21 files changed, 546 insertions, 39 deletions

diff --git a/embassy-executor/CHANGELOG.md b/embassy-executor/CHANGELOG.md
index 608c67724..914863a83 100644
--- a/embassy-executor/CHANGELOG.md
+++ b/embassy-executor/CHANGELOG.md
@@ -5,9 +5,22 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## unreleased
-
+<!-- next-header -->
+## Unreleased - ReleaseDate
+
+- Added `SpawnToken::id`
+- Task pools are now statically allocated on stable rust. All `task-arena-size-*` features have been removed and are no longer necessary.
+- New trace hooks: `_embassy_trace_poll_start` & `_embassy_trace_task_end`
+- Added task naming capability to tracing infrastructure
+- Added `Executor::id` & `Spawner::executor_id`
+- Disable `critical-section/std` for arch-std
+- Added possibility to select an executor in `#[embassy_executor::main]`
+- Fix AVR executor
+- executor: Make state implementations and their conditions match
 - Added support for Cortex-A and Cortex-R
+- Added support for `-> impl Future<Output = ()>` in `#[task]`
+- Fixed `Send` unsoundness with `-> impl Future` tasks
+- Marked `Spawner::for_current_executor` as `unsafe`
 
 ## 0.7.0 - 2025-01-02
 
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/build_common.rs b/embassy-executor/build_common.rs
index b15a8369f..4f24e6d37 100644
--- a/embassy-executor/build_common.rs
+++ b/embassy-executor/build_common.rs
@@ -92,35 +92,3 @@ pub fn set_target_cfgs(cfgs: &mut CfgSet) {
 
     cfgs.set("has_fpu", target.ends_with("-eabihf"));
 }
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
-pub struct CompilerDate {
-    year: u16,
-    month: u8,
-    day: u8,
-}
-
-impl CompilerDate {
-    fn parse(date: &str) -> Option<Self> {
-        let mut parts = date.split('-');
-        let year = parts.next()?.parse().ok()?;
-        let month = parts.next()?.parse().ok()?;
-        let day = parts.next()?.parse().ok()?;
-        Some(Self { year, month, day })
-    }
-}
-
-impl PartialEq<&str> for CompilerDate {
-    fn eq(&self, other: &&str) -> bool {
-        let Some(other) = Self::parse(other) else {
-            return false;
-        };
-        self.eq(&other)
-    }
-}
-
-impl PartialOrd<&str> for CompilerDate {
-    fn partial_cmp(&self, other: &&str) -> Option<std::cmp::Ordering> {
-        Self::parse(other).map(|other| self.cmp(&other))
-    }
-}
diff --git a/embassy-executor/release.toml b/embassy-executor/release.toml
new file mode 100644
index 000000000..fb6feaf21
--- /dev/null
+++ b/embassy-executor/release.toml
@@ -0,0 +1,5 @@
+pre-release-replacements = [
+    {file="CHANGELOG.md", search="Unreleased", replace="{{version}}", min=1},
+    {file="CHANGELOG.md", search="ReleaseDate", replace="{{date}}", min=1},
+    {file="CHANGELOG.md", search="<!-- next-header -->", replace="<!-- next-header -->\n## Unreleased - ReleaseDate\n", exactly=1},
+]
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/src/raw/trace.rs b/embassy-executor/src/raw/trace.rs
index 6c9cfda25..aa27ab37e 100644
--- a/embassy-executor/src/raw/trace.rs
+++ b/embassy-executor/src/raw/trace.rs
@@ -283,7 +283,17 @@ pub(crate) fn task_new(executor: &SyncExecutor, task: &TaskRef) {
     }
 
     #[cfg(feature = "rtos-trace")]
-    rtos_trace::trace::task_new(task.as_ptr() as u32);
+    {
+        rtos_trace::trace::task_new(task.as_ptr() as u32);
+        let name = task.name().unwrap_or("unnamed task\0");
+        let info = rtos_trace::TaskInfo {
+            name,
+            priority: 0,
+            stack_base: 0,
+            stack_size: 0,
+        };
+        rtos_trace::trace::task_send_info(task.id(), info);
+    }
 
