From c1a4a2f6c50f1879b57be281af18399d8d17757b Mon Sep 17 00:00:00 2001
From: Daniel McNab <36049421+DJMcNab@users.noreply.github.com>
Date: Fri, 25 Feb 2022 03:10:59 +0000
Subject: [PATCH] Remove the config api (#3633)

# Objective

- Fix the ugliness of the `config` api.
- Supercedes #2440, #2463, #2491

## Solution

- Since #2398, capturing closure systems have worked.
- Use those instead where we needed config before
- Remove the rest of the config api.
- Related: #2777
---
 crates/bevy_core/src/time/fixed_timestep.rs   |  33 ++--
 crates/bevy_ecs/macros/src/lib.rs             |  14 +-
 crates/bevy_ecs/src/lib.rs                    |   4 +-
 crates/bevy_ecs/src/schedule/state.rs         |  84 +++++----
 crates/bevy_ecs/src/system/function_system.rs |  74 +-------
 crates/bevy_ecs/src/system/mod.rs             |  19 +-
 crates/bevy_ecs/src/system/system_param.rs    | 163 +++++-------------
 7 files changed, 107 insertions(+), 284 deletions(-)

diff --git a/crates/bevy_core/src/time/fixed_timestep.rs b/crates/bevy_core/src/time/fixed_timestep.rs
index bd3384d4ca..fa39ced311 100644
--- a/crates/bevy_core/src/time/fixed_timestep.rs
+++ b/crates/bevy_core/src/time/fixed_timestep.rs
@@ -4,7 +4,7 @@ use bevy_ecs::{
     component::ComponentId,
     query::Access,
     schedule::ShouldRun,
-    system::{ConfigurableSystem, IntoSystem, Local, Res, ResMut, System},
+    system::{IntoSystem, Res, ResMut, System},
     world::World,
 };
 use bevy_utils::HashMap;
@@ -79,7 +79,9 @@ impl Default for FixedTimestep {
     fn default() -> Self {
         Self {
             state: LocalFixedTimestepState::default(),
-            internal_system: Box::new(IntoSystem::into_system(Self::prepare_system)),
+            internal_system: Box::new(IntoSystem::into_system(Self::prepare_system(
+                Default::default(),
+            ))),
         }
     }
 }
@@ -116,18 +118,18 @@ impl FixedTimestep {
     }
 
     fn prepare_system(
-        mut state: Local<LocalFixedTimestepState>,
-        time: Res<Time>,
-        mut fixed_timesteps: ResMut<FixedTimesteps>,
-    ) -> ShouldRun {
-        let should_run = state.update(&time);
-        if let Some(ref label) = state.label {
-            let res_state = fixed_timesteps.fixed_timesteps.get_mut(label).unwrap();
-            res_state.step = state.step;
-            res_state.accumulator = state.accumulator;
-        }
+        mut state: LocalFixedTimestepState,
+    ) -> impl FnMut(Res<Time>, ResMut<FixedTimesteps>) -> ShouldRun {
+        move |time, mut fixed_timesteps| {
+            let should_run = state.update(&time);
+            if let Some(ref label) = state.label {
+                let res_state = fixed_timesteps.fixed_timesteps.get_mut(label).unwrap();
+                res_state.step = state.step;
+                res_state.accumulator = state.accumulator;
+            }
 
-        should_run
+            should_run
+        }
     }
 }
 
@@ -202,8 +204,9 @@ impl System for FixedTimestep {
     }
 
     fn initialize(&mut self, world: &mut World) {
-        self.internal_system =
-            Box::new(Self::prepare_system.config(|c| c.0 = Some(self.state.clone())));
+        self.internal_system = Box::new(IntoSystem::into_system(Self::prepare_system(
+            self.state.clone(),
+        )));
         self.internal_system.initialize(world);
         if let Some(ref label) = self.state.label {
             let mut fixed_timesteps = world.get_resource_mut::<FixedTimesteps>().unwrap();
diff --git a/crates/bevy_ecs/macros/src/lib.rs b/crates/bevy_ecs/macros/src/lib.rs
index b1f1f63d6c..679df90154 100644
--- a/crates/bevy_ecs/macros/src/lib.rs
+++ b/crates/bevy_ecs/macros/src/lib.rs
@@ -222,8 +222,7 @@ pub fn impl_query_set(_input: TokenStream) -> TokenStream {
             unsafe impl<#(#query: WorldQuery + 'static,)* #(#filter: WorldQuery + 'static,)*> SystemParamState for QuerySetState<(#(QueryState<#query, #filter>,)*)>
                 where #(#filter::Fetch: FilterFetch,)*
             {
-                type Config = ();
-                fn init(world: &mut World, system_meta: &mut SystemMeta, config: Self::Config) -> Self {
+                fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
                     #(
                         let mut #query = QueryState::<#query, #filter>::new(world);
                         assert_component_access_compatibility(
@@ -255,8 +254,6 @@ pub fn impl_query_set(_input: TokenStream) -> TokenStream {
                             .extend(&#query.archetype_component_access);
                     )*
                 }
-
-                fn default_config() {}
             }
 
