System param for dynamic resources (#15189)

# Objective

Support accessing dynamic resources in a dynamic system, including
accessing them by component id. This is similar to how dynamic
components can be queried using `Query<FilteredEntityMut>`.

## Solution

Create `FilteredResources` and `FilteredResourcesMut` types that act
similar to `FilteredEntityRef` and `FilteredEntityMut` and that can be
used as system parameters.

## Example

```rust
// Use `FilteredResourcesParamBuilder` to declare access to resources.
let system = (FilteredResourcesParamBuilder::new(|builder| {
    builder.add_read::<B>().add_read::<C>();
}),)
    .build_state(&mut world)
    .build_system(resource_system);

world.init_resource::<A>();
world.init_resource::<C>();

fn resource_system(res: FilteredResources) {
    // The resource exists, but we have no access, so we can't read it.
    assert!(res.get::<A>().is_none());
    // The resource doesn't exist, so we can't read it.
    assert!(res.get::<B>().is_none());
    // The resource exists and we have access, so we can read it.
    let c = res.get::<C>().unwrap();
    // The type parameter can be left out if it can be determined from use.
    let c: Res<C> = res.get().unwrap();
}
```

## Future Work

As a follow-up PR, `ReflectResource` can be modified to take `impl
Into<FilteredResources>`, similar to how `ReflectComponent` takes `impl
Into<FilteredEntityRef>`. That will allow dynamic resources to be
accessed using reflection.

crates/bevy_ecs/src/lib.rs

@@ -63,8 +63,8 @@ pub mod prelude {
             SystemParamFunction, WithParamWarnPolicy,
         },
         world::{
-            Command, EntityMut, EntityRef, EntityWorldMut, FromWorld, OnAdd, OnInsert, OnRemove,
+            Command, EntityMut, EntityRef, EntityWorldMut, FilteredResources, FilteredResourcesMut,
-            OnReplace, World,
+            FromWorld, OnAdd, OnInsert, OnRemove, OnReplace, World,
         },
     };
 

crates/bevy_ecs/src/query/access.rs

@@ -1,4 +1,6 @@
+use crate::component::ComponentId;
 use crate::storage::SparseSetIndex;
+use crate::world::World;
 use core::{fmt, fmt::Debug, marker::PhantomData};
 use fixedbitset::FixedBitSet;
 
@@ -727,6 +729,25 @@ impl<T: SparseSetIndex> Access<T> {
         AccessConflicts::Individual(conflicts)
     }
 
+    /// Returns the indices of the resources this has access to.
+    pub fn resource_reads_and_writes(&self) -> impl Iterator<Item = T> + '_ {
+        self.resource_read_and_writes
+            .ones()
+            .map(T::get_sparse_set_index)
+    }
+
+    /// Returns the indices of the resources this has non-exclusive access to.
+    pub fn resource_reads(&self) -> impl Iterator<Item = T> + '_ {
+        self.resource_read_and_writes
+            .difference(&self.resource_writes)
+            .map(T::get_sparse_set_index)
+    }
+
+    /// Returns the indices of the resources this has exclusive access to.
+    pub fn resource_writes(&self) -> impl Iterator<Item = T> + '_ {
+        self.resource_writes.ones().map(T::get_sparse_set_index)
+    }
+
     /// Returns the indices of the components that this has an archetypal access to.
     ///
     /// These are components whose values are not accessed (and thus will never cause conflicts),
@@ -863,6 +884,24 @@ impl AccessConflicts {
         }
     }
 
+    pub(crate) fn format_conflict_list(&self, world: &World) -> String {
+        match self {
+            AccessConflicts::All => String::new(),
+            AccessConflicts::Individual(indices) => format!(
+                " {}",
+                indices
+                    .ones()
+                    .map(|index| world
+                        .components
+                        .get_info(ComponentId::get_sparse_set_index(index))
+                        .unwrap()
+                        .name())
+                    .collect::<Vec<&str>>()
+                    .join(", ")
+            ),
+        }
+    }
+
     /// An [`AccessConflicts`] which represents the absence of any conflict
     pub(crate) fn empty() -> Self {
         Self::Individual(FixedBitSet::new())
@@ -1239,6 +1278,20 @@ impl<T: SparseSetIndex> FilteredAccessSet<T> {
         self.add(filter);
     }
 
+    /// Adds read access to all resources to the set.
+    pub(crate) fn add_unfiltered_read_all_resources(&mut self) {
+        let mut filter = FilteredAccess::default();
+        filter.access.read_all_resources();
+        self.add(filter);
+    }
+
+    /// Adds write access to all resources to the set.
+    pub(crate) fn add_unfiltered_write_all_resources(&mut self) {
+        let mut filter = FilteredAccess::default();
+        filter.access.write_all_resources();
+        self.add(filter);
+    }
+
     /// Adds all of the accesses from the passed set to `self`.
     pub fn extend(&mut self, filtered_access_set: FilteredAccessSet<T>) {
         self.combined_access

crates/bevy_ecs/src/system/builder.rs

@@ -6,7 +6,10 @@ use crate::{
     system::{
         DynSystemParam, DynSystemParamState, Local, ParamSet, Query, SystemMeta, SystemParam,
     },
-    world::{FromWorld, World},
+    world::{
+        FilteredResources, FilteredResourcesBuilder, FilteredResourcesMut,
+        FilteredResourcesMutBuilder, FromWorld, World,
+    },
 };
 use core::fmt::Debug;
 