     #[cfg(feature = "rtos-trace")]
     TASK_TRACKER.add(*task);
diff --git a/embassy-executor/src/spawner.rs b/embassy-executor/src/spawner.rs
index 522d97db3..2909d19a0 100644
--- a/embassy-executor/src/spawner.rs
+++ b/embassy-executor/src/spawner.rs
@@ -122,10 +122,26 @@ impl Spawner {
     /// This function is `async` just to get access to the current async
     /// context. It returns instantly, it does not block/yield.
     ///
+    /// Using this method is discouraged due to it being unsafe. Consider the following
+    /// alternatives instead:
+    ///
+    /// - Pass the initial `Spawner` as an argument to tasks. Note that it's `Copy`, so you can
+    ///   make as many copies of it as you want.
+    /// - Use `SendSpawner::for_current_executor()` instead, which is safe but can only be used
+    ///   if task arguments are `Send`.
+    ///
+    /// The only case where using this method is absolutely required is obtaining the `Spawner`
+    /// for an `InterruptExecutor`.
+    ///
+    /// # Safety
+    ///
+    /// You must only execute this with an async `Context` created by the Embassy executor.
+    /// You must not execute it with manually-created `Context`s.
+    ///
     /// # Panics
     ///
     /// Panics if the current executor is not an Embassy executor.
-    pub fn for_current_executor() -> impl Future<Output = Self> {
+    pub unsafe fn for_current_executor() -> impl Future<Output = Self> {
         poll_fn(|cx| {
             let task = raw::task_from_waker(cx.waker());
             let executor = unsafe {
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..7757775ee 100644
--- a/embassy-executor/tests/ui.rs
+++ b/embassy-executor/tests/ui.rs
@@ -17,6 +17,17 @@ 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");
+    t.compile_fail("tests/ui/spawn_nonsend.rs");
+    t.compile_fail("tests/ui/return_impl_future_nonsend.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..57f147714
--- /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_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)
+
+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)
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_future_nonsend.rs b/embassy-executor/tests/ui/return_impl_future_nonsend.rs
new file mode 100644
index 000000000..b8c184b21
--- /dev/null
+++ b/embassy-executor/tests/ui/return_impl_future_nonsend.rs
@@ -0,0 +1,21 @@
+#![cfg_attr(feature = "nightly", feature(impl_trait_in_assoc_type))]
+
+use core::future::Future;
+
+use embassy_executor::SendSpawner;
+
+#[embassy_executor::task]
+fn task() -> impl Future<Output = ()> {
+    // runs in spawning thread
+    let non_send: *mut () = core::ptr::null_mut();
+    async move {
+        // runs in executor thread
+        println!("{}", non_send as usize);
+    }
+}
+
+fn send_spawn(s: SendSpawner) {
+    s.spawn(task()).unwrap();
+}
+
+fn main() {}
diff --git a/embassy-executor/tests/ui/return_impl_future_nonsend.stderr b/embassy-executor/tests/ui/return_impl_future_nonsend.stderr
new file mode 100644
index 000000000..8aeb9738a
--- /dev/null
+++ b/embassy-executor/tests/ui/return_impl_future_nonsend.stderr
@@ -0,0 +1,17 @@
+error: future cannot be sent between threads safely
+  --> tests/ui/return_impl_future_nonsend.rs:18:13
+   |
+18 |     s.spawn(task()).unwrap();
+   |             ^^^^^^ future created by async block is not `Send`
+   |
+   = help: within `impl Sized`, the trait `Send` is not implemented for `*mut ()`
+note: captured value is not `Send`
+  --> tests/ui/return_impl_future_nonsend.rs:13:24
+   |
+13 |         println!("{}", non_send as usize);
+   |                        ^^^^^^^^ has type `*mut ()` which is not `Send`
+note: required by a bound in `SendSpawner::spawn`
+  --> src/spawner.rs
+   |
+   |     pub fn spawn<S: Send>(&self, token: SpawnToken<S>) -> Result<(), SpawnError> {
+   |                     ^^^^ required by this bound in `SendSpawner::spawn`
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..759be1cde
--- /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_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`
+ --> src/raw/mod.rs
+  |
+  | impl<F: Future + 'static, const N: usize> TaskPool<F, N> {
+  |         ^^^^^^ required by this bound in `TaskPool::<F, N>::spawn`
+...
+  |     pub fn spawn(&'static self, future: impl FnOnce() -> F) -> 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)
+
+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)
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`
diff --git a/embassy-executor/tests/ui/spawn_nonsend.rs b/embassy-executor/tests/ui/spawn_nonsend.rs
new file mode 100644
index 000000000..4c4cc7697
--- /dev/null
+++ b/embassy-executor/tests/ui/spawn_nonsend.rs
@@ -0,0 +1,16 @@
+#![cfg_attr(feature = "nightly", feature(impl_trait_in_assoc_type))]
+
+use core::future::Future;
+
+use embassy_executor::SendSpawner;
+
+#[embassy_executor::task]
+async fn task(non_send: *mut ()) {
+    println!("{}", non_send as usize);
+}
+
+fn send_spawn(s: SendSpawner) {
+    s.spawn(task(core::ptr::null_mut())).unwrap();
+}
+
+fn main() {}
diff --git a/embassy-executor/tests/ui/spawn_nonsend.stderr b/embassy-executor/tests/ui/spawn_nonsend.stderr
new file mode 100644
index 000000000..2a06c8b94
--- /dev/null
+++ b/embassy-executor/tests/ui/spawn_nonsend.stderr
@@ -0,0 +1,41 @@
+warning: unused import: `core::future::Future`
+ --> tests/ui/spawn_nonsend.rs:3:5
+  |
+3 | use core::future::Future;
+  |     ^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: `#[warn(unused_imports)]` on by default
+
+error[E0277]: `*mut ()` cannot be sent between threads safely
+  --> tests/ui/spawn_nonsend.rs:13:13
+   |
+7  | #[embassy_executor::task]
+   | ------------------------- within this `impl Sized`
+...
+13 |     s.spawn(task(core::ptr::null_mut())).unwrap();
+   |       ----- ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `*mut ()` cannot be sent between threads safely
+   |       |
+   |       required by a bound introduced by this call
+   |
+   = help: within `impl Sized`, the trait `Send` is not implemented for `*mut ()`
+note: required because it's used within this closure
+  --> tests/ui/spawn_nonsend.rs:7:1
+   |
+7  | #[embassy_executor::task]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required because it appears within the type `impl Sized`
+  --> src/raw/mod.rs
+   |
+   |     pub unsafe fn _spawn_async_fn<FutFn>(&'static self, future: FutFn) -> SpawnToken<impl Sized>
+   |                                                                                      ^^^^^^^^^^
+note: required because it appears within the type `impl Sized`
+  --> tests/ui/spawn_nonsend.rs:7:1
+   |
+7  | #[embassy_executor::task]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+note: required by a bound in `SendSpawner::spawn`
+  --> src/spawner.rs
+   |
+   |     pub fn spawn<S: Send>(&self, token: SpawnToken<S>) -> Result<(), SpawnError> {
+   |                     ^^^^ required by this bound in `SendSpawner::spawn`
+   = note: this error originates in the attribute macro `embassy_executor::task` (in Nightly builds, run with -Z macro-backtrace for more info)