             impl<'w, 's, #(#query: WorldQuery + 'static,)* #(#filter: WorldQuery + 'static,)*> SystemParamFetch<'w, 's> for QuerySetState<(#(QueryState<#query, #filter>,)*)>
@@ -389,10 +386,9 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
         }
 
         unsafe impl<TSystemParamState: #path::system::SystemParamState, #punctuated_generics> #path::system::SystemParamState for #fetch_struct_name<TSystemParamState, #punctuated_generic_idents> {
-            type Config = TSystemParamState::Config;
-            fn init(world: &mut #path::world::World, system_meta: &mut #path::system::SystemMeta, config: Self::Config) -> Self {
+            fn init(world: &mut #path::world::World, system_meta: &mut #path::system::SystemMeta) -> Self {
                 Self {
-                    state: TSystemParamState::init(world, system_meta, config),
+                    state: TSystemParamState::init(world, system_meta),
                     marker: std::marker::PhantomData,
                 }
             }
@@ -401,10 +397,6 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
                 self.state.new_archetype(archetype, system_meta)
             }
 
-            fn default_config() -> TSystemParamState::Config {
-                TSystemParamState::default_config()
-            }
-
             fn apply(&mut self, world: &mut #path::world::World) {
                 self.state.apply(world)
             }
diff --git a/crates/bevy_ecs/src/lib.rs b/crates/bevy_ecs/src/lib.rs
index eea47ce304..84a331be48 100644
--- a/crates/bevy_ecs/src/lib.rs
+++ b/crates/bevy_ecs/src/lib.rs
@@ -33,8 +33,8 @@ pub mod prelude {
             Schedule, Stage, StageLabel, State, SystemLabel, SystemSet, SystemStage,
         },
         system::{
-            Commands, ConfigurableSystem, In, IntoChainSystem, IntoExclusiveSystem, IntoSystem,
-            Local, NonSend, NonSendMut, Query, QuerySet, RemovedComponents, Res, ResMut, System,
+            Commands, In, IntoChainSystem, IntoExclusiveSystem, IntoSystem, Local, NonSend,
+            NonSendMut, Query, QuerySet, RemovedComponents, Res, ResMut, System,
         },
         world::{FromWorld, Mut, World},
     };
diff --git a/crates/bevy_ecs/src/schedule/state.rs b/crates/bevy_ecs/src/schedule/state.rs
index 7dcbefb6af..eb37c97d49 100644
--- a/crates/bevy_ecs/src/schedule/state.rs
+++ b/crates/bevy_ecs/src/schedule/state.rs
@@ -3,7 +3,7 @@ use crate::{
         RunCriteriaDescriptor, RunCriteriaDescriptorCoercion, RunCriteriaLabel, ShouldRun,
         SystemSet,
     },
-    system::{ConfigurableSystem, In, IntoChainSystem, Local, Res, ResMut},
+    system::{In, IntoChainSystem, Local, Res, ResMut},
 };
 use std::{any::TypeId, fmt::Debug, hash::Hash};
 use thiserror::Error;
@@ -98,23 +98,22 @@ impl<T> State<T>
 where
     T: StateData,
 {
-    pub fn on_update(s: T) -> RunCriteriaDescriptor {
-        (|state: Res<State<T>>, pred: Local<Option<T>>| {
-            state.stack.last().unwrap() == pred.as_ref().unwrap() && state.transition.is_none()
+    pub fn on_update(pred: T) -> RunCriteriaDescriptor {
+        let pred_clone = pred.clone();
+        (move |state: Res<State<T>>| {
+            state.stack.last().unwrap() == &pred && state.transition.is_none()
         })
-        .config(|(_, pred)| *pred = Some(Some(s.clone())))
         .chain(should_run_adapter::<T>)
         .after(DriverLabel::of::<T>())
-        .label_discard_if_duplicate(StateCallback::Update.into_label(s))
+        .label_discard_if_duplicate(StateCallback::Update.into_label(pred_clone))
     }
 
