Replace nostd_async with a simple test executor

on-target-tests/Cargo.toml

@@ -60,7 +60,7 @@ futures = {version = "0.3.30", default-features = false, features = ["async-awai
 heapless = {version = "0.8.0", features = ["portable-atomic-critical-section", "defmt-03"]}
 i2c-write-iter = {version = "1.0.0", features = ["async"]}
 itertools = {version = "0.12.0", default-features = false}
-nostd_async = {version = "0.6.1", features = ["wfe"]}
+once_cell = { version = "1.19.0", default-features = false, features = ["critical-section"] }
 panic-probe = {version = "0.3", features = ["print-defmt"]}
 rp2040-boot2 = "0.3.0"
 rp2040-hal = {path = "../rp2040-hal", features = ["critical-section-impl", "defmt", "rt", "i2c-write-iter"]}

on-target-tests/tests/i2c_tests/mod.rs

@@ -12,6 +12,7 @@ use rp2040_hal::{
 
 pub mod blocking;
 pub mod non_blocking;
+pub mod test_executor;
 
 pub const ADDR_7BIT: u8 = 0x2c;
 pub const ADDR_10BIT: u16 = 0x12c;

on-target-tests/tests/i2c_tests/non_blocking.rs

@@ -30,8 +30,7 @@ pub struct State {
 }
 
 pub fn run_test(f: impl Future) {
-    let runtime = nostd_async::Runtime::new();
-    nostd_async::Task::new(f).spawn(&runtime).join();
+    super::test_executor::execute(f);
 }
 async fn wait_with(payload: &RefCell<TargetState>, mut f: impl FnMut(&TargetState) -> bool) {
     while f(payload.borrow().deref()) {

on-target-tests/tests/i2c_tests/test_executor.rs

@@ -0,0 +1,79 @@
+//! Simplistic test executor
+//!
+//! Compared to a real executor, this has some limitations:
+//!
+//! - Can only run to completion (like block_on, but without busy polling)
+//! - Can't spawn additional tasks
+//! - Must not be called multiple times concurrently
+
+use core::{
+    future::Future,
+    pin::{self, Pin},
+    ptr,
+    sync::atomic::{AtomicBool, Ordering},
+    task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
+};
+
+use once_cell::sync::OnceCell;
+
+static WOKE: AtomicBool = AtomicBool::new(false);
+static POLLING: AtomicBool = AtomicBool::new(false);
+
+static VTABLE: RawWakerVTable = RawWakerVTable::new(clone_fn, wake_fn, wake_fn, drop_fn);
+
+fn wake_fn(_data: *const ()) {
+    if !POLLING.load(Ordering::Relaxed) {
+        defmt::info!("waker called while not polling");
+    }
+    WOKE.store(true, Ordering::Relaxed);
+}
+
+fn clone_fn(data: *const ()) -> RawWaker {
+    RawWaker::new(data, &VTABLE)
+}
+
+fn drop_fn(_data: *const ()) {}
+
+fn context() -> Context<'static> {
+    static WAKER: OnceCell<Waker> = OnceCell::new();
+    // Safety: The functions in the vtable of this executor only modify static atomics.
+    let waker =
+        WAKER.get_or_init(|| unsafe { Waker::from_raw(RawWaker::new(ptr::null(), &VTABLE)) });
+
+    // Starting from rust 1.82.0, this could be used:
+    // static WAKER: Waker = unsafe { Waker::from_raw(RawWaker::new(ptr::null(), &VTABLE)) };
+    // (stabilized by https://github.com/rust-lang/rust/pull/128228)
+
+    Context::from_waker(waker)
+}
+
+/// Run future to completion
+///
+/// poll() will only be called when the waker was invoked, so this is suitable to test
+/// if the waker is properly triggered from an interrupt.
+///
+/// This won't work as expected of multiple calls to `execute` happen concurrently.
+///
+/// (Calling this function from multiple threads concurrently doesn't violate any
+/// safety guarantees, but wakers may wake the wrong task, making futures stall.)
+pub fn execute<T>(future: impl Future<Output = T>) -> T {
+    let mut pinned: Pin<&mut _> = pin::pin!(future);
+    if WOKE.load(Ordering::Relaxed) {
+        defmt::info!("woken before poll - ignoring");
+    }
+    POLLING.store(true, Ordering::Relaxed);
+    loop {
+        WOKE.store(false, Ordering::Relaxed);
+        if let Poll::Ready(result) = pinned.as_mut().poll(&mut context()) {
+            WOKE.store(false, Ordering::Relaxed);
+            POLLING.store(false, Ordering::Relaxed);
+            break result;
+        }
+        while !WOKE.load(Ordering::Relaxed) {
+            // In a real executor, there should be a WFI/WFE or similar here, to avoid
+            // busy looping.
+            // As this is only a test executor, we don't care.
+            core::hint::spin_loop();
+        }
+    }
+}