Merge pull request #4266 from 0e4ef622/task-rpit

Allow `-> impl Future<Output = ()>` in #[task]

13 files changed, 421 insertions, 17 deletions

diff --git a/embassy-executor-macros/src/macros/task.rs b/embassy-executor-macros/src/macros/task.rs
index 91d6beee8..91bf8e940 100644
--- a/embassy-executor-macros/src/macros/task.rs
+++ b/embassy-executor-macros/src/macros/task.rs
@@ -51,7 +51,11 @@ pub fn run(args: TokenStream, item: TokenStream) -> TokenStream {
         .embassy_executor
         .unwrap_or(Expr::Verbatim(TokenStream::from_str("::embassy_executor").unwrap()));
 
-    if f.sig.asyncness.is_none() {
+    let returns_impl_trait = match &f.sig.output {
+        ReturnType::Type(_, ty) => matches!(**ty, Type::ImplTrait(_)),
+        _ => false,
+    };
+    if f.sig.asyncness.is_none() && !returns_impl_trait {
         error(&mut errors, &f.sig, "task functions must be async");
     }
     if !f.sig.generics.params.is_empty() {
@@ -66,17 +70,19 @@ pub fn run(args: TokenStream, item: TokenStream) -> TokenStream {
     if !f.sig.variadic.is_none() {
         error(&mut errors, &f.sig, "task functions must not be variadic");
     }
-    match &f.sig.output {
-        ReturnType::Default => {}
-        ReturnType::Type(_, ty) => match &**ty {
-            Type::Tuple(tuple) if tuple.elems.is_empty() => {}
-            Type::Never(_) => {}
-            _ => error(
-                &mut errors,
-                &f.sig,
-                "task functions must either not return a value, return `()` or return `!`",
-            ),
-        },
+    if f.sig.asyncness.is_some() {
+        match &f.sig.output {
+            ReturnType::Default => {}
+            ReturnType::Type(_, ty) => match &**ty {
+                Type::Tuple(tuple) if tuple.elems.is_empty() => {}
+                Type::Never(_) => {}
+                _ => error(
+                    &mut errors,
+                    &f.sig,
+                    "task functions must either not return a value, return `()` or return `!`",
+                ),
+            },
+        }
     }
 
     let mut args = Vec::new();
@@ -128,12 +134,12 @@ pub fn run(args: TokenStream, item: TokenStream) -> TokenStream {
     #[cfg(feature = "nightly")]
     let mut task_outer_body = quote! {
         trait _EmbassyInternalTaskTrait {
-            type Fut: ::core::future::Future + 'static;
+            type Fut: ::core::future::Future<Output: #embassy_executor::_export::TaskReturnValue> + 'static;
             fn construct(#fargs) -> Self::Fut;
         }
 
         impl _EmbassyInternalTaskTrait for () {
-            type Fut = impl core::future::Future + 'static;
+            type Fut = impl core::future::Future<Output: #embassy_executor::_export::TaskReturnValue> + 'static;
             fn construct(#fargs) -> Self::Fut {
                 #task_inner_ident(#(#full_args,)*)
             }
diff --git a/embassy-executor/Cargo.toml b/embassy-executor/Cargo.toml
index f014ccf30..2dbf2c29a 100644
--- a/embassy-executor/Cargo.toml
+++ b/embassy-executor/Cargo.toml
@@ -59,6 +59,7 @@ avr-device = { version = "0.7.0", optional = true }
 critical-section = { version = "1.1", features = ["std"] }
 trybuild = "1.0"
 embassy-sync = { path = "../embassy-sync" }
+rustversion = "1.0.21"
 
 [features]
 
diff --git a/embassy-executor/src/lib.rs b/embassy-executor/src/lib.rs
index dfe420bab..e174a0594 100644
--- a/embassy-executor/src/lib.rs
+++ b/embassy-executor/src/lib.rs
@@ -65,8 +65,17 @@ pub mod _export {
 
     use crate::raw::TaskPool;
 