-    pub fn on_inactive_update(s: T) -> RunCriteriaDescriptor {
-        (|state: Res<State<T>>, mut is_inactive: Local<bool>, pred: Local<Option<T>>| match &state
-            .transition
-        {
+    pub fn on_inactive_update(pred: T) -> RunCriteriaDescriptor {
+        let pred_clone = pred.clone();
+        (move |state: Res<State<T>>, mut is_inactive: Local<bool>| match &state.transition {
             Some(StateTransition::Pausing(ref relevant, _))
             | Some(StateTransition::Resuming(_, ref relevant)) => {
-                if relevant == pred.as_ref().unwrap() {
+                if relevant == &pred {
                     *is_inactive = !*is_inactive;
                 }
                 false
@@ -122,31 +121,29 @@ where
             Some(_) => false,
             None => *is_inactive,
         })
-        .config(|(_, _, pred)| *pred = Some(Some(s.clone())))
         .chain(should_run_adapter::<T>)
         .after(DriverLabel::of::<T>())
-        .label_discard_if_duplicate(StateCallback::InactiveUpdate.into_label(s))
+        .label_discard_if_duplicate(StateCallback::InactiveUpdate.into_label(pred_clone))
     }
 
-    pub fn on_in_stack_update(s: T) -> RunCriteriaDescriptor {
-        (|state: Res<State<T>>, mut is_in_stack: Local<bool>, pred: Local<Option<T>>| match &state
-            .transition
-        {
+    pub fn on_in_stack_update(pred: T) -> RunCriteriaDescriptor {
+        let pred_clone = pred.clone();
+        (move |state: Res<State<T>>, mut is_in_stack: Local<bool>| match &state.transition {
             Some(StateTransition::Entering(ref relevant, _))
             | Some(StateTransition::ExitingToResume(_, ref relevant)) => {
-                if relevant == pred.as_ref().unwrap() {
+                if relevant == &pred {
                     *is_in_stack = !*is_in_stack;
                 }
                 false
             }
             Some(StateTransition::ExitingFull(_, ref relevant)) => {
-                if relevant == pred.as_ref().unwrap() {
+                if relevant == &pred {
                     *is_in_stack = !*is_in_stack;
                 }
                 false
             }
             Some(StateTransition::Startup) => {
-                if state.stack.last().unwrap() == pred.as_ref().unwrap() {
+                if state.stack.last().unwrap() == &pred {
                     *is_in_stack = !*is_in_stack;
                 }
                 false
@@ -154,78 +151,75 @@ where
             Some(_) => false,
             None => *is_in_stack,
         })
-        .config(|(_, _, pred)| *pred = Some(Some(s.clone())))
         .chain(should_run_adapter::<T>)
         .after(DriverLabel::of::<T>())
-        .label_discard_if_duplicate(StateCallback::InStackUpdate.into_label(s))
+        .label_discard_if_duplicate(StateCallback::InStackUpdate.into_label(pred_clone))
     }
 
-    pub fn on_enter(s: T) -> RunCriteriaDescriptor {
-        (|state: Res<State<T>>, pred: Local<Option<T>>| {
+    pub fn on_enter(pred: T) -> RunCriteriaDescriptor {
+        let pred_clone = pred.clone();
+        (move |state: Res<State<T>>| {
             state
                 .transition
                 .as_ref()
                 .map_or(false, |transition| match transition {
-                    StateTransition::Entering(_, entering) => entering == pred.as_ref().unwrap(),
-                    StateTransition::Startup => {
-                        state.stack.last().unwrap() == pred.as_ref().unwrap()
-                    }
+                    StateTransition::Entering(_, entering) => entering == &pred,
+                    StateTransition::Startup => state.stack.last().unwrap() == &pred,
                     _ => false,
                 })
         })
-        .config(|(_, pred)| *pred = Some(Some(s.clone())))
         .chain(should_run_adapter::<T>)
         .after(DriverLabel::of::<T>())
-        .label_discard_if_duplicate(StateCallback::Enter.into_label(s))
+        .label_discard_if_duplicate(StateCallback::Enter.into_label(pred_clone))
     }
 
