From 04a263c7001dc259a18fc5aac10d1f9a939a517c Mon Sep 17 00:00:00 2001
From: alexmoon <alex.r.moon@gmail.com>
Date: Mon, 4 Apr 2022 19:30:16 -0400
Subject: [PATCH 1/2] no_std version of `futures::future::select_all`

---
 embassy/src/util/mod.rs        |  2 ++
 embassy/src/util/select_all.rs | 56 ++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)
 create mode 100644 embassy/src/util/select_all.rs

diff --git a/embassy/src/util/mod.rs b/embassy/src/util/mod.rs
index 7744778b..f5cc9623 100644
--- a/embassy/src/util/mod.rs
+++ b/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`.
diff --git a/embassy/src/util/select_all.rs b/embassy/src/util/select_all.rs
new file mode 100644
index 00000000..1b9bddd6
--- /dev/null
+++ b/embassy/src/util/select_all.rs
@@ -0,0 +1,56 @@
+#![allow(dead_code)]
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+use futures::future::FutureExt;
+
+/// 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 + Unpin, const N: usize>(arr: [Fut; N]) -> Option<SelectAll<Fut, N>> {
+    assert!(N > 0);
+    Some(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 + Unpin, const N: usize> Future for SelectAll<Fut, N> {
+    type Output = (Fut::Output, usize);
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let item = self
+            .inner
+            .iter_mut()
+            .enumerate()
+            .find_map(|(i, f)| match f.poll_unpin(cx) {
+                Poll::Pending => None,
+                Poll::Ready(e) => Some((i, e)),
+            });
+        match item {
+            Some((idx, res)) => Poll::Ready((res, idx)),
+            None => Poll::Pending,
+        }
+    }
+}

From e42295c4c51a35132595a7579de8467a38bc8171 Mon Sep 17 00:00:00 2001
From: alexmoon <alex.r.moon@gmail.com>
Date: Mon, 4 Apr 2022 21:24:10 -0400
Subject: [PATCH 2/2] Remove Unpin bound from SelectAll

---
 embassy/src/util/select_all.rs | 34 ++++++++++++++++++----------------
 1 file changed, 18 insertions(+), 16 deletions(-)

diff --git a/embassy/src/util/select_all.rs b/embassy/src/util/select_all.rs
index 1b9bddd6..aef22d89 100644
--- a/embassy/src/util/select_all.rs
+++ b/embassy/src/util/select_all.rs
@@ -1,11 +1,7 @@
-#![allow(dead_code)]
-
 use core::future::Future;
 use core::pin::Pin;
 use core::task::{Context, Poll};
 
-use futures::future::FutureExt;
-
 /// Future for the [`select_all`] function.
 #[derive(Debug)]
 #[must_use = "futures do nothing unless you `.await` or poll them"]
@@ -24,9 +20,9 @@ impl<Fut: Unpin, const N: usize> Unpin for SelectAll<Fut, N> {}
 /// # Panics
 ///
 /// This function will panic if the array specified contains no items.
-pub fn select_all<Fut: Future + Unpin, const N: usize>(arr: [Fut; N]) -> Option<SelectAll<Fut, N>> {
+pub fn select_all<Fut: Future, const N: usize>(arr: [Fut; N]) -> SelectAll<Fut, N> {
     assert!(N > 0);
-    Some(SelectAll { inner: arr })
+    SelectAll { inner: arr }
 }
 
 impl<Fut, const N: usize> SelectAll<Fut, N> {
@@ -36,18 +32,24 @@ impl<Fut, const N: usize> SelectAll<Fut, N> {
     }
 }
 
-impl<Fut: Future + Unpin, const N: usize> Future for SelectAll<Fut, N> {
+impl<Fut: Future, const N: usize> Future for SelectAll<Fut, N> {
     type Output = (Fut::Output, usize);
 
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let item = self
-            .inner
-            .iter_mut()
-            .enumerate()
-            .find_map(|(i, f)| match f.poll_unpin(cx) {
-                Poll::Pending => None,
-                Poll::Ready(e) => Some((i, e)),
-            });
+    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,