@@ -77,6 +80,10 @@ use super::{init_query_param, Res, ResMut, Resource, SystemState};
 ///
 /// [`LocalBuilder`] can build a [`Local`] to supply the initial value for the `Local`.
 ///
+/// [`FilteredResourcesParamBuilder`] can build a [`FilteredResources`],
+/// and [`FilteredResourcesMutParamBuilder`] can build a [`FilteredResourcesMut`],
+/// to configure the resources that can be accessed.
+///
 /// [`DynParamBuilder`] can build a [`DynSystemParam`] to determine the type of the inner parameter,
 /// and to supply any `SystemParamBuilder` it needs.
 ///
@@ -526,6 +533,147 @@ unsafe impl<'s, T: FromWorld + Send + 'static> SystemParamBuilder<Local<'s, T>>
     }
 }
 
+/// A [`SystemParamBuilder`] for a [`FilteredResources`].
+/// See the [`FilteredResources`] docs for examples.
+pub struct FilteredResourcesParamBuilder<T>(T);
+
+impl<T> FilteredResourcesParamBuilder<T> {
+    /// Creates a [`SystemParamBuilder`] for a [`FilteredResources`] that accepts a callback to configure the [`FilteredResourcesBuilder`].
+    pub fn new(f: T) -> Self
+    where
+        T: FnOnce(&mut FilteredResourcesBuilder),
+    {
+        Self(f)
+    }
+}
+
+impl<'a> FilteredResourcesParamBuilder<Box<dyn FnOnce(&mut FilteredResourcesBuilder) + 'a>> {
+    /// Creates a [`SystemParamBuilder`] for a [`FilteredResources`] that accepts a callback to configure the [`FilteredResourcesBuilder`].
+    /// This boxes the callback so that it has a common type.
+    pub fn new_box(f: impl FnOnce(&mut FilteredResourcesBuilder) + 'a) -> Self {
+        Self(Box::new(f))
+    }
+}
+
+// SAFETY: Resource ComponentId and ArchetypeComponentId access is applied to SystemMeta. If this FilteredResources
+// conflicts with any prior access, a panic will occur.
+unsafe impl<'w, 's, T: FnOnce(&mut FilteredResourcesBuilder)>
+    SystemParamBuilder<FilteredResources<'w, 's>> for FilteredResourcesParamBuilder<T>
+{
+    fn build(
+        self,
+        world: &mut World,
+        meta: &mut SystemMeta,
+    ) -> <FilteredResources<'w, 's> as SystemParam>::State {
+        let mut builder = FilteredResourcesBuilder::new(world);
+        (self.0)(&mut builder);
+        let access = builder.build();
+
+        let combined_access = meta.component_access_set.combined_access();
+        let conflicts = combined_access.get_conflicts(&access);
+        if !conflicts.is_empty() {
+            let accesses = conflicts.format_conflict_list(world);
+            let system_name = &meta.name;
+            panic!("error[B0002]: FilteredResources in system {system_name} accesses resources(s){accesses} in a way that conflicts with a previous system parameter. Consider removing the duplicate access. See: https://bevyengine.org/learn/errors/#b0002");
+        }
+
+        if access.has_read_all_resources() {
+            meta.component_access_set
+                .add_unfiltered_read_all_resources();
+            meta.archetype_component_access.read_all_resources();
+        } else {
+            for component_id in access.resource_reads_and_writes() {
+                meta.component_access_set
+                    .add_unfiltered_resource_read(component_id);
+
+                let archetype_component_id = world.initialize_resource_internal(component_id).id();
+                meta.archetype_component_access
+                    .add_resource_read(archetype_component_id);
+            }
+        }
+
+        access
+    }
+}
+
+/// A [`SystemParamBuilder`] for a [`FilteredResourcesMut`].
+/// See the [`FilteredResourcesMut`] docs for examples.
+pub struct FilteredResourcesMutParamBuilder<T>(T);
+
+impl<T> FilteredResourcesMutParamBuilder<T> {
+    /// Creates a [`SystemParamBuilder`] for a [`FilteredResourcesMut`] that accepts a callback to configure the [`FilteredResourcesMutBuilder`].
+    pub fn new(f: T) -> Self
+    where
+        T: FnOnce(&mut FilteredResourcesMutBuilder),
+    {
+        Self(f)
+    }
+}
+
+impl<'a> FilteredResourcesMutParamBuilder<Box<dyn FnOnce(&mut FilteredResourcesMutBuilder) + 'a>> {
+    /// Creates a [`SystemParamBuilder`] for a [`FilteredResourcesMut`] that accepts a callback to configure the [`FilteredResourcesMutBuilder`].
+    /// This boxes the callback so that it has a common type.
+    pub fn new_box(f: impl FnOnce(&mut FilteredResourcesMutBuilder) + 'a) -> Self {
+        Self(Box::new(f))
+    }
+}
+
+// SAFETY: Resource ComponentId and ArchetypeComponentId access is applied to SystemMeta. If this FilteredResources
+// conflicts with any prior access, a panic will occur.
+unsafe impl<'w, 's, T: FnOnce(&mut FilteredResourcesMutBuilder)>
+    SystemParamBuilder<FilteredResourcesMut<'w, 's>> for FilteredResourcesMutParamBuilder<T>
+{
+    fn build(
+        self,
+        world: &mut World,
+        meta: &mut SystemMeta,
+    ) -> <FilteredResourcesMut<'w, 's> as SystemParam>::State {
+        let mut builder = FilteredResourcesMutBuilder::new(world);
+        (self.0)(&mut builder);
+        let access = builder.build();
+
+        let combined_access = meta.component_access_set.combined_access();
+        let conflicts = combined_access.get_conflicts(&access);
+        if !conflicts.is_empty() {
+            let accesses = conflicts.format_conflict_list(world);
+            let system_name = &meta.name;
+            panic!("error[B0002]: FilteredResourcesMut in system {system_name} accesses resources(s){accesses} in a way that conflicts with a previous system parameter. Consider removing the duplicate access. See: https://bevyengine.org/learn/errors/#b0002");
+        }
+
+        if access.has_read_all_resources() {
+            meta.component_access_set
+                .add_unfiltered_read_all_resources();
+            meta.archetype_component_access.read_all_resources();
+        } else {
+            for component_id in access.resource_reads() {
+                meta.component_access_set
+                    .add_unfiltered_resource_read(component_id);
+
+                let archetype_component_id = world.initialize_resource_internal(component_id).id();
+                meta.archetype_component_access
+                    .add_resource_read(archetype_component_id);
+            }
+        }
+
+        if access.has_write_all_resources() {
+            meta.component_access_set
+                .add_unfiltered_write_all_resources();
+            meta.archetype_component_access.write_all_resources();
+        } else {
+            for component_id in access.resource_writes() {
+                meta.component_access_set
+                    .add_unfiltered_resource_write(component_id);
+
+                let archetype_component_id = world.initialize_resource_internal(component_id).id();
+                meta.archetype_component_access
+                    .add_resource_write(archetype_component_id);
+            }
+        }
+
+        access
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate as bevy_ecs;
@@ -546,6 +694,9 @@ mod tests {
     #[derive(Component)]
     struct C;
 