-    pub fn on_exit(s: T) -> RunCriteriaDescriptor {
-        (|state: Res<State<T>>, pred: Local<Option<T>>| {
+    pub fn on_exit(pred: T) -> RunCriteriaDescriptor {
+        let pred_clone = pred.clone();
+        (move |state: Res<State<T>>| {
             state
                 .transition
                 .as_ref()
                 .map_or(false, |transition| match transition {
                     StateTransition::ExitingToResume(exiting, _)
-                    | StateTransition::ExitingFull(exiting, _) => exiting == pred.as_ref().unwrap(),
+                    | StateTransition::ExitingFull(exiting, _) => exiting == &pred,
                     _ => false,
                 })
         })
-        .config(|(_, pred)| *pred = Some(Some(s.clone())))
         .chain(should_run_adapter::<T>)
         .after(DriverLabel::of::<T>())
-        .label_discard_if_duplicate(StateCallback::Exit.into_label(s))
+        .label_discard_if_duplicate(StateCallback::Exit.into_label(pred_clone))
     }
 
-    pub fn on_pause(s: T) -> RunCriteriaDescriptor {
-        (|state: Res<State<T>>, pred: Local<Option<T>>| {
+    pub fn on_pause(pred: T) -> RunCriteriaDescriptor {
+        let pred_clone = pred.clone();
+        (move |state: Res<State<T>>| {
             state
                 .transition
                 .as_ref()
                 .map_or(false, |transition| match transition {
-                    StateTransition::Pausing(pausing, _) => pausing == pred.as_ref().unwrap(),
+                    StateTransition::Pausing(pausing, _) => pausing == &pred,
                     _ => false,
                 })
         })
-        .config(|(_, pred)| *pred = Some(Some(s.clone())))
         .chain(should_run_adapter::<T>)
         .after(DriverLabel::of::<T>())
-        .label_discard_if_duplicate(StateCallback::Pause.into_label(s))
+        .label_discard_if_duplicate(StateCallback::Pause.into_label(pred_clone))
     }
 
-    pub fn on_resume(s: T) -> RunCriteriaDescriptor {
-        (|state: Res<State<T>>, pred: Local<Option<T>>| {
+    pub fn on_resume(pred: T) -> RunCriteriaDescriptor {
+        let pred_clone = pred.clone();
+        (move |state: Res<State<T>>| {
             state
                 .transition
                 .as_ref()
                 .map_or(false, |transition| match transition {
-                    StateTransition::Resuming(_, resuming) => resuming == pred.as_ref().unwrap(),
+                    StateTransition::Resuming(_, resuming) => resuming == &pred,
                     _ => false,
                 })
         })
-        .config(|(_, pred)| *pred = Some(Some(s.clone())))
         .chain(should_run_adapter::<T>)
         .after(DriverLabel::of::<T>())
-        .label_discard_if_duplicate(StateCallback::Resume.into_label(s))
+        .label_discard_if_duplicate(StateCallback::Resume.into_label(pred_clone))
     }
 
     pub fn on_update_set(s: T) -> SystemSet {
diff --git a/crates/bevy_ecs/src/system/function_system.rs b/crates/bevy_ecs/src/system/function_system.rs
index a92f89bc7a..233b7ed6e2 100644
--- a/crates/bevy_ecs/src/system/function_system.rs
+++ b/crates/bevy_ecs/src/system/function_system.rs
@@ -141,16 +141,8 @@ pub struct SystemState<Param: SystemParam> {
 
 impl<Param: SystemParam> SystemState<Param> {
     pub fn new(world: &mut World) -> Self {
-        let config = <Param::Fetch as SystemParamState>::default_config();
-        Self::with_config(world, config)
-    }
-
-    pub fn with_config(
-        world: &mut World,
-        config: <Param::Fetch as SystemParamState>::Config,
-    ) -> Self {
         let mut meta = SystemMeta::new::<Param>();
-        let param_state = <Param::Fetch as SystemParamState>::init(world, &mut meta, config);
+        let param_state = <Param::Fetch as SystemParamState>::init(world, &mut meta);
         Self {
             meta,
             param_state,
@@ -407,73 +399,11 @@ where
     func: F,
     param_state: Option<Param::Fetch>,
     system_meta: SystemMeta,
-    config: Option<<Param::Fetch as SystemParamState>::Config>,
     // NOTE: PhantomData<fn()-> T> gives this safe Send/Sync impls
     #[allow(clippy::type_complexity)]
     marker: PhantomData<fn() -> (In, Out, Marker)>,
 }
 
