Explain why borrows can't be held across yield point in async blocks

compiler/rustc_error_codes/src/error_codes/E0373.md

@@ -50,3 +50,25 @@ fn foo() -> Box<Fn(u32) -> u32> {
 
 Now that the closure has its own copy of the data, there's no need to worry
 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
+
+async fn f() {
+    let room_ref = Arc::new(Vec::new());
+
+    let gameloop_handle = Runtime::new().unwrap().spawn(async {
+        game_loop(Arc::clone(&room_ref))
+    });
+    gameloop_handle.await;
+}
+
+fn game_loop(v: Arc<Vec<usize>>) {}
+```
+
+Similarly to closures, `async` blocks are not executed immediately and may
+capture closed-over data by reference. For more information, see
+https://rust-lang.github.io/async-book/03_async_await/01_chapter.html.

compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs

@@ -1324,21 +1324,32 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             Applicability::MachineApplicable,
         );
 
-        let msg = match category {
+        match category {
             ConstraintCategory::Return(_) | ConstraintCategory::OpaqueType => {
-                format!("{} is returned here", kind)
+                let msg = format!("{} is returned here", kind);
+                err.span_note(constraint_span, &msg);
             }
             ConstraintCategory::CallArgument => {
                 fr_name.highlight_region_name(&mut err);
-                format!("function requires argument type to outlive `{}`", fr_name)
+                if matches!(use_span.generator_kind(), Some(generator_kind)
+                    if matches!(generator_kind, GeneratorKind::Async(_)))
+                {
+                    err.note(
+                        "async blocks are not executed immediately and must either take a \
+                    reference or ownership of outside variables they use",
+                    );
+                } else {
+                    let msg = format!("function requires argument type to outlive `{}`", fr_name);
+                    err.span_note(constraint_span, &msg);
+                }
             }
             _ => bug!(
                 "report_escaping_closure_capture called with unexpected constraint \
                  category: `{:?}`",
                 category
             ),
-        };
-        err.span_note(constraint_span, &msg);
+        }
+
         err
     }
 

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

@@ -0,0 +1,33 @@
+// edition:2018
+
+use std::{sync::Arc, future::Future, pin::Pin, task::{Context,Poll}};
+
+async fn f() {
+    let room_ref = Arc::new(Vec::new());
+
+    let gameloop_handle = spawn(async { //~ ERROR E0373
+        game_loop(Arc::clone(&room_ref))
+    });
+    gameloop_handle.await;
+}
+
+fn game_loop(v: Arc<Vec<usize>>) {}
+
+fn spawn<F>(future: F) -> JoinHandle
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    loop {}
+}
+
+struct JoinHandle;
+
+impl Future for JoinHandle {
+    type Output = ();
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        loop {}
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,21 @@
+error[E0373]: async block may outlive the current function, but it borrows `room_ref`, which is owned by the current function
+  --> $DIR/issue-78938-async-block.rs:8:39
+   |
+LL |       let gameloop_handle = spawn(async {
+   |  _______________________________________^
+LL | |         game_loop(Arc::clone(&room_ref))
+   | |                               -------- `room_ref` is borrowed here
+LL | |     });
+   | |_____^ may outlive borrowed value `room_ref`
+   |
+   = note: async blocks are not executed immediately and must either take a reference or ownership of outside variables they use
+help: to force the async block to take ownership of `room_ref` (and any other referenced variables), use the `move` keyword
+   |
+LL |     let gameloop_handle = spawn(async move {
+LL |         game_loop(Arc::clone(&room_ref))
+LL |     });
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0373`.