Merge #693

693: no_std version of `futures::future::select_all` r=Dirbaio a=alexmoon

Here's a no-std compatible version of `select_all`. It's not quite as useful as the original because it requires an array of Unpin futures to be pre-constructed instead of taking an iterator (which could return `Pin<Box<_>>` in `std`). And, of course, you don't get a `Vec` of the unfinished futures returned at completion. Still, I think it's cleaner than a long cons of select calls.

I'll leave it up to you whether this is sufficiently general purpose to include in Embassy or not.


Co-authored-by: alexmoon <alex.r.moon@gmail.com>

embassy/src/util/mod.rs

@@ -1,9 +1,11 @@
 //! Misc utilities
 
 mod forever;
+mod select_all;
 mod yield_now;
 
 pub use forever::*;
+pub use select_all::*;
 pub use yield_now::*;
 
 /// Unsafely unborrow an owned singleton out of a `&mut`.

embassy/src/util/select_all.rs

@@ -0,0 +1,58 @@
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Future for the [`select_all`] function.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct SelectAll<Fut, const N: usize> {
+    inner: [Fut; N],
+}
+
+impl<Fut: Unpin, const N: usize> Unpin for SelectAll<Fut, N> {}
+
+/// Creates a new future which will select over a list of futures.
+///
+/// The returned future will wait for any future within `iter` to be ready. Upon
+/// completion the item resolved will be returned, along with the index of the
+/// future that was ready.
+///
+/// # Panics
+///
+/// This function will panic if the array specified contains no items.
+pub fn select_all<Fut: Future, const N: usize>(arr: [Fut; N]) -> SelectAll<Fut, N> {
+    assert!(N > 0);
+    SelectAll { inner: arr }
+}
+
+impl<Fut, const N: usize> SelectAll<Fut, N> {
+    /// Consumes this combinator, returning the underlying futures.
+    pub fn into_inner(self) -> [Fut; N] {
+        self.inner
+    }
+}
+
+impl<Fut: Future, const N: usize> Future for SelectAll<Fut, N> {
+    type Output = (Fut::Output, usize);
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        // Safety: Since `self` is pinned, `inner` cannot move. Since `inner` cannot move,
+        // its elements also cannot move. Therefore it is safe to access `inner` and pin
+        // references to the contained futures.
+        let item = unsafe {
+            self.get_unchecked_mut()
+                .inner
+                .iter_mut()
+                .enumerate()
+                .find_map(|(i, f)| match Pin::new_unchecked(f).poll(cx) {
+                    Poll::Pending => None,
+                    Poll::Ready(e) => Some((i, e)),
+                })
+        };
+
+        match item {
+            Some((idx, res)) => Poll::Ready((res, idx)),
+            None => Poll::Pending,
+        }
+    }
+}