+    #[derive(Resource, Default)]
+    struct R;
+
     fn local_system(local: Local<u64>) -> u64 {
         *local
     }
@@ -774,4 +925,114 @@ mod tests {
         let output = world.run_system_once(system).unwrap();
         assert_eq!(output, 101);
     }
+
+    #[test]
+    fn filtered_resource_conflicts_read_with_res() {
+        let mut world = World::new();
+        (
+            ParamBuilder::resource(),
+            FilteredResourcesParamBuilder::new(|builder| {
+                builder.add_read::<R>();
+            }),
+        )
+            .build_state(&mut world)
+            .build_system(|_r: Res<R>, _fr: FilteredResources| {});
+    }
+
+    #[test]
+    #[should_panic]
+    fn filtered_resource_conflicts_read_with_resmut() {
+        let mut world = World::new();
+        (
+            ParamBuilder::resource_mut(),
+            FilteredResourcesParamBuilder::new(|builder| {
+                builder.add_read::<R>();
+            }),
+        )
+            .build_state(&mut world)
+            .build_system(|_r: ResMut<R>, _fr: FilteredResources| {});
+    }
+
+    #[test]
+    #[should_panic]
+    fn filtered_resource_conflicts_read_all_with_resmut() {
+        let mut world = World::new();
+        (
+            ParamBuilder::resource_mut(),
+            FilteredResourcesParamBuilder::new(|builder| {
+                builder.add_read_all();
+            }),
+        )
+            .build_state(&mut world)
+            .build_system(|_r: ResMut<R>, _fr: FilteredResources| {});
+    }
+
+    #[test]
+    fn filtered_resource_mut_conflicts_read_with_res() {
+        let mut world = World::new();
+        (
+            ParamBuilder::resource(),
+            FilteredResourcesMutParamBuilder::new(|builder| {
+                builder.add_read::<R>();
+            }),
+        )
+            .build_state(&mut world)
+            .build_system(|_r: Res<R>, _fr: FilteredResourcesMut| {});
+    }
+
+    #[test]
+    #[should_panic]
+    fn filtered_resource_mut_conflicts_read_with_resmut() {
+        let mut world = World::new();
+        (
+            ParamBuilder::resource_mut(),
+            FilteredResourcesMutParamBuilder::new(|builder| {
+                builder.add_read::<R>();
+            }),
+        )
+            .build_state(&mut world)
+            .build_system(|_r: ResMut<R>, _fr: FilteredResourcesMut| {});
+    }
+
+    #[test]
+    #[should_panic]
+    fn filtered_resource_mut_conflicts_write_with_res() {
+        let mut world = World::new();
+        (
+            ParamBuilder::resource(),
+            FilteredResourcesMutParamBuilder::new(|builder| {
+                builder.add_write::<R>();
+            }),
+        )
+            .build_state(&mut world)
+            .build_system(|_r: Res<R>, _fr: FilteredResourcesMut| {});
+    }
+
+    #[test]
+    #[should_panic]
+    fn filtered_resource_mut_conflicts_write_all_with_res() {
+        let mut world = World::new();
+        (
+            ParamBuilder::resource(),
+            FilteredResourcesMutParamBuilder::new(|builder| {
+                builder.add_write_all();
+            }),
+        )
+            .build_state(&mut world)
+            .build_system(|_r: Res<R>, _fr: FilteredResourcesMut| {});
+    }
+
+    #[test]
+    #[should_panic]
+    fn filtered_resource_mut_conflicts_write_with_resmut() {
+        let mut world = World::new();
+        (
+            ParamBuilder::resource_mut(),
+            FilteredResourcesMutParamBuilder::new(|builder| {
+                builder.add_write::<R>();
+            }),
+        )
+            .build_state(&mut world)
+            .build_system(|_r: ResMut<R>, _fr: FilteredResourcesMut| {});
+    }
 }

crates/bevy_ecs/src/system/mod.rs

@@ -105,6 +105,8 @@
 //! In addition, the following parameters can be used when constructing a dynamic system with [`SystemParamBuilder`],
 //! but will only provide an empty value when used with an ordinary system:
 //!
+//! - [`FilteredResources`](crate::world::FilteredResources)
+//! - [`FilteredResourcesMut`](crate::world::FilteredResourcesMut)
 //! - [`DynSystemParam`]
 //! - [`Vec<P>`] where `P: SystemParam`
 //! - [`ParamSet<Vec<P>>`] where `P: SystemParam`

