Use UnsafeWorldCell to increase code quality for SystemParam (#8174)

# Objective

The type `&World` is currently in an awkward place, since it has two
meanings:
1. Read-only access to the entire world.
2. Interior mutable access to the world; immutable and/or mutable access
to certain portions of world data.

This makes `&World` difficult to reason about, and surprising to see in
function signatures if one does not know about the interior mutable
property.

The type `UnsafeWorldCell` was added in #6404, which is meant to
alleviate this confusion by adding a dedicated type for interior mutable
world access. However, much of the engine still treats `&World` as an
interior mutable-ish type. One of those places is `SystemParam`.

## Solution

Modify `SystemParam::get_param` to accept `UnsafeWorldCell` instead of
`&World`. Simplify the safety invariants, since the `UnsafeWorldCell`
type encapsulates the concept of constrained world access.

---

## Changelog

`SystemParam::get_param` now accepts an `UnsafeWorldCell` instead of
`&World`. This type provides a high-level API for unsafe interior
mutable world access.

## Migration Guide

For manual implementers of `SystemParam`: the function `get_item` now
takes `UnsafeWorldCell` instead of `&World`. To access world data, use:

* `.get_entity()`, which returns an `UnsafeEntityCell` which can be used
to access component data.
* `get_resource()` and its variants, to access resource data.

crates/bevy_ecs/macros/src/lib.rs

@@ -229,7 +229,7 @@ pub fn impl_param_set(_input: TokenStream) -> TokenStream {
                 unsafe fn get_param<'w, 's>(
                     state: &'s mut Self::State,
                     system_meta: &SystemMeta,
-                    world: &'w World,
+                    world: UnsafeWorldCell<'w>,
                     change_tick: Tick,
                 ) -> Self::Item<'w, 's> {
                     ParamSet {
@@ -407,7 +407,7 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
                 unsafe fn get_param<'w, 's>(
                     state: &'s mut Self::State,
                     system_meta: &#path::system::SystemMeta,
-                    world: &'w #path::world::World,
+                    world: #path::world::unsafe_world_cell::UnsafeWorldCell<'w>,
                     change_tick: #path::component::Tick,
                 ) -> Self::Item<'w, 's> {
                     let (#(#tuple_patterns,)*) = <

crates/bevy_ecs/src/removal_detection.rs

@@ -8,7 +8,7 @@ use crate::{
     prelude::Local,
     storage::SparseSet,
     system::{ReadOnlySystemParam, SystemMeta, SystemParam},
-    world::World,
+    world::{unsafe_world_cell::UnsafeWorldCell, World},
 };
 
 use std::{
@@ -264,9 +264,9 @@ unsafe impl<'a> SystemParam for &'a RemovedComponentEvents {
     unsafe fn get_param<'w, 's>(
         _state: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
-        world.removed_components()
+        world.world_metadata().removed_components()
     }
 }

crates/bevy_ecs/src/system/function_system.rs

@@ -291,7 +291,12 @@ impl<Param: SystemParam> SystemState<Param> {
         world: &'w World,
         change_tick: Tick,
     ) -> SystemParamItem<'w, 's, Param> {
-        let param = Param::get_param(&mut self.param_state, &self.meta, world, change_tick);
+        let param = Param::get_param(
+            &mut self.param_state,
+            &self.meta,
+            world.as_unsafe_world_cell_migration_internal(),
+            change_tick,
+        );
         self.meta.last_run = change_tick;
         param
     }
@@ -481,7 +486,7 @@ where
         let params = F::Param::get_param(
             self.param_state.as_mut().expect(Self::PARAM_MESSAGE),
             &self.system_meta,
-            world,
+            world.as_unsafe_world_cell_migration_internal(),
             change_tick,
         );
         let out = self.func.run(input, params);

crates/bevy_ecs/src/system/system_param.rs

@@ -9,7 +9,7 @@ use crate::{
         Access, FilteredAccess, FilteredAccessSet, QueryState, ReadOnlyWorldQuery, WorldQuery,
     },
     system::{Query, SystemMeta},
-    world::{FromWorld, World},
+    world::{unsafe_world_cell::UnsafeWorldCell, FromWorld, World},
 };
 use bevy_ecs_macros::impl_param_set;
 pub use bevy_ecs_macros::Resource;
@@ -127,14 +127,13 @@ pub unsafe trait SystemParam: Sized {
 