+    trait TaskReturnValue {}
+    impl TaskReturnValue for () {}
+    impl TaskReturnValue for Never {}
+
+    #[diagnostic::on_unimplemented(
+        message = "task futures must resolve to `()` or `!`",
+        note = "use `async fn` or change the return type to `impl Future<Output = ()>`"
+    )]
+    #[allow(private_bounds)]
     pub trait TaskFn<Args>: Copy {
-        type Fut: Future + 'static;
+        type Fut: Future<Output: TaskReturnValue> + 'static;
     }
 
     macro_rules! task_fn_impl {
@@ -74,7 +83,7 @@ pub mod _export {
             impl<F, Fut, $($Tn,)*> TaskFn<($($Tn,)*)> for F
             where
                 F: Copy + FnOnce($($Tn,)*) -> Fut,
-                Fut: Future + 'static,
+                Fut: Future<Output: TaskReturnValue> + 'static,
             {
                 type Fut = Fut;
             }
@@ -205,4 +214,42 @@ pub mod _export {
         Align268435456: 268435456,
         Align536870912: 536870912,
     );
+
+    #[allow(dead_code)]
+    trait HasOutput {
+        type Output;
+    }
+
+    impl<O> HasOutput for fn() -> O {
+        type Output = O;
+    }
+
+    #[allow(dead_code)]
+    type Never = <fn() -> ! as HasOutput>::Output;
+}
+
+/// Implementation details for embassy macros.
+/// Do not use. Used for macros and HALs only. Not covered by semver guarantees.
+#[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 {}
+
+    #[allow(dead_code)]
+    trait HasOutput {
+        type Output;
+    }
+
+    impl<O> HasOutput for fn() -> O {
+        type Output = O;
+    }
+
+    #[allow(dead_code)]
+    type Never = <fn() -> ! as HasOutput>::Output;
 }
diff --git a/embassy-executor/tests/test.rs b/embassy-executor/tests/test.rs
index 78c49c071..c1e7ec5d7 100644
--- a/embassy-executor/tests/test.rs
+++ b/embassy-executor/tests/test.rs
@@ -1,7 +1,8 @@
 #![cfg_attr(feature = "nightly", feature(impl_trait_in_assoc_type))]
+#![cfg_attr(feature = "nightly", feature(never_type))]
 
 use std::boxed::Box;
-use std::future::poll_fn;
+use std::future::{poll_fn, Future};
 use std::sync::{Arc, Mutex};
 use std::task::Poll;
 
@@ -58,6 +59,39 @@ fn executor_task() {
         trace.push("poll task1")
     }
 
+    #[task]
+    async fn task2() -> ! {
+        panic!()
+    }
+
+    let (executor, trace) = setup();
+    executor.spawner().spawn(task1(trace.clone())).unwrap();
+
+    unsafe { executor.poll() };
+    unsafe { executor.poll() };
+
+    assert_eq!(
+        trace.get(),
+        &[
+            "pend",       // spawning a task pends the executor
+            "poll task1", // poll only once.
+        ]
+    )
+}
+
+#[test]
+fn executor_task_rpit() {
+    #[task]
+    fn task1(trace: Trace) -> impl Future<Output = ()> {
+        async move { trace.push("poll task1") }
+    }
+
+    #[cfg(feature = "nightly")]
+    #[task]
+    fn task2() -> impl Future<Output = !> {
+        async { panic!() }
+    }
+
     let (executor, trace) = setup();
     executor.spawner().spawn(task1(trace.clone())).unwrap();
 