crates/bevy_ecs/src/system/system_param.rs

@@ -6,12 +6,15 @@ use crate::{
     component::{ComponentId, ComponentTicks, Components, Tick},
     entity::Entities,
     query::{
-        Access, AccessConflicts, FilteredAccess, FilteredAccessSet, QueryData, QueryFilter,
+        Access, FilteredAccess, FilteredAccessSet, QueryData, QueryFilter, QuerySingleError,
-        QuerySingleError, QueryState, ReadOnlyQueryData,
+        QueryState, ReadOnlyQueryData,
     },
-    storage::{ResourceData, SparseSetIndex},
+    storage::ResourceData,
     system::{Query, Single, SystemMeta},
-    world::{unsafe_world_cell::UnsafeWorldCell, DeferredWorld, FromWorld, World},
+    world::{
+        unsafe_world_cell::UnsafeWorldCell, DeferredWorld, FilteredResources, FilteredResourcesMut,
+        FromWorld, World,
+    },
 };
 use bevy_ecs_macros::impl_param_set;
 pub use bevy_ecs_macros::{Resource, SystemParam};
@@ -349,21 +352,7 @@ fn assert_component_access_compatibility(
     if conflicts.is_empty() {
         return;
     }
-    let accesses = match conflicts {
+    let accesses = conflicts.format_conflict_list(world);
-        AccessConflicts::All => "",
-        AccessConflicts::Individual(indices) => &format!(
-            " {}",
-            indices
-                .ones()
-                .map(|index| world
-                    .components
-                    .get_info(ComponentId::get_sparse_set_index(index))
-                    .unwrap()
-                    .name())
-                .collect::<Vec<&str>>()
-                .join(", ")
-        ),
-    };
     panic!("error[B0001]: Query<{query_type}, {filter_type}> in system {system_name} accesses component(s){accesses} in a way that conflicts with a previous system parameter. Consider using `Without<T>` to create disjoint Queries or merging conflicting Queries into a `ParamSet`. See: https://bevyengine.org/learn/errors/b0001");
 }
 
@@ -2447,6 +2436,57 @@ unsafe impl SystemParam for DynSystemParam<'_, '_> {
     }
 }
 
+// SAFETY: When initialized with `init_state`, `get_param` returns a `FilteredResources` with no access.
+// Therefore, `init_state` trivially registers all access, and no accesses can conflict.
+// Note that the safety requirements for non-empty access are handled by the `SystemParamBuilder` impl that builds them.
+unsafe impl SystemParam for FilteredResources<'_, '_> {
+    type State = Access<ComponentId>;
+
+    type Item<'world, 'state> = FilteredResources<'world, 'state>;
+
+    fn init_state(_world: &mut World, _system_meta: &mut SystemMeta) -> Self::State {
+        Access::new()
+    }
+
+    unsafe fn get_param<'world, 'state>(
+        state: &'state mut Self::State,
+        system_meta: &SystemMeta,
+        world: UnsafeWorldCell<'world>,
+        change_tick: Tick,
+    ) -> Self::Item<'world, 'state> {
+        // SAFETY: The caller ensures that `world` has access to anything registered in `init_state` or `build`,
+        // and the builder registers `access` in `build`.
+        unsafe { FilteredResources::new(world, state, system_meta.last_run, change_tick) }
+    }
+}
+
+// SAFETY: FilteredResources only reads resources.
+unsafe impl ReadOnlySystemParam for FilteredResources<'_, '_> {}
+
+// SAFETY: When initialized with `init_state`, `get_param` returns a `FilteredResourcesMut` with no access.
+// Therefore, `init_state` trivially registers all access, and no accesses can conflict.
+// Note that the safety requirements for non-empty access are handled by the `SystemParamBuilder` impl that builds them.
+unsafe impl SystemParam for FilteredResourcesMut<'_, '_> {
+    type State = Access<ComponentId>;
+
+    type Item<'world, 'state> = FilteredResourcesMut<'world, 'state>;
+
+    fn init_state(_world: &mut World, _system_meta: &mut SystemMeta) -> Self::State {
+        Access::new()
+    }
+
+    unsafe fn get_param<'world, 'state>(
+        state: &'state mut Self::State,
+        system_meta: &SystemMeta,
+        world: UnsafeWorldCell<'world>,
+        change_tick: Tick,
+    ) -> Self::Item<'world, 'state> {
+        // SAFETY: The caller ensures that `world` has access to anything registered in `init_state` or `build`,
+        // and the builder registers `access` in `build`.
+        unsafe { FilteredResourcesMut::new(world, state, system_meta.last_run, change_tick) }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