     /// # Safety
     ///
-    /// This call might use any of the [`World`] accesses that were registered in [`Self::init_state`].
-    /// - None of those accesses may conflict with any other [`SystemParam`]s
-    ///   that exist at the same time, including those on other threads.
+    /// - The passed [`UnsafeWorldCell`] must have access to any world data
+    ///   registered in [`init_state`](SystemParam::init_state).
     /// - `world` must be the same `World` that was used to initialize [`state`](SystemParam::init_state).
     unsafe fn get_param<'world, 'state>(
         state: &'state mut Self::State,
         system_meta: &SystemMeta,
-        world: &'world World,
+        world: UnsafeWorldCell<'world>,
         change_tick: Tick,
     ) -> Self::Item<'world, 'state>;
 }
@@ -192,10 +191,19 @@ unsafe impl<Q: WorldQuery + 'static, F: ReadOnlyWorldQuery + 'static> SystemPara
     unsafe fn get_param<'w, 's>(
         state: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
-        Query::new(world, state, system_meta.last_run, change_tick, false)
+        Query::new(
+            // SAFETY: We have registered all of the query's world accesses,
+            // so the caller ensures that `world` has permission to access any
+            // world data that the query needs.
+            world.unsafe_world(),
+            state,
+            system_meta.last_run,
+            change_tick,
+            false,
+        )
     }
 }
 
@@ -329,7 +337,7 @@ fn assert_component_access_compatibility(
 /// ```
 pub struct ParamSet<'w, 's, T: SystemParam> {
     param_states: &'s mut T::State,
-    world: &'w World,
+    world: UnsafeWorldCell<'w>,
     system_meta: SystemMeta,
     change_tick: Tick,
 }
@@ -435,11 +443,10 @@ unsafe impl<'a, T: Resource> SystemParam for Res<'a, T> {
     unsafe fn get_param<'w, 's>(
         &mut component_id: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         let (ptr, ticks) = world
-            .as_unsafe_world_cell_migration_internal()
             .get_resource_with_ticks(component_id)
             .unwrap_or_else(|| {
                 panic!(
@@ -476,11 +483,10 @@ unsafe impl<'a, T: Resource> SystemParam for Option<Res<'a, T>> {
     unsafe fn get_param<'w, 's>(
         &mut component_id: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         world
-            .as_unsafe_world_cell_migration_internal()
             .get_resource_with_ticks(component_id)
             .map(|(ptr, ticks)| Res {
                 value: ptr.deref(),
@@ -530,11 +536,10 @@ unsafe impl<'a, T: Resource> SystemParam for ResMut<'a, T> {
     unsafe fn get_param<'w, 's>(
         &mut component_id: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         let value = world
-            .as_unsafe_world_cell_migration_internal()
             .get_resource_mut_by_id(component_id)
             .unwrap_or_else(|| {
                 panic!(
@@ -568,11 +573,10 @@ unsafe impl<'a, T: Resource> SystemParam for Option<ResMut<'a, T>> {
     unsafe fn get_param<'w, 's>(
         &mut component_id: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         world
-            .as_unsafe_world_cell_migration_internal()
             .get_resource_mut_by_id(component_id)
             .map(|value| ResMut {
                 value: value.value.deref_mut::<T>(),
@@ -621,10 +625,11 @@ unsafe impl SystemParam for &'_ World {
     unsafe fn get_param<'w, 's>(
         _state: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
-        world
+        // SAFETY: Read-only access to the entire world was registerd in `init_state`.
+        world.world()
     }
 }
 
@@ -742,7 +747,7 @@ unsafe impl<'a, T: FromWorld + Send + 'static> SystemParam for Local<'a, T> {
     unsafe fn get_param<'w, 's>(
         state: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        _world: &'w World,
+        _world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         Local(state.get())
@@ -916,7 +921,7 @@ unsafe impl<T: SystemBuffer> SystemParam for Deferred<'_, T> {
     unsafe fn get_param<'w, 's>(
         state: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        _world: &'w World,
+        _world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         Deferred(state.get())
@@ -1022,11 +1027,10 @@ unsafe impl<'a, T: 'static> SystemParam for NonSend<'a, T> {
     unsafe fn get_param<'w, 's>(
         &mut component_id: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         let (ptr, ticks) = world
-            .as_unsafe_world_cell_migration_internal()
             .get_non_send_with_ticks(component_id)
             .unwrap_or_else(|| {
                 panic!(
@@ -1061,11 +1065,10 @@ unsafe impl<T: 'static> SystemParam for Option<NonSend<'_, T>> {
     unsafe fn get_param<'w, 's>(
         &mut component_id: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         world
-            .as_unsafe_world_cell_migration_internal()
             .get_non_send_with_ticks(component_id)
             .map(|(ptr, ticks)| NonSend {
                 value: ptr.deref(),
@@ -1114,11 +1117,10 @@ unsafe impl<'a, T: 'static> SystemParam for NonSendMut<'a, T> {
     unsafe fn get_param<'w, 's>(
         &mut component_id: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         let (ptr, ticks) = world
-            .as_unsafe_world_cell_migration_internal()
             .get_non_send_with_ticks(component_id)
             .unwrap_or_else(|| {
                 panic!(
@@ -1147,11 +1149,10 @@ unsafe impl<'a, T: 'static> SystemParam for Option<NonSendMut<'a, T>> {
     unsafe fn get_param<'w, 's>(
         &mut component_id: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         world
-            .as_unsafe_world_cell_migration_internal()
             .get_non_send_with_ticks(component_id)
             .map(|(ptr, ticks)| NonSendMut {
                 value: ptr.assert_unique().deref_mut(),
@@ -1174,7 +1175,7 @@ unsafe impl<'a> SystemParam for &'a Archetypes {
     unsafe fn get_param<'w, 's>(
         _state: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         world.archetypes()
@@ -1195,7 +1196,7 @@ unsafe impl<'a> SystemParam for &'a Components {
     unsafe fn get_param<'w, 's>(
         _state: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         world.components()
@@ -1216,7 +1217,7 @@ unsafe impl<'a> SystemParam for &'a Entities {
     unsafe fn get_param<'w, 's>(
         _state: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         world.entities()
@@ -1237,7 +1238,7 @@ unsafe impl<'a> SystemParam for &'a Bundles {
     unsafe fn get_param<'w, 's>(
         _state: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         world.bundles()
@@ -1286,7 +1287,7 @@ unsafe impl SystemParam for SystemChangeTick {
     unsafe fn get_param<'w, 's>(
         _state: &'s mut Self::State,
         system_meta: &SystemMeta,
-        _world: &'w World,
+        _world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         SystemChangeTick {
@@ -1356,7 +1357,7 @@ unsafe impl SystemParam for SystemName<'_> {
     unsafe fn get_param<'w, 's>(
         name: &'s mut Self::State,
         _system_meta: &SystemMeta,
-        _world: &'w World,
+        _world: UnsafeWorldCell<'w>,
         _change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         SystemName { name }
@@ -1398,7 +1399,7 @@ macro_rules! impl_system_param_tuple {
             unsafe fn get_param<'w, 's>(
                 state: &'s mut Self::State,
                 _system_meta: &SystemMeta,
-                _world: &'w World,
+                _world: UnsafeWorldCell<'w>,
                 _change_tick: Tick,
             ) -> Self::Item<'w, 's> {
 
