Create new NonSendMarker (#18301)

# Objective

Create new `NonSendMarker` that does not depend on `NonSend`.

Required, in order to accomplish #17682. In that issue, we are trying to
replace `!Send` resources with `thread_local!` in order to unblock the
resources-as-components effort. However, when we remove all the `!Send`
resources from a system, that allows the system to run on a thread other
than the main thread, which is against the design of the system. So this
marker gives us the control to require a system to run on the main
thread without depending on `!Send` resources.

## Solution

Create a new `NonSendMarker` to replace the existing one that does not
depend on `NonSend`.

## Testing

Other than running tests, I ran a few examples:
- `window_resizing`
- `wireframe`
- `volumetric_fog` (looks so cool)
- `rotation`
- `button`

There is a Mac/iOS-specific change and I do not have a Mac or iOS device
to test it. I am doubtful that it would cause any problems for 2
reasons:
1. The change is the same as the non-wasm change which I did test
2. The Pixel Eagle tests run Mac tests

But it wouldn't hurt if someone wanted to spin up an example that
utilizes the `bevy_render` crate, which is where the Mac/iSO change was.

## Migration Guide

If `NonSendMarker` is being used from `bevy_app::prelude::*`, replace it
with `bevy_ecs::system::NonSendMarker` or use it from
`bevy_ecs::prelude::*`. In addition to that, `NonSendMarker` does not
need to be wrapped like so:
```rust
fn my_system(_non_send_marker: Option<NonSend<NonSendMarker>>) {
    ...
}
```

Instead, it can be used without any wrappers:
```rust
fn my_system(_non_send_marker: NonSendMarker) {
    ...
}
```

---------

Co-authored-by: Chris Russell <8494645+chescock@users.noreply.github.com>

Author: joshua-holmes

crates/bevy_app/src/lib.rs

@@ -58,6 +58,6 @@ pub mod prelude {
             RunFixedMainLoopSystem, SpawnScene, Startup, Update,
         },
         sub_app::SubApp,
-        NonSendMarker, Plugin, PluginGroup, TaskPoolOptions, TaskPoolPlugin,
+        Plugin, PluginGroup, TaskPoolOptions, TaskPoolPlugin,
     };
 }

crates/bevy_app/src/task_pool_plugin.rs

@@ -3,18 +3,19 @@ use crate::{App, Plugin};
 use alloc::string::ToString;
 use bevy_platform_support::sync::Arc;
 use bevy_tasks::{AsyncComputeTaskPool, ComputeTaskPool, IoTaskPool, TaskPoolBuilder};
-use core::{fmt::Debug, marker::PhantomData};
+use core::fmt::Debug;
 use log::trace;
 
 cfg_if::cfg_if! {
     if #[cfg(not(all(target_arch = "wasm32", feature = "web")))] {
-        use {crate::Last, bevy_ecs::prelude::NonSend, bevy_tasks::tick_global_task_pools_on_main_thread};
+        use {crate::Last, bevy_tasks::tick_global_task_pools_on_main_thread};
+        use bevy_ecs::system::NonSendMarker;
 
         /// A system used to check and advanced our task pools.
         ///
         /// Calls [`tick_global_task_pools_on_main_thread`],
         /// and uses [`NonSendMarker`] to ensure that this system runs on the main thread
-        fn tick_global_task_pools(_main_thread_marker: Option<NonSend<NonSendMarker>>) {
+        fn tick_global_task_pools(_main_thread_marker: NonSendMarker) {
             tick_global_task_pools_on_main_thread();
         }
     }
@@ -36,8 +37,6 @@ impl Plugin for TaskPoolPlugin {
         _app.add_systems(Last, tick_global_task_pools);
     }
 }
-/// A dummy type that is [`!Send`](Send), to force systems to run on the main thread.
-pub struct NonSendMarker(PhantomData<*mut ()>);
 
 /// Defines a simple way to determine how many threads to use given the number of remaining cores
 /// and number of total cores

crates/bevy_ecs/src/lib.rs

@@ -91,8 +91,8 @@ pub mod prelude {
         spawn::{Spawn, SpawnRelated},
         system::{
             Command, Commands, Deferred, EntityCommand, EntityCommands, In, InMut, InRef,
-            IntoSystem, Local, NonSend, NonSendMut, ParamSet, Populated, Query, ReadOnlySystem,
-            Res, ResMut, Single, System, SystemIn, SystemInput, SystemParamBuilder,
+            IntoSystem, Local, NonSend, NonSendMarker, NonSendMut, ParamSet, Populated, Query,
+            ReadOnlySystem, Res, ResMut, Single, System, SystemIn, SystemInput, SystemParamBuilder,
             SystemParamFunction, WithParamWarnPolicy,
         },
         world::{

crates/bevy_ecs/src/system/system_param.rs

@@ -1420,6 +1420,33 @@ unsafe impl<T: SystemBuffer> SystemParam for Deferred<'_, T> {
     }
 }
 
+/// A dummy type that is [`!Send`](Send), to force systems to run on the main thread.
+pub struct NonSendMarker;
+
+// SAFETY: No world access.
+unsafe impl SystemParam for NonSendMarker {
+    type State = ();
+    type Item<'w, 's> = Self;
+
+    #[inline]
+    fn init_state(_world: &mut World, system_meta: &mut SystemMeta) -> Self::State {
+        system_meta.set_non_send();
+    }
+
+    #[inline]
+    unsafe fn get_param<'world, 'state>(
+        _state: &'state mut Self::State,
+        _system_meta: &SystemMeta,
+        _world: UnsafeWorldCell<'world>,
+        _change_tick: Tick,
+    ) -> Self::Item<'world, 'state> {
+        Self
+    }
+}
+
+// SAFETY: Does not read any world state
+unsafe impl ReadOnlySystemParam for NonSendMarker {}
+
 /// Shared borrow of a non-[`Send`] resource.
 ///
 /// Only `Send` resources may be accessed with the [`Res`] [`SystemParam`]. In case that the

crates/bevy_render/src/view/window/mod.rs

@@ -304,9 +304,7 @@ const DEFAULT_DESIRED_MAXIMUM_FRAME_LATENCY: u32 = 2;
 pub fn create_surfaces(
     // By accessing a NonSend resource, we tell the scheduler to put this system on the main thread,
     // which is necessary for some OS's
-    #[cfg(any(target_os = "macos", target_os = "ios"))] _marker: Option<
-        NonSend<bevy_app::NonSendMarker>,
-    >,
+    #[cfg(any(target_os = "macos", target_os = "ios"))] _marker: NonSendMarker,
     windows: Res<ExtractedWindows>,
     mut window_surfaces: ResMut<WindowSurfaces>,
     render_instance: Res<RenderInstance>,