crates/bevy_ecs/src/world/filtered_resource.rs

@@ -0,0 +1,625 @@
+use std::sync::OnceLock;
+
+use crate::{
+    change_detection::{Mut, MutUntyped, Ref, Ticks, TicksMut},
+    component::{ComponentId, Tick},
+    query::Access,
+    system::Resource,
+    world::{unsafe_world_cell::UnsafeWorldCell, World},
+};
+use bevy_ptr::Ptr;
+#[cfg(feature = "track_change_detection")]
+use bevy_ptr::UnsafeCellDeref;
+
+/// Provides read-only access to a set of [`Resource`]s defined by the contained [`Access`].
+///
+/// Use [`FilteredResourcesMut`] if you need mutable access to some resources.
+///
+/// To be useful as a [`SystemParam`](crate::system::SystemParam),
+/// this must be configured using a [`FilteredResourcesParamBuilder`](crate::system::FilteredResourcesParamBuilder)
+/// to build the system using a [`SystemParamBuilder`](crate::prelude::SystemParamBuilder).
+///
+/// # Examples
+///
+/// ```
+/// # use bevy_ecs::{prelude::*, system::*};
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct A;
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct B;
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct C;
+/// #
+/// # let mut world = World::new();
+/// // Use `FilteredResourcesParamBuilder` to declare access to resources.
+/// let system = (FilteredResourcesParamBuilder::new(|builder| {
+///     builder.add_read::<B>().add_read::<C>();
+/// }),)
+///     .build_state(&mut world)
+///     .build_system(resource_system);
+///
+/// world.init_resource::<A>();
+/// world.init_resource::<C>();
+///
+/// fn resource_system(res: FilteredResources) {
+///     // The resource exists, but we have no access, so we can't read it.
+///     assert!(res.get::<A>().is_none());
+///     // The resource doesn't exist, so we can't read it.
+///     assert!(res.get::<B>().is_none());
+///     // The resource exists and we have access, so we can read it.
+///     let c = res.get::<C>().unwrap();
+///     // The type parameter can be left out if it can be determined from use.
+///     let c: Ref<C> = res.get().unwrap();
+/// }
+/// #
+/// # world.run_system_once(system);
+/// ```
+///
+/// This can be used alongside ordinary [`Res`](crate::system::Res) and [`ResMut`](crate::system::ResMut) parameters if they do not conflict.
+///
+/// ```
+/// # use bevy_ecs::{prelude::*, system::*};
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct A;
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct B;
+/// #
+/// # let mut world = World::new();
+/// # world.init_resource::<A>();
+/// # world.init_resource::<B>();
+/// #
+/// let system = (
+///     FilteredResourcesParamBuilder::new(|builder| {
+///         builder.add_read::<A>();
+///     }),
+///     ParamBuilder,
+///     ParamBuilder,
+/// )
+///     .build_state(&mut world)
+///     .build_system(resource_system);
+///
+/// // Read access to A does not conflict with read access to A or write access to B.
+/// fn resource_system(filtered: FilteredResources, res_a: Res<A>, res_mut_b: ResMut<B>) {
+///     let res_a_2: Ref<A> = filtered.get::<A>().unwrap();
+/// }
+/// #
+/// # world.run_system_once(system);
+/// ```
+///
+/// But it will conflict if it tries to read the same resource that another parameter writes.
+///
+/// ```should_panic
+/// # use bevy_ecs::{prelude::*, system::*};
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct A;
+/// #
+/// # let mut world = World::new();
+/// # world.init_resource::<A>();
+/// #
+/// let system = (
+///     FilteredResourcesParamBuilder::new(|builder| {
+///         builder.add_read::<A>();
+///     }),
+///     ParamBuilder,
+/// )
+///     .build_state(&mut world)
+///     .build_system(invalid_resource_system);
+///
+/// // Read access to A conflicts with write access to A.
+/// fn invalid_resource_system(filtered: FilteredResources, res_mut_a: ResMut<A>) { }
+/// #
+/// # world.run_system_once(system);
+/// ```
+#[derive(Clone, Copy)]
+pub struct FilteredResources<'w, 's> {
+    world: UnsafeWorldCell<'w>,
+    access: &'s Access<ComponentId>,
+    last_run: Tick,
+    this_run: Tick,
+}
+
+impl<'w, 's> FilteredResources<'w, 's> {
+    /// Creates a new [`FilteredResources`].
+    /// # Safety
+    /// It is the callers responsibility to ensure that nothing else may access the any resources in the `world` in a way that conflicts with `access`.
+    pub(crate) unsafe fn new(
+        world: UnsafeWorldCell<'w>,
+        access: &'s Access<ComponentId>,
+        last_run: Tick,
+        this_run: Tick,
+    ) -> Self {
+        Self {
+            world,
+            access,
+            last_run,
+            this_run,
+        }
+    }
+
+    /// Returns a reference to the underlying [`Access`].
+    pub fn access(&self) -> &Access<ComponentId> {
+        self.access
+    }
+
+    /// Returns `true` if the `FilteredResources` has access to the given resource.
+    /// Note that [`Self::get()`] may still return `None` if the resource does not exist.
+    pub fn has_read<R: Resource>(&self) -> bool {
+        let component_id = self.world.components().resource_id::<R>();
+        component_id.is_some_and(|component_id| self.access.has_resource_read(component_id))
+    }
+
+    /// Gets a reference to the resource of the given type if it exists and the `FilteredResources` has access to it.
+    pub fn get<R: Resource>(&self) -> Option<Ref<'w, R>> {
+        let component_id = self.world.components().resource_id::<R>()?;
+        if !self.access.has_resource_read(component_id) {
+            return None;
+        }
+        // SAFETY: We have read access to this resource
+        unsafe { self.world.get_resource_with_ticks(component_id) }.map(
+            |(value, ticks, _caller)| Ref {
+                // SAFETY: `component_id` was obtained from the type ID of `R`.
+                value: unsafe { value.deref() },
+                // SAFETY: We have read access to the resource, so no mutable reference can exist.
+                ticks: unsafe { Ticks::from_tick_cells(ticks, self.last_run, self.this_run) },
+                #[cfg(feature = "track_change_detection")]
+                // SAFETY: We have read access to the resource, so no mutable reference can exist.