@@ -1521,7 +1522,7 @@ unsafe impl<P: SystemParam + 'static> SystemParam for StaticSystemParam<'_, '_,
     unsafe fn get_param<'world, 'state>(
         state: &'state mut Self::State,
         system_meta: &SystemMeta,
-        world: &'world World,
+        world: UnsafeWorldCell<'world>,
         change_tick: Tick,
     ) -> Self::Item<'world, 'state> {
         // SAFETY: Defer to the safety of P::SystemParam
@@ -1539,7 +1540,7 @@ unsafe impl<T: ?Sized> SystemParam for PhantomData<T> {
     unsafe fn get_param<'world, 'state>(
         _state: &'state mut Self::State,
         _system_meta: &SystemMeta,
-        _world: &'world World,
+        _world: UnsafeWorldCell<'world>,
         _change_tick: Tick,
     ) -> Self::Item<'world, 'state> {
         PhantomData

crates/bevy_ecs/src/world/identifier.rs

@@ -6,6 +6,8 @@ use crate::{
 };
 use std::sync::atomic::{AtomicUsize, Ordering};
 
+use super::unsafe_world_cell::UnsafeWorldCell;
+
 #[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
 // We use usize here because that is the largest `Atomic` we want to require
 /// A unique identifier for a [`World`].
@@ -56,10 +58,10 @@ unsafe impl SystemParam for WorldId {
     unsafe fn get_param<'world, 'state>(
         _: &'state mut Self::State,
         _: &crate::system::SystemMeta,
-        world: &'world super::World,
+        world: UnsafeWorldCell<'world>,
         _: Tick,
     ) -> Self::Item<'world, 'state> {
-        world.id
+        world.world_metadata().id()
     }
 }
 

crates/bevy_ecs/src/world/unsafe_world_cell.rs

@@ -144,7 +144,7 @@ impl<'w> UnsafeWorldCell<'w> {
     /// # Safety
     /// - must not be used in a way that would conflict with any
     ///   live exclusive borrows on world data
-    unsafe fn unsafe_world(self) -> &'w World {
+    pub(crate) unsafe fn unsafe_world(self) -> &'w World {
         // SAFETY:
         // - caller ensures that the returned `&World` is not used in a way that would conflict
         //   with any existing mutable borrows of world data

crates/bevy_render/src/extract_param.rs

@@ -3,6 +3,7 @@ use bevy_ecs::{
     component::Tick,
     prelude::*,
     system::{ReadOnlySystemParam, SystemMeta, SystemParam, SystemParamItem, SystemState},
+    world::unsafe_world_cell::UnsafeWorldCell,
 };
 use std::ops::{Deref, DerefMut};
 
@@ -76,7 +77,7 @@ where
     unsafe fn get_param<'w, 's>(
         state: &'s mut Self::State,
         system_meta: &SystemMeta,
-        world: &'w World,
+        world: UnsafeWorldCell<'w>,
         change_tick: Tick,
     ) -> Self::Item<'w, 's> {
         // SAFETY: