros::Component;
    use bevy_platform::collections::{HashMap, HashSet};
    use bevy_utils::prelude::DebugName;
    use core::{
        any::TypeId,
        panic,
        sync::atomic::{AtomicBool, AtomicU32, Ordering},
    };
    use std::{println, sync::Mutex};

    type ID = u8;

    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
    enum DropLogItem {
        Create(ID),
        Drop(ID),
    }

    #[derive(Component)]
    struct MayPanicInDrop {
        drop_log: Arc<Mutex<Vec<DropLogItem>>>,
        expected_panic_flag: Arc<AtomicBool>,
        should_panic: bool,
        id: u8,
    }

    impl MayPanicInDrop {
        fn new(
            drop_log: &Arc<Mutex<Vec<DropLogItem>>>,
            expected_panic_flag: &Arc<AtomicBool>,
            should_panic: bool,
            id: u8,
        ) -> Self {
            println!("creating component with id {id}");
            drop_log.lock().unwrap().push(DropLogItem::Create(id));

            Self {
                drop_log: Arc::clone(drop_log),
                expected_panic_flag: Arc::clone(expected_panic_flag),
                should_panic,
                id,
            }
        }
    }

    impl Drop for MayPanicInDrop {
        fn drop(&mut self) {
            println!("dropping component with id {}", self.id);

            {
                let mut drop_log = self.drop_log.lock().unwrap();
                drop_log.push(DropLogItem::Drop(self.id));
                // Don't keep the mutex while panicking, or we'll poison it.
                drop(drop_log);
            }

            if self.should_panic {
                self.expected_panic_flag.store(true, Ordering::SeqCst);
                panic!("testing what happens on panic inside drop");
            }
        }
    }

    struct DropTestHelper {
        drop_log: Arc<Mutex<Vec<DropLogItem>>>,
        /// Set to `true` right before we intentionally panic, so that if we get
        /// a panic, we know if it was intended or not.
        expected_panic_flag: Arc<AtomicBool>,
    }

    impl DropTestHelper {
        pub fn new() -> Self {
            Self {
                drop_log: Arc::new(Mutex::new(Vec::<DropLogItem>::new())),
                expected_panic_flag: Arc::new(AtomicBool::new(false)),
            }
        }

        pub fn make_component(&self, should_panic: bool, id: ID) -> MayPanicInDrop {
            MayPanicInDrop::new(&self.drop_log, &self.expected_panic_flag, should_panic, id)
        }

        pub fn finish(self, panic_res: std::thread::Result<()>) -> Vec<DropLogItem> {
            let drop_log = self.drop_log.lock().unwrap();
            let expected_panic_flag = self.expected_panic_flag.load(Ordering::SeqCst);

            if !expected_panic_flag {
                match panic_res {
                    Ok(()) => panic!("Expected a panic but it didn't happen"),
                    Err(e) => std::panic::resume_unwind(e),
                }
            }

            drop_log.to_owned()
        }
    }

    #[test]
    fn panic_while_overwriting_component() {
        let helper = DropTestHelper::new();

        let res = std::panic::catch_unwind(|| {
            let mut world = World::new();
            world
                .spawn_empty()
                .insert(helper.make_component(true, 0))
                .insert(helper.make_component(false, 1));

            println!("Done inserting! Dropping world...");
        });

        let drop_log = helper.finish(res);

        assert_eq!(
            &*drop_log,
            [
                DropLogItem::Create(0),
                DropLogItem::Create(1),
                DropLogItem::Drop(0),
                DropLogItem::Drop(1),
            ]
        );
    }

    #[derive(Resource)]
    struct TestResource(u32);

    #[derive(Resource)]
    struct TestResource2(String);

    #[derive(Resource)]
    struct TestResource3;

    #[test]
    fn get_resource_by_id() {
        let mut world = World::new();
        world.insert_resource(TestResource(42));
        let component_id = world
            .components()
            .get_valid_id(TypeId::of::<TestResource>())
            .unwrap();

        let resource = world.get_resource_by_id(component_id).unwrap();
        // SAFETY: `TestResource` is the correct resource type
        let resource = unsafe { resource.deref::<TestResource>() };

        assert_eq!(resource.0, 42);
    }

    #[test]
    fn get_resource_mut_by_id() {
        let mut world = World::new();
        world.insert_resource(TestResource(42));
        let component_id = world
            .components()
            .get_valid_id(TypeId::of::<TestResource>())
            .unwrap();

        {
            let mut resource = world.get_resource_mut_by_id(component_id).unwrap();
            resource.set_changed();
            // SAFETY: `TestResource` is the correct resource type
            let resource = unsafe { resource.into_inner().deref_mut::<TestResource>() };
            resource.0 = 43;
        }

        let resource = world.get_resource_by_id(component_id).unwrap();
        // SAFETY: `TestResource` is the correct resource type
        let resource = unsafe { resource.deref::<TestResource>() };

        assert_eq!(resource.0, 43);
    }

    #[test]
    fn iter_resources() {
        let mut world = World::new();
        // Remove DefaultQueryFilters so it doesn't show up in the iterator
        world.remove_resource::<DefaultQueryFilters>();
        world.insert_resource(TestResource(42));
        world.insert_resource(TestResource2("Hello, world!".to_string()));
        world.insert_resource(TestResource3);
        world.remove_resource::<TestResource3>();

        let id1 = world.component_id::<TestResource>().unwrap();
        let id2 = world.component_id::<TestResource2>().unwrap();

        let mut iter = world.iter_resources();

        let (id, info, ptr) = iter.next().unwrap();
        assert_eq!(id, id1);
        assert_eq!(info.name(), DebugName::type_name::<TestResource>());
        // SAFETY: We know that the resource is of type `TestResource`
        assert_eq!(unsafe { ptr.deref::<TestResource>().0 }, 42);

        let (id, info, ptr) = iter.next().unwrap();
        assert_eq!(id, id2);
        assert_eq!(info.name(), DebugName::type_name::<TestResource2>());
        assert_eq!(
            // SAFETY: We know that the resource is of type `TestResource2`
            unsafe { &ptr.deref::<TestResource2>().0 },
            &"Hello, world!".to_string()
        );

        assert!(iter.next().is_none());
    }

    #[test]
    fn iter_resources_mut() {
        let mut world = World::new();
        // Remove DefaultQueryFilters so it doesn't show up in the iterator
        world.remove_resource::<DefaultQueryFilters>();
        world.insert_resource(TestResource(42));
        world.insert_resource(TestResource2("Hello, world!".to_string()));
        world.insert_resource(TestResource3);
        world.remove_resource::<TestResource3>();

        let id1 = world.component_id::<TestResource>().unwrap();
        let id2 = world.component_id::<TestResource2>().unwrap();

        let mut iter = world.iter_resources_mut();

        let (id, info, mut mut_untyped) = iter.next().unwrap();
        assert_eq!(id, id1);
        assert_eq!(info.name(), DebugName::type_name::<TestResource>());
        // SAFETY: We know that the resource is of type `TestResource`
        unsafe {
            mut_untyped.as_mut().deref_mut::<TestResource>().0 = 43;
        };

        let (id, info, mut mut_untyped) = iter.next().unwrap();
        assert_eq!(id, id2);
        assert_eq!(info.name(), DebugName::type_name::<TestResource2>());
        // SAFETY: We know that the resource is of type `TestResource2`
        unsafe {
            mut_untyped.as_mut().deref_mut::<TestResource2>().0 = "Hello, world?".to_string();
        };

        assert!(iter.next().is_none());
        drop(iter);

        assert_eq!(world.resource::<TestResource>().0, 43);
        assert_eq!(
            world.resource::<TestResource2>().0,
            "Hello, world?".to_string()
        );
    }

