Test that async blocks are UnwindSafe

This was a regression from the reverted #105250 which is now covered by a test.

src/test/ui/async-await/async-is-unwindsafe.rs

@@ -0,0 +1,30 @@
+// edition:2018
+
+fn is_unwindsafe(_: impl std::panic::UnwindSafe) {}
+
+fn main() {
+    // A normal future created by an async block takes a `&mut Context<'_>` argument.
+    // That should not leak through to the whole async block.
+    is_unwindsafe(async {
+        async {}.await; // this needs an inner await point
+    });
+
+    is_unwindsafe(async {
+        //~^ ERROR the type `&mut Context<'_>` may not be safely transferred across an unwind boundary
+        use std::ptr::null;
+        use std::task::{Context, RawWaker, RawWakerVTable, Waker};
+        let waker = unsafe {
+            Waker::from_raw(RawWaker::new(
+                null(),
+                &RawWakerVTable::new(|_| todo!(), |_| todo!(), |_| todo!(), |_| todo!()),
+            ))
+        };
+        let mut cx = Context::from_waker(&waker);
+        let cx_ref = &mut cx;
+
+        async {}.await; // this needs an inner await point
+
+        // in this case, `&mut Context<'_>` is *truly* alive across an await point
+        drop(cx_ref);
+    });
+}

src/test/ui/async-await/async-is-unwindsafe.stderr

@@ -0,0 +1,38 @@
+error[E0277]: the type `&mut Context<'_>` may not be safely transferred across an unwind boundary
+  --> $DIR/async-is-unwindsafe.rs:12:19
+   |
+LL |       is_unwindsafe(async {
+   |  ___________________^
+LL | |
+LL | |         use std::ptr::null;
+LL | |         use std::task::{Context, RawWaker, RawWakerVTable, Waker};
+...  |
+LL | |         drop(cx_ref);
+LL | |     });
+   | |     ^
+   | |     |
+   | |_____`&mut Context<'_>` may not be safely transferred across an unwind boundary
+   |       within this `[async block@$DIR/async-is-unwindsafe.rs:12:19: 29:6]`
+   |
+   = help: within `[async block@$DIR/async-is-unwindsafe.rs:12:19: 29:6]`, the trait `UnwindSafe` is not implemented for `&mut Context<'_>`
+   = note: `UnwindSafe` is implemented for `&std::task::Context<'_>`, but not for `&mut std::task::Context<'_>`
+note: future does not implement `UnwindSafe` as this value is used across an await
+  --> $DIR/async-is-unwindsafe.rs:25:17
+   |
+LL |         let cx_ref = &mut cx;
+   |             ------ has type `&mut Context<'_>` which does not implement `UnwindSafe`
+LL |
+LL |         async {}.await; // this needs an inner await point
+   |                 ^^^^^^ await occurs here, with `cx_ref` maybe used later
+...
+LL |     });
+   |     - `cx_ref` is later dropped here
+note: required by a bound in `is_unwindsafe`
+  --> $DIR/async-is-unwindsafe.rs:3:26
+   |
+LL | fn is_unwindsafe(_: impl std::panic::UnwindSafe) {}
+   |                          ^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `is_unwindsafe`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.