diff --git a/embassy-executor/tests/ui.rs b/embassy-executor/tests/ui.rs
index 278a4b903..c4a1a601c 100644
--- a/embassy-executor/tests/ui.rs
+++ b/embassy-executor/tests/ui.rs
@@ -17,6 +17,15 @@ fn ui() {
     t.compile_fail("tests/ui/nonstatic_struct_elided.rs");
     t.compile_fail("tests/ui/nonstatic_struct_generic.rs");
     t.compile_fail("tests/ui/not_async.rs");
+    if rustversion::cfg!(stable) {
+        // output is slightly different on nightly
+        t.compile_fail("tests/ui/bad_return_impl_future.rs");
+        t.compile_fail("tests/ui/return_impl_send.rs");
+    }
+    if cfg!(feature = "nightly") {
+        t.compile_fail("tests/ui/bad_return_impl_future_nightly.rs");
+        t.compile_fail("tests/ui/return_impl_send_nightly.rs");
+    }
     t.compile_fail("tests/ui/self_ref.rs");
     t.compile_fail("tests/ui/self.rs");
     t.compile_fail("tests/ui/type_error.rs");
diff --git a/embassy-executor/tests/ui/bad_return_impl_future.rs b/embassy-executor/tests/ui/bad_return_impl_future.rs
new file mode 100644
index 000000000..baaa7dc5a
--- /dev/null
+++ b/embassy-executor/tests/ui/bad_return_impl_future.rs
@@ -0,0 +1,9 @@
+#![cfg_attr(feature = "nightly", feature(impl_trait_in_assoc_type))]
+use core::future::Future;
+
+#[embassy_executor::task]
+fn task() -> impl Future<Output = u32> {
+    async { 5 }
+}
+
+fn main() {}
diff --git a/embassy-executor/tests/ui/bad_return_impl_future.stderr b/embassy-executor/tests/ui/bad_return_impl_future.stderr
new file mode 100644
index 000000000..2980fd18b
--- /dev/null
+++ b/embassy-executor/tests/ui/bad_return_impl_future.stderr
@@ -0,0 +1,120 @@
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/bad_return_impl_future.rs:5:4
+  |
+4 | #[embassy_executor::task]
+  | ------------------------- required by a bound introduced by this call
+5 | fn task() -> impl Future<Output = u32> {
+  |    ^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Future<Output = u32> {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_size`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_size<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> usize
+  |                  -------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |                         ^^^^^^^^^ required by this bound in `task_pool_size`
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/bad_return_impl_future.rs:4:1
+  |
+4 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Future<Output = u32> {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_size`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_size<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> usize
+  |                  -------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |            ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `task_pool_size`
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/bad_return_impl_future.rs:5:4
+  |
+4 | #[embassy_executor::task]
+  | ------------------------- required by a bound introduced by this call
+5 | fn task() -> impl Future<Output = u32> {
+  |    ^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Future<Output = u32> {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_align`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_align<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> usize
+  |                  --------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |                         ^^^^^^^^^ required by this bound in `task_pool_align`
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/bad_return_impl_future.rs:4:1
+  |
+4 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Future<Output = u32> {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_align`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_align<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> usize
+  |                  --------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |            ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `task_pool_align`
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/bad_return_impl_future.rs:5:4
+  |
+4 | #[embassy_executor::task]
+  | ------------------------- required by a bound introduced by this call
+5 | fn task() -> impl Future<Output = u32> {
+  |    ^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Future<Output = u32> {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_new`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_new<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> TaskPool<Fut, POOL_SIZE>
+  |                  ------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |                         ^^^^^^^^^ required by this bound in `task_pool_new`
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/bad_return_impl_future.rs:4:1
+  |
+4 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Future<Output = u32> {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_new`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_new<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> TaskPool<Fut, POOL_SIZE>
+  |                  ------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |            ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `task_pool_new`
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/bad_return_impl_future.rs:5:4
+  |
+4 | #[embassy_executor::task]
+  | ------------------------- required by a bound introduced by this call
+5 | fn task() -> impl Future<Output = u32> {
+  |    ^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Future<Output = u32> {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `__task_pool_get`
+ --> tests/ui/bad_return_impl_future.rs:4:1
+  |
+4 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `__task_pool_get`
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/embassy-executor/tests/ui/bad_return_impl_future_nightly.rs b/embassy-executor/tests/ui/bad_return_impl_future_nightly.rs
new file mode 100644
index 000000000..baaa7dc5a
--- /dev/null
+++ b/embassy-executor/tests/ui/bad_return_impl_future_nightly.rs
@@ -0,0 +1,9 @@
+#![cfg_attr(feature = "nightly", feature(impl_trait_in_assoc_type))]
+use core::future::Future;
+
+#[embassy_executor::task]
+fn task() -> impl Future<Output = u32> {
+    async { 5 }
+}
+
+fn main() {}
diff --git a/embassy-executor/tests/ui/bad_return_impl_future_nightly.stderr b/embassy-executor/tests/ui/bad_return_impl_future_nightly.stderr
new file mode 100644
index 000000000..73ceb989d
--- /dev/null
+++ b/embassy-executor/tests/ui/bad_return_impl_future_nightly.stderr
@@ -0,0 +1,10 @@