    #[test]
    fn custom_non_send_with_layout() {
        static DROP_COUNT: AtomicU32 = AtomicU32::new(0);

        let mut world = World::new();

        // SAFETY: the drop function is valid for the layout and the data will be safe to access from any thread
        let descriptor = unsafe {
            ComponentDescriptor::new_with_layout(
                "Custom Test Component".to_string(),
                StorageType::Table,
                core::alloc::Layout::new::<[u8; 8]>(),
                Some(|ptr| {
                    let data = ptr.read::<[u8; 8]>();
                    assert_eq!(data, [0, 1, 2, 3, 4, 5, 6, 7]);
                    DROP_COUNT.fetch_add(1, Ordering::SeqCst);
                }),
                true,
                false,
                ComponentCloneBehavior::Default,
                None,
            )
        };

        let component_id = world.register_component_with_descriptor(descriptor);

        let value: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
        OwningPtr::make(value, |ptr| {
            // SAFETY: value is valid for the component layout
            unsafe {
                world.insert_non_send_by_id(component_id, ptr, MaybeLocation::caller());
            }
        });

        // SAFETY: [u8; 8] is the correct type for the resource
        let data = unsafe {
            world
                .get_non_send_by_id(component_id)
                .unwrap()
                .deref::<[u8; 8]>()
        };
        assert_eq!(*data, [0, 1, 2, 3, 4, 5, 6, 7]);

        assert!(world.remove_non_send_by_id(component_id).is_some());

        assert_eq!(DROP_COUNT.load(Ordering::SeqCst), 1);
    }

    #[derive(Resource)]
    struct TestFromWorld(u32);
    impl FromWorld for TestFromWorld {
        fn from_world(world: &mut World) -> Self {
            let b = world.resource::<TestResource>();
            Self(b.0)
        }
    }

    #[test]
    fn init_resource_does_not_overwrite() {
        let mut world = World::new();
        world.insert_resource(TestResource(0));
        world.init_resource::<TestFromWorld>();
        world.insert_resource(TestResource(1));
        world.init_resource::<TestFromWorld>();

        let resource = world.resource::<TestFromWorld>();

        assert_eq!(resource.0, 0);
    }

    #[test]
    fn init_non_send_does_not_overwrite() {
        let mut world = World::new();
        world.insert_resource(TestResource(0));
        world.init_non_send::<TestFromWorld>();
        world.insert_resource(TestResource(1));
        world.init_non_send::<TestFromWorld>();

        let resource = world.non_send::<TestFromWorld>();

        assert_eq!(resource.0, 0);
    }

    #[derive(Component)]
    struct Foo;

    #[derive(Component)]
    struct Bar;

    #[derive(Component)]
    struct Baz;

    #[test]
    fn inspect_entity_components() {
        let mut world = World::new();
        let ent0 = world.spawn((Foo, Bar, Baz)).id();
        let ent1 = world.spawn((Foo, Bar)).id();
        let ent2 = world.spawn((Bar, Baz)).id();
        let ent3 = world.spawn((Foo, Baz)).id();
        let ent4 = world.spawn(Foo).id();
        let ent5 = world.spawn(Bar).id();
        let ent6 = world.spawn(Baz).id();

        fn to_type_ids(
            component_infos: Vec<(ComponentId, &ComponentInfo)>,
        ) -> HashSet<Option<TypeId>> {
            component_infos
                .into_iter()
                .map(|(_, info)| info.type_id())
                .collect()
        }