-impl<In, Out, Param: SystemParam, Marker, F> FunctionSystem<In, Out, Param, Marker, F> {
-    /// Gives mutable access to the systems config via a callback. This is useful to set up system
-    /// [`Local`](crate::system::Local)s.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// # use bevy_ecs::prelude::*;
-    /// # let world = &mut World::default();
-    /// fn local_is_42(local: Local<usize>) {
-    ///     assert_eq!(*local, 42);
-    /// }
-    /// let mut system = local_is_42.config(|config| config.0 = Some(42));
-    /// system.initialize(world);
-    /// system.run((), world);
-    /// ```
-    #[must_use]
-    pub fn config(
-        mut self,
-        f: impl FnOnce(&mut <Param::Fetch as SystemParamState>::Config),
-    ) -> Self {
-        f(self.config.as_mut().unwrap());
-        self
-    }
-}
-
-/// Provides `my_system.config(...)` API.
-pub trait ConfigurableSystem<In, Out, Param: SystemParam, Marker>:
-    IntoSystem<In, Out, (IsFunctionSystem, Param, Marker)>
-{
-    /// See [`FunctionSystem::config()`](crate::system::FunctionSystem::config).
-    fn config(
-        self,
-        f: impl FnOnce(&mut <Param::Fetch as SystemParamState>::Config),
-    ) -> Self::System;
-}
-
-impl<In, Out, Param: SystemParam, Marker, F> ConfigurableSystem<In, Out, Param, Marker> for F
-where
-    In: 'static,
-    Out: 'static,
-    Param: SystemParam + 'static,
-    Marker: 'static,
-    F: SystemParamFunction<In, Out, Param, Marker>
-        + IntoSystem<
-            In,
-            Out,
-            (IsFunctionSystem, Param, Marker),
-            System = FunctionSystem<In, Out, Param, Marker, F>,
-        > + Send
-        + Sync
-        + 'static,
-{
-    fn config(
-        self,
-        f: impl FnOnce(&mut <<Param as SystemParam>::Fetch as SystemParamState>::Config),
-    ) -> Self::System {
-        IntoSystem::into_system(self).config(f)
-    }
-}
-
 pub struct IsFunctionSystem;
 
 impl<In, Out, Param, Marker, F> IntoSystem<In, Out, (IsFunctionSystem, Param, Marker)> for F
@@ -489,7 +419,6 @@ where
         FunctionSystem {
             func,
             param_state: None,
-            config: Some(<Param::Fetch as SystemParamState>::default_config()),
             system_meta: SystemMeta::new::<F>(),
             marker: PhantomData,
         }
@@ -558,7 +487,6 @@ where
         self.param_state = Some(<Param::Fetch as SystemParamState>::init(
             world,
             &mut self.system_meta,
-            self.config.take().unwrap(),
         ));
     }
 
diff --git a/crates/bevy_ecs/src/system/mod.rs b/crates/bevy_ecs/src/system/mod.rs
index 1d489d6f97..dfbd98a310 100644
--- a/crates/bevy_ecs/src/system/mod.rs
+++ b/crates/bevy_ecs/src/system/mod.rs
@@ -103,8 +103,8 @@ mod tests {
         query::{Added, Changed, Or, QueryState, With, Without},
         schedule::{Schedule, Stage, SystemStage},
         system::{
-            ConfigurableSystem, IntoExclusiveSystem, IntoSystem, Local, NonSend, NonSendMut, Query,
-            QuerySet, RemovedComponents, Res, ResMut, System, SystemState,
+            IntoExclusiveSystem, IntoSystem, Local, NonSend, NonSendMut, Query, QuerySet,
+            RemovedComponents, Res, ResMut, System, SystemState,
         },
         world::{FromWorld, World},
     };
@@ -525,21 +525,6 @@ mod tests {
         assert_eq!(world.get_resource::<NSystems>().unwrap().0, 2);
     }
 
