switch dynsystemparam to shared and block it from being used with exc…

…lusive parameters

crates/bevy_ecs/src/system/builder.rs

@@ -8,6 +8,7 @@ use crate::{
     resource::Resource,
     system::{
         DynSystemParam, DynSystemParamState, Local, ParamSet, Query, SystemMeta, SystemParam,
+        WorldAccessLevel,
     },
     world::{
         FilteredResources, FilteredResourcesBuilder, FilteredResourcesMut,
@@ -513,6 +514,11 @@ impl<'a> DynParamBuilder<'a> {
     /// Creates a new [`DynParamBuilder`] by wrapping a [`SystemParamBuilder`] of any type.
     /// The built [`DynSystemParam`] can be downcast to `T`.
     pub fn new<T: SystemParam + 'static>(builder: impl SystemParamBuilder<T> + 'a) -> Self {
+        assert_ne!(
+            T::world_access_level(),
+            WorldAccessLevel::Exclusive,
+            "DynSystemParam cannot hold exclusive system parameters"
+        );
         Self(Box::new(|world, meta| {
             DynSystemParamState::new::<T>(builder.build(world, meta))
         }))

crates/bevy_ecs/src/system/system_param.rs

@@ -2825,8 +2825,11 @@ unsafe impl SystemParam for DynSystemParam<'_, '_> {
     }
 
     fn world_access_level() -> WorldAccessLevel {
-        // TODO: Don't assume exclusive access
-        WorldAccessLevel::Exclusive
+        // We don't allow users to create DynSystemParam's that hold exclusive
+        // params, so we can safely return Shared here. This is technically
+        // over-restrictive when storing `WorldAccessLevel::None` params, but we
+        // can't conditionally return based on the held state.
+        WorldAccessLevel::Shared
     }
 }