        let foo_id = TypeId::of::<Foo>();
        let bar_id = TypeId::of::<Bar>();
        let baz_id = TypeId::of::<Baz>();
        assert_eq!(
            to_type_ids(world.inspect_entity(ent0).unwrap().collect()),
            [Some(foo_id), Some(bar_id), Some(baz_id)]
                .into_iter()
                .collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent1).unwrap().collect()),
            [Some(foo_id), Some(bar_id)]
                .into_iter()
                .collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent2).unwrap().collect()),
            [Some(bar_id), Some(baz_id)]
                .into_iter()
                .collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent3).unwrap().collect()),
            [Some(foo_id), Some(baz_id)]
                .into_iter()
                .collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent4).unwrap().collect()),
            [Some(foo_id)].into_iter().collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent5).unwrap().collect()),
            [Some(bar_id)].into_iter().collect::<HashSet<_>>()
        );
        assert_eq!(
            to_type_ids(world.inspect_entity(ent6).unwrap().collect()),
            [Some(baz_id)].into_iter().collect::<HashSet<_>>()
        );
    }

    #[test]
    fn iterate_entities() {
        let mut world = World::new();
        let mut entity_counters = <HashMap<_, _>>::default();

        let iterate_and_count_entities = |world: &World, entity_counters: &mut HashMap<_, _>| {
            entity_counters.clear();
            for entity in world.iter_entities() {
                let counter = entity_counters.entry(entity.id()).or_insert(0);
                *counter += 1;
            }
        };

        // Adding one entity and validating iteration
        let ent0 = world.spawn((Foo, Bar, Baz)).id();

        iterate_and_count_entities(&world, &mut entity_counters);
        assert_eq!(entity_counters[&ent0], 1);
        assert_eq!(entity_counters.len(), 2);

        // Spawning three more entities and then validating iteration
        let ent1 = world.spawn((Foo, Bar)).id();
        let ent2 = world.spawn((Bar, Baz)).id();
        let ent3 = world.spawn((Foo, Baz)).id();

        iterate_and_count_entities(&world, &mut entity_counters);

        assert_eq!(entity_counters[&ent0], 1);
        assert_eq!(entity_counters[&ent1], 1);
        assert_eq!(entity_counters[&ent2], 1);
        assert_eq!(entity_counters[&ent3], 1);
        assert_eq!(entity_counters.len(), 5);

        // Despawning first entity and then validating the iteration
        assert!(world.despawn(ent0));

        iterate_and_count_entities(&world, &mut entity_counters);

        assert_eq!(entity_counters[&ent1], 1);
        assert_eq!(entity_counters[&ent2], 1);
        assert_eq!(entity_counters[&ent3], 1);
        assert_eq!(entity_counters.len(), 4);

        // Spawning three more entities, despawning three and then validating the iteration
        let ent4 = world.spawn(Foo).id();
        let ent5 = world.spawn(Bar).id();
        let ent6 = world.spawn(Baz).id();

        assert!(world.despawn(ent2));
        assert!(world.despawn(ent3));
        assert!(world.despawn(ent4));

        iterate_and_count_entities(&world, &mut entity_counters);

        assert_eq!(entity_counters[&ent1], 1);
        assert_eq!(entity_counters[&ent5], 1);
        assert_eq!(entity_counters[&ent6], 1);
        assert_eq!(entity_counters.len(), 4);

        // Despawning remaining entities and then validating the iteration
        assert!(world.despawn(ent1));
        assert!(world.despawn(ent5));
        assert!(world.despawn(ent6));

        iterate_and_count_entities(&world, &mut entity_counters);

        assert_eq!(entity_counters.len(), 1);
    }

    #[test]
    fn spawn_empty_bundle() {
        let mut world = World::new();
        world.spawn(());
    }