+                changed_by: unsafe { _caller.deref() },
+            },
+        )
+    }
+
+    /// Gets a pointer to the resource with the given [`ComponentId`] if it exists and the `FilteredResources` has access to it.
+    pub fn get_by_id(&self, component_id: ComponentId) -> Option<Ptr<'w>> {
+        if !self.access.has_resource_read(component_id) {
+            return None;
+        }
+        // SAFETY: We have read access to this resource
+        unsafe { self.world.get_resource_by_id(component_id) }
+    }
+}
+
+impl<'w, 's> From<FilteredResourcesMut<'w, 's>> for FilteredResources<'w, 's> {
+    fn from(resources: FilteredResourcesMut<'w, 's>) -> Self {
+        // SAFETY:
+        // - `FilteredResourcesMut` guarantees exclusive access to all resources in the new `FilteredResources`.
+        unsafe {
+            FilteredResources::new(
+                resources.world,
+                resources.access,
+                resources.last_run,
+                resources.this_run,
+            )
+        }
+    }
+}
+
+impl<'w, 's> From<&'w FilteredResourcesMut<'_, 's>> for FilteredResources<'w, 's> {
+    fn from(resources: &'w FilteredResourcesMut<'_, 's>) -> Self {
+        // SAFETY:
+        // - `FilteredResourcesMut` guarantees exclusive access to all components in the new `FilteredResources`.
+        unsafe {
+            FilteredResources::new(
+                resources.world,
+                resources.access,
+                resources.last_run,
+                resources.this_run,
+            )
+        }
+    }
+}
+
+impl<'w> From<&'w World> for FilteredResources<'w, 'static> {
+    fn from(value: &'w World) -> Self {
+        static READ_ALL_RESOURCES: OnceLock<Access<ComponentId>> = OnceLock::new();
+        let access = READ_ALL_RESOURCES.get_or_init(|| {
+            let mut access = Access::new();
+            access.read_all_resources();
+            access
+        });
+
+        let last_run = value.last_change_tick();
+        let this_run = value.read_change_tick();
+        // SAFETY: We have a reference to the entire world, so nothing else can alias with read access to all resources.
+        unsafe {
+            Self::new(
+                value.as_unsafe_world_cell_readonly(),
+                access,
+                last_run,
+                this_run,
+            )
+        }
+    }
+}
+
+impl<'w> From<&'w mut World> for FilteredResources<'w, 'static> {
+    fn from(value: &'w mut World) -> Self {
+        Self::from(&*value)
+    }
+}
+
+/// Provides mutable access to a set of [`Resource`]s defined by the contained [`Access`].
+///
+/// Use [`FilteredResources`] if you only need read-only access to resources.
+///
+/// To be useful as a [`SystemParam`](crate::system::SystemParam),
+/// this must be configured using a [`FilteredResourcesMutParamBuilder`](crate::system::FilteredResourcesMutParamBuilder)
+/// to build the system using a [`SystemParamBuilder`](crate::prelude::SystemParamBuilder).
+///
+/// # Examples
+///
+/// ```
+/// # use bevy_ecs::{prelude::*, system::*};
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct A;
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct B;
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct C;
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct D;
+/// #
+/// # let mut world = World::new();
+/// // Use `FilteredResourcesMutParamBuilder` to declare access to resources.
+/// let system = (FilteredResourcesMutParamBuilder::new(|builder| {
+///     builder.add_write::<B>().add_read::<C>().add_write::<D>();
+/// }),)
+///     .build_state(&mut world)
+///     .build_system(resource_system);
+///
+/// world.init_resource::<A>();
+/// world.init_resource::<C>();
+/// world.init_resource::<D>();
+///
+/// fn resource_system(mut res: FilteredResourcesMut) {
+///     // The resource exists, but we have no access, so we can't read it or write it.
+///     assert!(res.get::<A>().is_none());
+///     assert!(res.get_mut::<A>().is_none());
+///     // The resource doesn't exist, so we can't read it or write it.
+///     assert!(res.get::<B>().is_none());
+///     assert!(res.get_mut::<B>().is_none());
+///     // The resource exists and we have read access, so we can read it but not write it.
+///     let c = res.get::<C>().unwrap();
+///     assert!(res.get_mut::<C>().is_none());
+///     // The resource exists and we have write access, so we can read it or write it.
+///     let d = res.get::<D>().unwrap();
+///     let d = res.get_mut::<D>().unwrap();
+///     // The type parameter can be left out if it can be determined from use.
+///     let c: Ref<C> = res.get().unwrap();
+/// }
+/// #
+/// # world.run_system_once(system);
+/// ```
+///
+/// This can be used alongside ordinary [`Res`](crate::system::ResMut) and [`ResMut`](crate::system::ResMut) parameters if they do not conflict.
+///
+/// ```
+/// # use bevy_ecs::{prelude::*, system::*};
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct A;
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct B;
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct C;
+/// #
+/// # let mut world = World::new();
+/// # world.init_resource::<A>();
+/// # world.init_resource::<B>();
+/// # world.init_resource::<C>();
+/// #
+/// let system = (
+///     FilteredResourcesMutParamBuilder::new(|builder| {
+///         builder.add_read::<A>().add_write::<B>();
+///     }),
+///     ParamBuilder,
+///     ParamBuilder,
+/// )
+///     .build_state(&mut world)
+///     .build_system(resource_system);
+///
+/// // Read access to A does not conflict with read access to A or write access to C.
+/// // Write access to B does not conflict with access to A or C.
+/// fn resource_system(mut filtered: FilteredResourcesMut, res_a: Res<A>, res_mut_c: ResMut<C>) {
+///     let res_a_2: Ref<A> = filtered.get::<A>().unwrap();
+///     let res_mut_b: Mut<B> = filtered.get_mut::<B>().unwrap();
+/// }
+/// #
+/// # world.run_system_once(system);
+/// ```
+///
+/// But it will conflict if it tries to read the same resource that another parameter writes,
+/// or write the same resource that another parameter reads.
+///
+/// ```should_panic