-    #[test]
-    fn configure_system_local() {
-        let mut world = World::default();
-        world.insert_resource(false);
-        fn sys(local: Local<usize>, mut modified: ResMut<bool>) {
-            assert_eq!(*local, 42);
-            *modified = true;
-        }
-
-        run_system(&mut world, sys.config(|config| config.0 = Some(42)));
-
-        // ensure the system actually ran
-        assert!(*world.get_resource::<bool>().unwrap());
-    }
-
     #[test]
     fn world_collections_system() {
         let mut world = World::default();
diff --git a/crates/bevy_ecs/src/system/system_param.rs b/crates/bevy_ecs/src/system/system_param.rs
index 37d4c57b93..f242892b87 100644
--- a/crates/bevy_ecs/src/system/system_param.rs
+++ b/crates/bevy_ecs/src/system/system_param.rs
@@ -60,25 +60,11 @@ pub type SystemParamItem<'w, 's, P> = <<P as SystemParam>::Fetch as SystemParamF
 /// Additionally, it is the implementor's responsibility to ensure there is no
 /// conflicting access across all [`SystemParam`]'s.
 pub unsafe trait SystemParamState: Send + Sync + 'static {
-    /// Values of this type can be used to adjust the behavior of the
-    /// system parameter. For instance, this can be used to pass
-    /// values from a `Plugin` to a `System`, or to control the
-    /// behavior of the `System`.
-    ///
-    /// The default configuration of the parameter is set by
-    /// [`SystemParamState::default_config`]. To change it, invoke
-    /// [`FunctionSystem::config`](super::FunctionSystem::config) when
-    /// creating the system.
-    ///
-    /// See [`FunctionSystem::config`](super::FunctionSystem::config)
-    /// for more information and examples.
-    type Config: Send + Sync;
-    fn init(world: &mut World, system_meta: &mut SystemMeta, config: Self::Config) -> Self;
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self;
     #[inline]
     fn new_archetype(&mut self, _archetype: &Archetype, _system_meta: &mut SystemMeta) {}
     #[inline]
     fn apply(&mut self, _world: &mut World) {}
-    fn default_config() -> Self::Config;
 }
 
 /// A [`SystemParamFetch`] that only reads a given [`World`].
@@ -122,9 +108,7 @@ unsafe impl<Q: WorldQuery + 'static, F: WorldQuery + 'static> SystemParamState f
 where
     F::Fetch: FilterFetch,
 {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
         let state = QueryState::new(world);
         assert_component_access_compatibility(
             &system_meta.name,
@@ -149,8 +133,6 @@ where
             .archetype_component_access
             .extend(&self.archetype_component_access);
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, Q: WorldQuery + 'static, F: WorldQuery + 'static> SystemParamFetch<'w, 's>
@@ -295,9 +277,7 @@ impl<'a, T: Resource> SystemParam for Res<'a, T> {
 // SAFE: Res ComponentId and ArchetypeComponentId access is applied to SystemMeta. If this Res
 // conflicts with any prior access, a panic will occur.
 unsafe impl<T: Resource> SystemParamState for ResState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
         let component_id = world.initialize_resource::<T>();
         let combined_access = system_meta.component_access_set.combined_access_mut();
         assert!(
@@ -320,8 +300,6 @@ unsafe impl<T: Resource> SystemParamState for ResState<T> {
             marker: PhantomData,
         }
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: Resource> SystemParamFetch<'w, 's> for ResState<T> {
@@ -364,13 +342,9 @@ impl<'a, T: Resource> SystemParam for Option<Res<'a, T>> {
 unsafe impl<T: Resource> ReadOnlySystemParamFetch for OptionResState<T> {}
 
 unsafe impl<T: Resource> SystemParamState for OptionResState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
-        Self(ResState::init(world, system_meta, ()))
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
+        Self(ResState::init(world, system_meta))
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: Resource> SystemParamFetch<'w, 's> for OptionResState<T> {
@@ -407,9 +381,7 @@ impl<'a, T: Resource> SystemParam for ResMut<'a, T> {
 // SAFE: Res ComponentId and ArchetypeComponentId access is applied to SystemMeta. If this Res
 // conflicts with any prior access, a panic will occur.
 unsafe impl<T: Resource> SystemParamState for ResMutState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
         let component_id = world.initialize_resource::<T>();
         let combined_access = system_meta.component_access_set.combined_access_mut();
         if combined_access.has_write(component_id) {
@@ -435,8 +407,6 @@ unsafe impl<T: Resource> SystemParamState for ResMutState<T> {
             marker: PhantomData,
         }
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: Resource> SystemParamFetch<'w, 's> for ResMutState<T> {
@@ -478,13 +448,9 @@ impl<'a, T: Resource> SystemParam for Option<ResMut<'a, T>> {
 }
 
 unsafe impl<T: Resource> SystemParamState for OptionResMutState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
-        Self(ResMutState::init(world, system_meta, ()))
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
+        Self(ResMutState::init(world, system_meta))
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: Resource> SystemParamFetch<'w, 's> for OptionResMutState<T> {
@@ -519,17 +485,13 @@ unsafe impl ReadOnlySystemParamFetch for CommandQueue {}
 
 // SAFE: only local state is accessed
 unsafe impl SystemParamState for CommandQueue {
-    type Config = ();
-
-    fn init(_world: &mut World, _system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(_world: &mut World, _system_meta: &mut SystemMeta) -> Self {
         Default::default()
     }
 
     fn apply(&mut self, world: &mut World) {
         self.apply(world);
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's> SystemParamFetch<'w, 's> for CommandQueue {
@@ -557,9 +519,7 @@ impl<'w, 's> SystemParam for &'w World {
 }
 
 unsafe impl<'w, 's> SystemParamState for WorldState {
-    type Config = ();
-
-    fn init(_world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(_world: &mut World, system_meta: &mut SystemMeta) -> Self {
         let mut access = Access::default();
         access.read_all();
         if !system_meta
@@ -584,8 +544,6 @@ unsafe impl<'w, 's> SystemParamState for WorldState {
 
         WorldState
     }
-
-    fn default_config() -> Self::Config {}
 }
 
 impl<'w, 's> SystemParamFetch<'w, 's> for WorldState {
@@ -626,6 +584,22 @@ impl<'w, 's> SystemParamFetch<'w, 's> for WorldState {
 /// // Note how the read local is still 0 due to the locals not being shared.
 /// assert_eq!(read_system.run((), world), 0);
 /// ```
+///
+/// N.B. A [`Local`]s value cannot be read or written to outside of the containing system.
+/// To add configuration to a system, convert a capturing closure into the system instead:
+///
+/// ```
+/// # use bevy_ecs::prelude::*;
+/// # use bevy_ecs::system::assert_is_system;
+/// struct Config(u32);
+/// struct Myu32Wrapper(u32);
+/// fn reset_to_system(value: Config) -> impl FnMut(ResMut<Myu32Wrapper>) {
+///     move |mut val| val.0 = value.0
+/// }
+///
+/// // .add_system(reset_to_system(my_config))
+/// # assert_is_system(reset_to_system(Config(10)));
+/// ```
 pub struct Local<'a, T: Resource>(&'a mut T);
 
 // SAFE: Local only accesses internal state
@@ -665,14 +639,8 @@ impl<'a, T: Resource + FromWorld> SystemParam for Local<'a, T> {
 
 // SAFE: only local state is accessed
 unsafe impl<T: Resource + FromWorld> SystemParamState for LocalState<T> {
-    type Config = Option<T>;
-
-    fn init(world: &mut World, _system_meta: &mut SystemMeta, config: Self::Config) -> Self {
-        Self(config.unwrap_or_else(|| T::from_world(world)))
-    }
-
-    fn default_config() -> Option<T> {
-        None
+    fn init(world: &mut World, _system_meta: &mut SystemMeta) -> Self {
+        Self(T::from_world(world))
     }
 }
 
@@ -751,16 +719,12 @@ impl<'a, T: Component> SystemParam for RemovedComponents<'a, T> {
 // SAFE: no component access. removed component entity collections can be read in parallel and are
 // never mutably borrowed during system execution
 unsafe impl<T: Component> SystemParamState for RemovedComponentsState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, _system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(world: &mut World, _system_meta: &mut SystemMeta) -> Self {
         Self {
             component_id: world.init_component::<T>(),
             marker: PhantomData,
         }
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: Component> SystemParamFetch<'w, 's> for RemovedComponentsState<T> {
@@ -858,9 +822,7 @@ impl<'a, T: 'static> SystemParam for NonSend<'a, T> {
 // SAFE: NonSendComponentId and ArchetypeComponentId access is applied to SystemMeta. If this
 // NonSend conflicts with any prior access, a panic will occur.
 unsafe impl<T: 'static> SystemParamState for NonSendState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
         system_meta.set_non_send();
 
         let component_id = world.initialize_non_send_resource::<T>();
@@ -885,8 +847,6 @@ unsafe impl<T: 'static> SystemParamState for NonSendState<T> {
             marker: PhantomData,
         }
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: 'static> SystemParamFetch<'w, 's> for NonSendState<T> {
@@ -931,13 +891,9 @@ impl<'w, T: 'static> SystemParam for Option<NonSend<'w, T>> {
 unsafe impl<T: 'static> ReadOnlySystemParamFetch for OptionNonSendState<T> {}
 
 unsafe impl<T: 'static> SystemParamState for OptionNonSendState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
-        Self(NonSendState::init(world, system_meta, ()))
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
+        Self(NonSendState::init(world, system_meta))
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: 'static> SystemParamFetch<'w, 's> for OptionNonSendState<T> {
@@ -975,9 +931,7 @@ impl<'a, T: 'static> SystemParam for NonSendMut<'a, T> {
 // SAFE: NonSendMut ComponentId and ArchetypeComponentId access is applied to SystemMeta. If this
 // NonSendMut conflicts with any prior access, a panic will occur.
 unsafe impl<T: 'static> SystemParamState for NonSendMutState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
         system_meta.set_non_send();
 
