Fix error E0373 documentation

compiler/rustc_error_codes/src/error_codes/E0373.md

@@ -54,19 +54,35 @@ about safety.
 This error may also be encountered while using `async` blocks:
 
 ```compile_fail,E0373
-use std::sync::Arc;
-use tokio::runtime::Runtime; // 0.3.1
+use std::{sync::Arc, future::Future, pin::Pin, task::{Context, Poll}};
 
 async fn f() {
-    let room_ref = Arc::new(Vec::new());
+    let v = Arc::new(Vec::new());
 
-    let gameloop_handle = Runtime::new().unwrap().spawn(async {
-        game_loop(Arc::clone(&room_ref))
+    let handle = spawn(async { //~ ERROR E0373
+        g(Arc::clone(&v))
     });
-    gameloop_handle.await;
+    handle.await;
 }
 
-fn game_loop(v: Arc<Vec<usize>>) {}
+fn g(v: Arc<Vec<usize>>) {}
+
+fn spawn<F>(future: F) -> JoinHandle
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    unimplemented!()
+}
+
+struct JoinHandle;
+
+impl Future for JoinHandle {
+    type Output = ();
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        unimplemented!()
+    }
+}
 ```
 
 Similarly to closures, `async` blocks are not executed immediately and may

src/test/ui/async-await/issues/issue-78938-async-block.rs

@@ -1,6 +1,6 @@
 // edition:2018
 
-use std::{sync::Arc, future::Future, pin::Pin, task::{Context,Poll}};
+use std::{sync::Arc, future::Future, pin::Pin, task::{Context, Poll}};
 
 async fn f() {
     let room_ref = Arc::new(Vec::new());