Add diagnostic::on_unimplemented for nightly

2 files changed, 6 insertions, 1 deletions

diff --git a/embassy-executor/src/lib.rs b/embassy-executor/src/lib.rs
index e26e8ee7d..e174a0594 100644
--- a/embassy-executor/src/lib.rs
+++ b/embassy-executor/src/lib.rs
@@ -233,6 +233,10 @@ pub mod _export {
 #[doc(hidden)]
 #[cfg(feature = "nightly")]
 pub mod _export {
+    #[diagnostic::on_unimplemented(
+        message = "task futures must resolve to `()` or `!`",
+        note = "use `async fn` or change the return type to `impl Future<Output = ()>`"
+    )]
     pub trait TaskReturnValue {}
     impl TaskReturnValue for () {}
     impl TaskReturnValue for Never {}
diff --git a/embassy-executor/tests/ui/bad_return_impl_trait_nightly.stderr b/embassy-executor/tests/ui/bad_return_impl_trait_nightly.stderr
index 21e20d5c0..a51251bb8 100644
--- a/embassy-executor/tests/ui/bad_return_impl_trait_nightly.stderr
+++ b/embassy-executor/tests/ui/bad_return_impl_trait_nightly.stderr
@@ -1,9 +1,10 @@
-error[E0277]: the trait bound `u32: TaskReturnValue` is not satisfied
+error[E0277]: task futures must resolve to `()` or `!`
  --> tests/ui/bad_return_impl_trait_nightly.rs:4:1
   |
 4 | #[embassy_executor::task]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TaskReturnValue` is not implemented for `u32`
   |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
   = help: the following other types implement trait `TaskReturnValue`:
             ()
             <fn() -> ! as _export::HasOutput>::Output