Remove some superfluous unsafe code (#3297)

# Objective

- This `unsafe` is weird

## Solution

- Don't use `unsafe` here

Hopefully this isn't already in an open PR.

crates/bevy_ecs/src/schedule/executor_parallel.rs

@@ -175,7 +175,7 @@ impl ParallelExecutor {
     fn prepare_systems<'scope>(
         &mut self,
         scope: &mut Scope<'scope, ()>,
-        systems: &'scope [ParallelSystemContainer],
+        systems: &'scope mut [ParallelSystemContainer],
         world: &'scope World,
     ) {
         #[cfg(feature = "trace")]
@@ -183,13 +183,15 @@ impl ParallelExecutor {
         #[cfg(feature = "trace")]
         let _guard = span.enter();
         self.should_run.clear();
-        for (index, system_data) in self.system_metadata.iter_mut().enumerate() {
+        for (index, (system_data, system)) in
+            self.system_metadata.iter_mut().zip(systems).enumerate()
+        {
             // Spawn the system task.
-            if systems[index].should_run() {
+            if system.should_run() {
                 self.should_run.set(index, true);
                 let start_receiver = system_data.start_receiver.clone();
                 let finish_sender = self.finish_sender.clone();
-                let system = unsafe { systems[index].system_mut_unsafe() };
+                let system = system.system_mut();
                 #[cfg(feature = "trace")] // NB: outside the task to get the TLS current span
                 let system_span = bevy_utils::tracing::info_span!("system", name = &*system.name());
                 let task = async move {

crates/bevy_ecs/src/schedule/system_container.rs

@@ -7,7 +7,7 @@ use crate::{
     },
     system::{ExclusiveSystem, System},
 };
-use std::{borrow::Cow, cell::UnsafeCell};
+use std::borrow::Cow;
 
 /// System metadata like its name, labels, order requirements and component access.
 pub trait SystemContainer: GraphNode<Label = BoxedSystemLabel> {
@@ -106,7 +106,7 @@ impl SystemContainer for ExclusiveSystemContainer {
 }
 
 pub struct ParallelSystemContainer {
-    system: Box<UnsafeCell<dyn System<In = (), Out = ()>>>,
+    system: Box<dyn System<In = (), Out = ()>>,
     pub(crate) run_criteria_index: Option<usize>,
     pub(crate) run_criteria_label: Option<BoxedRunCriteriaLabel>,
     pub(crate) should_run: bool,
@@ -123,8 +123,7 @@ unsafe impl Sync for ParallelSystemContainer {}
 impl ParallelSystemContainer {
     pub(crate) fn from_descriptor(descriptor: ParallelSystemDescriptor) -> Self {
         ParallelSystemContainer {
-            // SAFE: it is fine to wrap inner value with UnsafeCell, as it is repr(transparent)
-            system: unsafe { Box::from_raw(Box::into_raw(descriptor.system) as *mut _) },
+            system: descriptor.system,
             should_run: false,
             run_criteria_index: None,
             run_criteria_label: None,
@@ -141,19 +140,11 @@ impl ParallelSystemContainer {
     }
 
     pub fn system(&self) -> &dyn System<In = (), Out = ()> {
-        // SAFE: statically enforced shared access
-        unsafe { self.system.get().as_ref().unwrap() }
+        &*self.system
     }
 
     pub fn system_mut(&mut self) -> &mut dyn System<In = (), Out = ()> {
-        self.system.get_mut()
-    }
-
-    /// # Safety
-    /// Ensure no other borrows exist along with this one.
-    #[allow(clippy::mut_from_ref)]
-    pub unsafe fn system_mut_unsafe(&self) -> &mut dyn System<In = (), Out = ()> {
-        self.system.get().as_mut().unwrap()
+        &mut *self.system
     }
 
     pub fn should_run(&self) -> bool {