+/// # use bevy_ecs::{prelude::*, system::*};
+/// #
+/// # #[derive(Default, Resource)]
+/// # struct A;
+/// #
+/// # let mut world = World::new();
+/// # world.init_resource::<A>();
+/// #
+/// let system = (
+///     FilteredResourcesMutParamBuilder::new(|builder| {
+///         builder.add_write::<A>();
+///     }),
+///     ParamBuilder,
+/// )
+///     .build_state(&mut world)
+///     .build_system(invalid_resource_system);
+///
+/// // Read access to A conflicts with write access to A.
+/// fn invalid_resource_system(filtered: FilteredResourcesMut, res_a: Res<A>) { }
+/// #
+/// # world.run_system_once(system);
+/// ```
+pub struct FilteredResourcesMut<'w, 's> {
+    world: UnsafeWorldCell<'w>,
+    access: &'s Access<ComponentId>,
+    last_run: Tick,
+    this_run: Tick,
+}
+
+impl<'w, 's> FilteredResourcesMut<'w, 's> {
+    /// Creates a new [`FilteredResources`].
+    /// # Safety
+    /// It is the callers responsibility to ensure that nothing else may access the any resources in the `world` in a way that conflicts with `access`.
+    pub(crate) unsafe fn new(
+        world: UnsafeWorldCell<'w>,
+        access: &'s Access<ComponentId>,
+        last_run: Tick,
+        this_run: Tick,
+    ) -> Self {
+        Self {
+            world,
+            access,
+            last_run,
+            this_run,
+        }
+    }
+
+    /// Gets read-only access to all of the resources this `FilteredResourcesMut` can access.
+    pub fn as_readonly(&self) -> FilteredResources<'_, 's> {
+        FilteredResources::from(self)
+    }
+
+    /// Returns a new instance with a shorter lifetime.
+    /// This is useful if you have `&mut FilteredResourcesMut`, but you need `FilteredResourcesMut`.
+    pub fn reborrow(&mut self) -> FilteredResourcesMut<'_, 's> {
+        // SAFETY: We have exclusive access to this access for the duration of `'_`, so there cannot be anything else that conflicts.
+        unsafe { Self::new(self.world, self.access, self.last_run, self.this_run) }
+    }
+
+    /// Returns a reference to the underlying [`Access`].
+    pub fn access(&self) -> &Access<ComponentId> {
+        self.access
+    }
+
+    /// Returns `true` if the `FilteredResources` has read access to the given resource.
+    /// Note that [`Self::get()`] may still return `None` if the resource does not exist.
+    pub fn has_read<R: Resource>(&self) -> bool {
+        let component_id = self.world.components().resource_id::<R>();
+        component_id.is_some_and(|component_id| self.access.has_resource_read(component_id))
+    }
+
+    /// Returns `true` if the `FilteredResources` has write access to the given resource.
+    /// Note that [`Self::get_mut()`] may still return `None` if the resource does not exist.
+    pub fn has_write<R: Resource>(&self) -> bool {
+        let component_id = self.world.components().resource_id::<R>();
+        component_id.is_some_and(|component_id| self.access.has_resource_write(component_id))
+    }
+
+    /// Gets a reference to the resource of the given type if it exists and the `FilteredResources` has access to it.
+    pub fn get<R: Resource>(&self) -> Option<Ref<'_, R>> {
+        self.as_readonly().get()
+    }
+
+    /// Gets a pointer to the resource with the given [`ComponentId`] if it exists and the `FilteredResources` has access to it.
+    pub fn get_by_id(&self, component_id: ComponentId) -> Option<Ptr<'_>> {
+        self.as_readonly().get_by_id(component_id)
+    }
+
+    /// Gets a mutable reference to the resource of the given type if it exists and the `FilteredResources` has access to it.
+    pub fn get_mut<R: Resource>(&mut self) -> Option<Mut<'_, R>> {
+        // SAFETY: We have exclusive access to the resources in `access` for `'_`, and we shorten the returned lifetime to that.
+        unsafe { self.get_mut_unchecked() }
+    }
+
+    /// Gets a mutable pointer to the resource with the given [`ComponentId`] if it exists and the `FilteredResources` has access to it.
+    pub fn get_mut_by_id(&mut self, component_id: ComponentId) -> Option<MutUntyped<'_>> {
+        // SAFETY: We have exclusive access to the resources in `access` for `'_`, and we shorten the returned lifetime to that.
+        unsafe { self.get_mut_by_id_unchecked(component_id) }
+    }
+
+    /// Consumes self and gets mutable access to resource of the given type with the world `'w` lifetime if it exists and the `FilteredResources` has access to it.
+    pub fn into_mut<R: Resource>(mut self) -> Option<Mut<'w, R>> {
+        // SAFETY: This consumes self, so we have exclusive access to the resources in `access` for the entirety of `'w`.
+        unsafe { self.get_mut_unchecked() }
+    }
+
+    /// Consumes self and gets mutable access to resource with the given [`ComponentId`] with the world `'w` lifetime if it exists and the `FilteredResources` has access to it.
+    pub fn into_mut_by_id(mut self, component_id: ComponentId) -> Option<MutUntyped<'w>> {
+        // SAFETY: This consumes self, so we have exclusive access to the resources in `access` for the entirety of `'w`.
+        unsafe { self.get_mut_by_id_unchecked(component_id) }
+    }
+
+    /// Gets a mutable pointer to the resource of the given type if it exists and the `FilteredResources` has access to it.
+    /// # Safety
+    /// It is the callers responsibility to ensure that there are no conflicting borrows of anything in `access` for the duration of the returned value.
+    unsafe fn get_mut_unchecked<R: Resource>(&mut self) -> Option<Mut<'w, R>> {
+        let component_id = self.world.components().resource_id::<R>()?;
+        // SAFETY: THe caller ensures that there are no conflicting borrows.
+        unsafe { self.get_mut_by_id_unchecked(component_id) }
+            // SAFETY: The underlying type of the resource is `R`.
+            .map(|ptr| unsafe { ptr.with_type::<R>() })
+    }
+
+    /// Gets a mutable pointer to the resource with the given [`ComponentId`] if it exists and the `FilteredResources` has access to it.
+    /// # Safety
+    /// It is the callers responsibility to ensure that there are no conflicting borrows of anything in `access` for the duration of the returned value.
