Remove some unsafe code (bevyengine#540)

crates/bevy_tasks/src/single_threaded_task_pool.rs

@@ -65,11 +65,8 @@ impl TaskPool {
     {
         let executor = async_executor::LocalExecutor::new();
 
-        let executor: &async_executor::LocalExecutor = &executor;
-        let executor: &'scope async_executor::LocalExecutor = unsafe { mem::transmute(executor) };
-
         let mut scope = Scope {
-            executor,
+            executor: &executor,
             results: Vec::new(),
         };
 
@@ -87,26 +84,18 @@ impl TaskPool {
 }
 
 pub struct Scope<'scope, T> {
-    executor: &'scope async_executor::LocalExecutor,
+    executor: &'scope async_executor::LocalExecutor<'scope>,
     // Vector to gather results of all futures spawned during scope run
     results: Vec<Arc<Mutex<Option<T>>>>,
 }
 
-impl<'scope, T: Send + 'static> Scope<'scope, T> {
+impl<'scope, T: Send + 'scope> Scope<'scope, T> {
     pub fn spawn<Fut: Future<Output = T> + 'scope + Send>(&mut self, f: Fut) {
         let result = Arc::new(Mutex::new(None));
         self.results.push(result.clone());
         let f = async move {
             result.lock().unwrap().replace(f.await);
         };
-
-        // SAFETY: This function blocks until all futures complete, so we do not read/write the
-        // data from futures outside of the 'scope lifetime. However, rust has no way of knowing
-        // this so we must convert to 'static here to appease the compiler as it is unable to
-        // validate safety.
-        let fut: Pin<Box<dyn Future<Output = ()> + 'scope>> = Box::pin(f);
-        let fut: Pin<Box<dyn Future<Output = ()> + 'static>> = unsafe { mem::transmute(fut) };
-
-        self.executor.spawn(fut).detach();
+        self.executor.spawn(f).detach();
     }
 }

crates/bevy_tasks/src/task.rs

@@ -38,8 +38,7 @@ impl<T> Task<T> {
 impl<T> Future for Task<T> {
     type Output = T;
 
-    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        // Safe because Task is pinned and contains async_executor::Task by value
-        unsafe { self.map_unchecked_mut(|x| &mut x.0).poll(cx) }
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut self.0).poll(cx)
     }
 }

crates/bevy_tasks/src/task_pool.rs

@@ -6,7 +6,7 @@ use std::{
     thread::{self, JoinHandle},
 };
 
-use futures_lite::future;
+use futures_lite::{future, pin};
 
 /// Used to create a TaskPool
 #[derive(Debug, Default, Clone)]
@@ -180,12 +180,8 @@ impl TaskPool {
             results
         };
 
-        // Move the value to ensure that it is owned
-        let mut fut = fut;
-
-        // Shadow the original binding so that it can't be directly accessed
-        // ever again.
-        let fut = unsafe { Pin::new_unchecked(&mut fut) };
+        // Pin the future on the stack.
+        pin!(fut);
 
         // SAFETY: This function blocks until all futures complete, so we do not read/write the
         // data from futures outside of the 'scope lifetime. However, rust has no way of knowing