Error on using yield without also using #[coroutine] on the closure

And suggest adding the `#[coroutine]` to the closure

Author: oli-obk

example/polymorphize_coroutine.rs

@@ -1,4 +1,4 @@
-#![feature(coroutines, coroutine_trait)]
+#![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
 
 use std::ops::Coroutine;
 use std::pin::Pin;
@@ -8,7 +8,8 @@ fn main() {
 }
 
 fn run_coroutine<T>() {
-    let mut coroutine = || {
+    let mut coroutine = #[coroutine]
+    || {
         yield;
         return;
     };

example/std_example.rs

@@ -1,6 +1,7 @@
 #![feature(
     core_intrinsics,
     coroutines,
+    stmt_expr_attributes,
     coroutine_trait,
     is_sorted,
     repr_simd,
@@ -123,9 +124,12 @@ fn main() {
         test_simd();
     }
 
-    Box::pin(move |mut _task_context| {
-        yield ();
-    })
+    Box::pin(
+        #[coroutine]
+        move |mut _task_context| {
+            yield ();
+        },
+    )
     .as_mut()
     .resume(0);