+    unsafe fn get_mut_by_id_unchecked(
+        &mut self,
+        component_id: ComponentId,
+    ) -> Option<MutUntyped<'w>> {
+        if !self.access.has_resource_write(component_id) {
+            return None;
+        }
+        // SAFETY: We have access to this resource in `access`, and the caller ensures that there are no conflicting borrows for the duration of the returned value.
+        unsafe { self.world.get_resource_with_ticks(component_id) }.map(
+            |(value, ticks, _caller)| MutUntyped {
+                // SAFETY: We have exclusive access to the underlying storage.
+                value: unsafe { value.assert_unique() },
+                // SAFETY: We have exclusive access to the underlying storage.
+                ticks: unsafe { TicksMut::from_tick_cells(ticks, self.last_run, self.this_run) },
+                #[cfg(feature = "track_change_detection")]
+                // SAFETY: We have exclusive access to the underlying storage.
+                changed_by: unsafe { _caller.deref_mut() },
+            },
+        )
+    }
+}
+
+impl<'w> From<&'w mut World> for FilteredResourcesMut<'w, 'static> {
+    fn from(value: &'w mut World) -> Self {
+        static WRITE_ALL_RESOURCES: OnceLock<Access<ComponentId>> = OnceLock::new();
+        let access = WRITE_ALL_RESOURCES.get_or_init(|| {
+            let mut access = Access::new();
+            access.write_all_resources();
+            access
+        });
+
+        let last_run = value.last_change_tick();
+        let this_run = value.change_tick();
+        // SAFETY: We have a mutable reference to the entire world, so nothing else can alias with mutable access to all resources.
+        unsafe {
+            Self::new(
+                value.as_unsafe_world_cell_readonly(),
+                access,
+                last_run,
+                this_run,
+            )
+        }
+    }
+}
+
+/// Builder struct to define the access for a [`FilteredResources`].
+///
+/// This is passed to a callback in [`FilteredResourcesParamBuilder`](crate::system::FilteredResourcesParamBuilder).
+pub struct FilteredResourcesBuilder<'w> {
+    world: &'w mut World,
+    access: Access<ComponentId>,
+}
+
+impl<'w> FilteredResourcesBuilder<'w> {
+    /// Creates a new builder with no access.
+    pub fn new(world: &'w mut World) -> Self {
+        Self {
+            world,
+            access: Access::new(),
+        }
+    }
+
+    /// Returns a reference to the underlying [`Access`].
+    pub fn access(&self) -> &Access<ComponentId> {
+        &self.access
+    }
+
+    /// Add accesses required to read all resources.
+    pub fn add_read_all(&mut self) -> &mut Self {
+        self.access.read_all_resources();
+        self
+    }
+
+    /// Add accesses required to read the resource of the given type.
+    pub fn add_read<R: Resource>(&mut self) -> &mut Self {
+        let component_id = self.world.components.register_resource::<R>();
+        self.add_read_by_id(component_id)
+    }
+
+    /// Add accesses required to read the resource with the given [`ComponentId`].
+    pub fn add_read_by_id(&mut self, component_id: ComponentId) -> &mut Self {
+        self.access.add_resource_read(component_id);
+        self
+    }
+
+    /// Create an [`Access`] that represents the accesses of the builder.
+    pub fn build(self) -> Access<ComponentId> {
+        self.access
+    }
+}
+
+/// Builder struct to define the access for a [`FilteredResourcesMut`].
+///
+/// This is passed to a callback in [`FilteredResourcesMutParamBuilder`](crate::system::FilteredResourcesMutParamBuilder).
+pub struct FilteredResourcesMutBuilder<'w> {
+    world: &'w mut World,
+    access: Access<ComponentId>,
+}
+
+impl<'w> FilteredResourcesMutBuilder<'w> {
+    /// Creates a new builder with no access.
+    pub fn new(world: &'w mut World) -> Self {
+        Self {
+            world,
+            access: Access::new(),
+        }
+    }
+
+    /// Returns a reference to the underlying [`Access`].
+    pub fn access(&self) -> &Access<ComponentId> {
+        &self.access
+    }
+
+    /// Add accesses required to read all resources.
+    pub fn add_read_all(&mut self) -> &mut Self {
+        self.access.read_all_resources();
+        self
+    }
+
+    /// Add accesses required to read the resource of the given type.
+    pub fn add_read<R: Resource>(&mut self) -> &mut Self {
+        let component_id = self.world.components.register_resource::<R>();
+        self.add_read_by_id(component_id)
+    }
+
+    /// Add accesses required to read the resource with the given [`ComponentId`].
+    pub fn add_read_by_id(&mut self, component_id: ComponentId) -> &mut Self {
+        self.access.add_resource_read(component_id);
+        self
+    }
+
+    /// Add accesses required to get mutable access to all resources.
+    pub fn add_write_all(&mut self) -> &mut Self {
+        self.access.write_all_resources();
+        self
+    }
+
+    /// Add accesses required to get mutable access to the resource of the given type.
+    pub fn add_write<R: Resource>(&mut self) -> &mut Self {
+        let component_id = self.world.components.register_resource::<R>();
+        self.add_write_by_id(component_id)
+    }
+
+    /// Add accesses required to get mutable access to the resource with the given [`ComponentId`].
+    pub fn add_write_by_id(&mut self, component_id: ComponentId) -> &mut Self {
+        self.access.add_resource_write(component_id);
+        self
+    }
+
+    /// Create an [`Access`] that represents the accesses of the builder.
+    pub fn build(self) -> Access<ComponentId> {
+        self.access
+    }
+}

crates/bevy_ecs/src/world/mod.rs

@@ -5,6 +5,7 @@ mod component_constants;
 mod deferred_world;
 mod entity_ref;
 pub mod error;
+mod filtered_resource;
 mod identifier;
 mod spawn_batch;
 pub mod unsafe_world_cell;
@@ -22,6 +23,7 @@ pub use entity_ref::{
     EntityMut, EntityMutExcept, EntityRef, EntityRefExcept, EntityWorldMut, Entry,
     FilteredEntityMut, FilteredEntityRef, OccupiedEntry, VacantEntry,
 };
+pub use filtered_resource::*;
 pub use identifier::WorldId;
 pub use spawn_batch::*;