         let component_id = world.initialize_non_send_resource::<T>();
@@ -1005,8 +959,6 @@ unsafe impl<T: 'static> SystemParamState for NonSendMutState<T> {
             marker: PhantomData,
         }
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: 'static> SystemParamFetch<'w, 's> for NonSendMutState<T> {
@@ -1049,13 +1001,9 @@ impl<'a, T: 'static> SystemParam for Option<NonSendMut<'a, T>> {
 }
 
 unsafe impl<T: 'static> SystemParamState for OptionNonSendMutState<T> {
-    type Config = ();
-
-    fn init(world: &mut World, system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
-        Self(NonSendMutState::init(world, system_meta, ()))
+    fn init(world: &mut World, system_meta: &mut SystemMeta) -> Self {
+        Self(NonSendMutState::init(world, system_meta))
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's, T: 'static> SystemParamFetch<'w, 's> for OptionNonSendMutState<T> {
@@ -1094,13 +1042,9 @@ pub struct ArchetypesState;
 
 // SAFE: no component value access
 unsafe impl SystemParamState for ArchetypesState {
-    type Config = ();
-
-    fn init(_world: &mut World, _system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(_world: &mut World, _system_meta: &mut SystemMeta) -> Self {
         Self
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's> SystemParamFetch<'w, 's> for ArchetypesState {
@@ -1129,13 +1073,9 @@ pub struct ComponentsState;
 
 // SAFE: no component value access
 unsafe impl SystemParamState for ComponentsState {
-    type Config = ();
-
-    fn init(_world: &mut World, _system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(_world: &mut World, _system_meta: &mut SystemMeta) -> Self {
         Self
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's> SystemParamFetch<'w, 's> for ComponentsState {
@@ -1164,13 +1104,9 @@ pub struct EntitiesState;
 
 // SAFE: no component value access
 unsafe impl SystemParamState for EntitiesState {
-    type Config = ();
-
-    fn init(_world: &mut World, _system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(_world: &mut World, _system_meta: &mut SystemMeta) -> Self {
         Self
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's> SystemParamFetch<'w, 's> for EntitiesState {
@@ -1199,13 +1135,9 @@ pub struct BundlesState;
 
 // SAFE: no component value access
 unsafe impl SystemParamState for BundlesState {
-    type Config = ();
-
-    fn init(_world: &mut World, _system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(_world: &mut World, _system_meta: &mut SystemMeta) -> Self {
         Self
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's> SystemParamFetch<'w, 's> for BundlesState {
@@ -1239,13 +1171,9 @@ impl SystemParam for SystemChangeTick {
 pub struct SystemChangeTickState {}
 
 unsafe impl SystemParamState for SystemChangeTickState {
-    type Config = ();
-
-    fn init(_world: &mut World, _system_meta: &mut SystemMeta, _config: Self::Config) -> Self {
+    fn init(_world: &mut World, _system_meta: &mut SystemMeta) -> Self {
         Self {}
     }
-
-    fn default_config() {}
 }
 
 impl<'w, 's> SystemParamFetch<'w, 's> for SystemChangeTickState {
@@ -1295,11 +1223,9 @@ macro_rules! impl_system_param_tuple {
         /// SAFE: implementors of each `SystemParamState` in the tuple have validated their impls
         #[allow(non_snake_case)]
         unsafe impl<$($param: SystemParamState),*> SystemParamState for ($($param,)*) {
-            type Config = ($(<$param as SystemParamState>::Config,)*);
             #[inline]
-            fn init(_world: &mut World, _system_meta: &mut SystemMeta, config: Self::Config) -> Self {
-                let ($($param,)*) = config;
-                (($($param::init(_world, _system_meta, $param),)*))
+            fn init(_world: &mut World, _system_meta: &mut SystemMeta) -> Self {
+                (($($param::init(_world, _system_meta),)*))
             }
 
             #[inline]
@@ -1313,11 +1239,6 @@ macro_rules! impl_system_param_tuple {
                 let ($($param,)*) = self;
                 $($param.apply(_world);)*
             }
-
-            #[allow(clippy::unused_unit)]
-            fn default_config() -> ($(<$param as SystemParamState>::Config,)*) {
-                ($(<$param as SystemParamState>::default_config(),)*)
-            }
         }
     };
 }