    #[test]
    fn get_entity() {
        let mut world = World::new();

        let e1 = world.spawn_empty().id();
        let e2 = world.spawn_empty().id();

        assert!(world.get_entity(e1).is_ok());
        assert!(world.get_entity([e1, e2]).is_ok());
        assert!(world
            .get_entity(&[e1, e2] /* this is an array not a slice */)
            .is_ok());
        assert!(world.get_entity(&vec![e1, e2][..]).is_ok());
        assert!(world
            .get_entity(&EntityHashSet::from_iter([e1, e2]))
            .is_ok());

        world.entity_mut(e1).despawn();

        assert_eq!(
            Err(e1),
            world.get_entity(e1).map(|_| {}).map_err(|e| e.entity())
        );
        assert_eq!(
            Err(e1),
            world
                .get_entity([e1, e2])
                .map(|_| {})
                .map_err(|e| e.entity())
        );
        assert_eq!(
            Err(e1),
            world
                .get_entity(&[e1, e2] /* this is an array not a slice */)
                .map(|_| {})
                .map_err(|e| e.entity())
        );
        assert_eq!(
            Err(e1),
            world
                .get_entity(&vec![e1, e2][..])
                .map(|_| {})
                .map_err(|e| e.entity())
        );
        assert_eq!(
            Err(e1),
            world
                .get_entity(&EntityHashSet::from_iter([e1, e2]))
                .map(|_| {})
                .map_err(|e| e.entity())
        );
    }

    #[test]
    fn get_entity_mut() {
        let mut world = World::new();

        let e1 = world.spawn_empty().id();
        let e2 = world.spawn_empty().id();

        assert!(world.get_entity_mut(e1).is_ok());
        assert!(world.get_entity_mut([e1, e2]).is_ok());
        assert!(world
            .get_entity_mut(&[e1, e2] /* this is an array not a slice */)
            .is_ok());
        assert!(world.get_entity_mut(&vec![e1, e2][..]).is_ok());
        assert!(world
            .get_entity_mut(&EntityHashSet::from_iter([e1, e2]))
            .is_ok());

        assert_eq!(
            Err(EntityMutableFetchError::AliasedMutability(e1)),
            world.get_entity_mut([e1, e2, e1]).map(|_| {})
        );
        assert_eq!(
            Err(EntityMutableFetchError::AliasedMutability(e1)),
            world
                .get_entity_mut(&[e1, e2, e1] /* this is an array not a slice */)
                .map(|_| {})
        );
        assert_eq!(
            Err(EntityMutableFetchError::AliasedMutability(e1)),
            world.get_entity_mut(&vec![e1, e2, e1][..]).map(|_| {})
        );
        // Aliased mutability isn't allowed by HashSets
        assert!(world
            .get_entity_mut(&EntityHashSet::from_iter([e1, e2, e1]))
            .is_ok());

        world.entity_mut(e1).despawn();
        assert!(world.get_entity_mut(e2).is_ok());

        assert!(matches!(
            world.get_entity_mut(e1).map(|_| {}),
            Err(EntityMutableFetchError::NotSpawned(e)) if e.entity() == e1
        ));
        assert!(matches!(
            world.get_entity_mut([e1, e2]).map(|_| {}),
            Err(EntityMutableFetchError::NotSpawned(e)) if e.entity() == e1));
        assert!(matches!(
            world
                .get_entity_mut(&[e1, e2] /* this is an array not a slice */)
                .map(|_| {}),
            Err(EntityMutableFetchError::NotSpawned(e)) if e.entity() == e1));
        assert!(matches!(
            world.get_entity_mut(&vec![e1, e2][..]).map(|_| {}),
            Err(EntityMutableFetchError::NotSpawned(e)) if e.entity() == e1,
        ));
        assert!(matches!(
            world
                .get_entity_mut(&EntityHashSet::from_iter([e1, e2]))
                .map(|_| {}),
            Err(EntityMutableFetchError::NotSpawned(e)) if e.entity() == e1));
    }

