turn check into explicit assert fn

Author: maniwani

crates/bevy_ecs/src/system/function_system.rs

@@ -6,8 +6,9 @@ use crate::{
     query::{Access, FilteredAccessSet},
     schedule::{SystemLabel, SystemLabelId},
     system::{
-        check_system_change_tick, MaybeUnsafeCell, ReadOnlySystemParamFetch, System, SystemParam,
-        SystemParamFetch, SystemParamItem, SystemParamState, WorldAccessLevel,
+        assert_valid_world_access_level, check_system_change_tick, MaybeUnsafeCell,
+        ReadOnlySystemParamFetch, System, SystemParam, SystemParamFetch, SystemParamItem,
+        SystemParamState, WorldAccessLevel,
     },
     world::{World, WorldId},
 };
@@ -140,7 +141,7 @@ pub struct SystemState<Param: SystemParam> {
 
 impl<Param: SystemParam> SystemState<Param> {
     pub fn new(world: &mut World) -> Self {
-        let _ = <Param::Fetch as SystemParamState>::world_access_level();
+        assert_valid_world_access_level::<Param>();
         let mut meta = SystemMeta::new::<Param>();
         meta.last_change_tick = world.change_tick().wrapping_sub(MAX_CHANGE_AGE);
         let param_state = <Param::Fetch as SystemParamState>::init(world, &mut meta);
@@ -335,7 +336,7 @@ where
 {
     type System = FunctionSystem<In, Out, Param, Marker, F>;
     fn into_system(func: Self) -> Self::System {
-        let _ = <Param::Fetch as SystemParamState>::world_access_level();
+        assert_valid_world_access_level::<Param>();
         FunctionSystem {
             func,
             param_state: None,

crates/bevy_ecs/src/system/system_param.rs

@@ -26,6 +26,12 @@ pub enum WorldAccessLevel {
     Exclusive,
 }
 
+/// Asserts that a given [`SystemParam`] (or tuple) has a valid [`WorldAccessLevel`].
+pub fn assert_valid_world_access_level<Params: SystemParam>() {
+    // this will panic on params/tuples whose fields/members have conflicting access levels
+    let _ = <Params::Fetch as SystemParamState>::world_access_level();
+}
+
 /// A parameter that can be used in a [`System`](super::System).
 ///
 /// # Derive