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/bad_return_impl_future_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
diff --git a/embassy-executor/tests/ui/return_impl_send.rs b/embassy-executor/tests/ui/return_impl_send.rs
new file mode 100644
index 000000000..6ddb0e722
--- /dev/null
+++ b/embassy-executor/tests/ui/return_impl_send.rs
@@ -0,0 +1,6 @@
+#![cfg_attr(feature = "nightly", feature(impl_trait_in_assoc_type))]
+
+#[embassy_executor::task]
+fn task() -> impl Send {}
+
+fn main() {}
diff --git a/embassy-executor/tests/ui/return_impl_send.stderr b/embassy-executor/tests/ui/return_impl_send.stderr
new file mode 100644
index 000000000..7e3e468b8
--- /dev/null
+++ b/embassy-executor/tests/ui/return_impl_send.stderr
@@ -0,0 +1,137 @@
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/return_impl_send.rs:4:4
+  |
+3 | #[embassy_executor::task]
+  | ------------------------- required by a bound introduced by this call
+4 | fn task() -> impl Send {}
+  |    ^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Send {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_size`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_size<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> usize
+  |                  -------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |                         ^^^^^^^^^ required by this bound in `task_pool_size`
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/return_impl_send.rs:3:1
+  |
+3 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Send {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_size`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_size<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> usize
+  |                  -------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |            ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `task_pool_size`
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/return_impl_send.rs:4:4
+  |
+3 | #[embassy_executor::task]
+  | ------------------------- required by a bound introduced by this call
+4 | fn task() -> impl Send {}
+  |    ^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Send {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_align`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_align<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> usize
+  |                  --------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |                         ^^^^^^^^^ required by this bound in `task_pool_align`
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/return_impl_send.rs:3:1
+  |
+3 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Send {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_align`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_align<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> usize
+  |                  --------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |            ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `task_pool_align`
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/return_impl_send.rs:4:4
+  |
+3 | #[embassy_executor::task]
+  | ------------------------- required by a bound introduced by this call
+4 | fn task() -> impl Send {}
+  |    ^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Send {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_new`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_new<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> TaskPool<Fut, POOL_SIZE>
+  |                  ------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |                         ^^^^^^^^^ required by this bound in `task_pool_new`
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/return_impl_send.rs:3:1
+  |
+3 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Send {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `task_pool_new`
+ --> src/lib.rs
+  |
+  |     pub const fn task_pool_new<F, Args, Fut, const POOL_SIZE: usize>(_: F) -> TaskPool<Fut, POOL_SIZE>
+  |                  ------------- required by a bound in this function
+  |     where
+  |         F: TaskFn<Args, Fut = Fut>,
+  |            ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `task_pool_new`
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: task futures must resolve to `()` or `!`
+ --> tests/ui/return_impl_send.rs:4:4
+  |
+3 | #[embassy_executor::task]
+  | ------------------------- required by a bound introduced by this call
+4 | fn task() -> impl Send {}
+  |    ^^^^ the trait `TaskFn<_>` is not implemented for fn item `fn() -> impl Send {__task_task}`
+  |
+  = note: use `async fn` or change the return type to `impl Future<Output = ()>`
+note: required by a bound in `__task_pool_get`
+ --> tests/ui/return_impl_send.rs:3:1
+  |
+3 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `__task_pool_get`
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: `impl Send` is not a future
+ --> tests/ui/return_impl_send.rs:3:1
+  |
+3 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^ `impl Send` is not a future
+  |
+  = help: the trait `Future` is not implemented for `impl Send`
+note: required by a bound in `TaskPool::<F, N>::_spawn_async_fn`
+ --> src/raw/mod.rs
+  |
+  | impl<F: Future + 'static, const N: usize> TaskPool<F, N> {
+  |         ^^^^^^ required by this bound in `TaskPool::<F, N>::_spawn_async_fn`
+...
+  |     pub unsafe fn _spawn_async_fn<FutFn>(&'static self, future: FutFn) -> SpawnToken<impl Sized>
+  |                   --------------- required by a bound in this associated function
+  = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/embassy-executor/tests/ui/return_impl_send_nightly.rs b/embassy-executor/tests/ui/return_impl_send_nightly.rs
new file mode 100644
index 000000000..6ddb0e722
--- /dev/null
+++ b/embassy-executor/tests/ui/return_impl_send_nightly.rs
@@ -0,0 +1,6 @@
+#![cfg_attr(feature = "nightly", feature(impl_trait_in_assoc_type))]
+
+#[embassy_executor::task]
+fn task() -> impl Send {}
+
+fn main() {}
diff --git a/embassy-executor/tests/ui/return_impl_send_nightly.stderr b/embassy-executor/tests/ui/return_impl_send_nightly.stderr
new file mode 100644
index 000000000..de9ba6243
--- /dev/null
+++ b/embassy-executor/tests/ui/return_impl_send_nightly.stderr
@@ -0,0 +1,10 @@
+error[E0277]: `impl Send` is not a future
+ --> tests/ui/return_impl_send_nightly.rs:3:1
+  |
+3 | #[embassy_executor::task]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^
+  | |
+  | `impl Send` is not a future
+  | return type was inferred to be `impl Send` here
+  |
+  = help: the trait `Future` is not implemented for `impl Send`