    #[test]
    #[track_caller]
    fn entity_spawn_despawn_tracking() {
        use core::panic::Location;

        let mut world = World::new();
        let entity = world.spawn_empty().id();
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_by(entity),
            MaybeLocation::new(Some(Location::caller()))
        );
        assert_eq!(
            world.entities.entity_get_spawn_or_despawn_tick(entity),
            Some(world.change_tick())
        );
        let new = world.despawn_no_free(entity).unwrap();
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_by(entity),
            MaybeLocation::new(Some(Location::caller()))
        );
        assert_eq!(
            world.entities.entity_get_spawn_or_despawn_tick(entity),
            Some(world.change_tick())
        );

        world.spawn_empty_at(new).unwrap();
        assert_eq!(entity.index(), new.index());
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_by(entity),
            MaybeLocation::new(None)
        );
        assert_eq!(
            world.entities.entity_get_spawn_or_despawn_tick(entity),
            None
        );
        world.despawn(new);
        assert_eq!(
            world.entities.entity_get_spawned_or_despawned_by(entity),
            MaybeLocation::new(None)
        );
        assert_eq!(
            world.entities.entity_get_spawn_or_despawn_tick(entity),
            None
        );
    }

    #[test]
    fn new_world_has_disabling() {
        let mut world = World::new();
        world.spawn(Foo);
        world.spawn((Foo, Disabled));
        assert_eq!(1, world.query::<&Foo>().iter(&world).count());

        // If we explicitly remove the resource, no entities should be filtered anymore
        world.remove_resource::<DefaultQueryFilters>();
        assert_eq!(2, world.query::<&Foo>().iter(&world).count());
    }

    #[test]
    fn entities_and_commands() {
        #[derive(Component, PartialEq, Debug)]
        struct Foo(u32);

        let mut world = World::new();

        let eid = world.spawn(Foo(35)).id();

        let (mut fetcher, mut commands) = world.entities_and_commands();
        let emut = fetcher.get_mut(eid).unwrap();
        commands.entity(eid).despawn();
        assert_eq!(emut.get::<Foo>().unwrap(), &Foo(35));

        world.flush();

        assert!(world.get_entity(eid).is_err());
    }

    #[test]
    fn resource_query_after_resource_scope() {
        #[derive(Event)]
        struct EventA;

        #[derive(Resource)]
        struct ResourceA;

        let mut world = World::default();

        world.insert_resource(ResourceA);
        world.add_observer(move |_event: On<EventA>, _res: Res<ResourceA>| {});
        world.resource_scope(|world, _res: Mut<ResourceA>| {
            // since we use commands, this should trigger outside of the resource_scope, so the observer should work.
            world.commands().trigger(EventA);
        });
    }

    #[test]
    fn entities_and_commands_deferred() {
        #[derive(Component, PartialEq, Debug)]
        struct Foo(u32);

        let mut world = World::new();

        let eid = world.spawn(Foo(1)).id();

        let mut dworld = DeferredWorld::from(&mut world);

        let (mut fetcher, mut commands) = dworld.entities_and_commands();
        let emut = fetcher.get_mut(eid).unwrap();
        commands.entity(eid).despawn();
        assert_eq!(emut.get::<Foo>().unwrap(), &Foo(1));

        world.flush();

        assert!(world.get_entity(eid).is_err());
    }

    #[test]
    fn resource_scope_ticks() {
        #[derive(Resource)]
        struct R;

        let mut world = World::new();
        world.insert_resource(R);
        world.resource_scope(|world, r: Mut<R>| {
            assert_eq!(world.change_tick(), r.added());
            assert_eq!(world.change_tick(), r.last_changed());
            world.increment_change_tick();
        });
        assert_eq!(world.change_tick(), world.resource_ref::<R>().added());
        assert_eq!(
            world.change_tick(),
            world.resource_ref::<R>().last_changed()
        );
    }

    #[test]
    fn world_resource_entity() {
        #[derive(Resource)]
        struct R1;

        #[derive(Resource)]
        struct R2;

        let mut world = World::new();
        world.insert_resource(R1);

        assert!(world.resource_entity::<R1>().is_some());
        assert!(world.resource_entity::<R2>().is_none());
    }
}