Fix stacked borrows violations in waker_ref and FuturesUnordered (#2550)

Author: taiki-e

.github/workflows/ci.yml

@@ -56,11 +56,11 @@ jobs:
         if: matrix.target != 'aarch64-unknown-linux-gnu'
 
   core-msrv:
-    name: cargo +${{ matrix.rust }} build (futures-{core, io, sink, task})
+    name: cargo +${{ matrix.rust }} build (futures-{core, io, sink})
     strategy:
       matrix:
         rust:
-          # This is the minimum Rust version supported by futures-core, futures-io, futures-sink, futures-task.
+          # This is the minimum Rust version supported by futures-core, futures-io, futures-sink.
           # When updating this, the reminder to update the minimum required version in README.md, Cargo.toml, and .clippy.toml.
           - 1.36
     runs-on: ubuntu-latest
@@ -76,22 +76,22 @@ jobs:
       # Check no-default-features
       - run: |
           cargo hack build --workspace --ignore-private --no-default-features \
-            --exclude futures --exclude futures-util --exclude futures-macro --exclude futures-executor --exclude futures-channel --exclude futures-test
+            --exclude futures --exclude futures-util --exclude futures-task --exclude futures-macro --exclude futures-executor --exclude futures-channel --exclude futures-test
       # Check alloc feature
       - run: |
           cargo hack build --workspace --ignore-private --no-default-features --features alloc --ignore-unknown-features \
-            --exclude futures --exclude futures-util --exclude futures-macro --exclude futures-executor --exclude futures-channel --exclude futures-test
+            --exclude futures --exclude futures-util --exclude futures-task --exclude futures-macro --exclude futures-executor --exclude futures-channel --exclude futures-test
       # Check std feature
       - run: |
           cargo hack build --workspace --ignore-private --no-default-features --features std \
-            --exclude futures --exclude futures-util --exclude futures-macro --exclude futures-executor --exclude futures-channel --exclude futures-test
+            --exclude futures --exclude futures-util --exclude futures-task --exclude futures-macro --exclude futures-executor --exclude futures-channel --exclude futures-test
 
   util-msrv:
     name: cargo +${{ matrix.rust }} build
     strategy:
       matrix:
         rust:
-          # This is the minimum Rust version supported by futures, futures-util, futures-macro, futures-executor, futures-channel, futures-test.
+          # This is the minimum Rust version supported by futures, futures-util, futures-task, futures-macro, futures-executor, futures-channel, futures-test.
           # When updating this, the reminder to update the minimum required version in README.md and Cargo.toml.
           - 1.45
     runs-on: ubuntu-latest

futures-task/src/waker_ref.rs

@@ -55,7 +55,7 @@ where
 {
     // simply copy the pointer instead of using Arc::into_raw,
     // as we don't actually keep a refcount by using ManuallyDrop.<
-    let ptr = (&**wake as *const W) as *const ();
+    let ptr = Arc::as_ptr(wake).cast::<()>();
 
     let waker =
         ManuallyDrop::new(unsafe { Waker::from_raw(RawWaker::new(ptr, waker_vtable::<W>())) });

futures-util/src/stream/futures_unordered/mod.rs

@@ -150,7 +150,7 @@ impl<Fut> FuturesUnordered<Fut> {
             queued: AtomicBool::new(true),
             ready_to_run_queue: Weak::new(),
         });
-        let stub_ptr = &*stub as *const Task<Fut>;
+        let stub_ptr = Arc::as_ptr(&stub);
         let ready_to_run_queue = Arc::new(ReadyToRunQueue {
             waker: AtomicWaker::new(),
             head: AtomicPtr::new(stub_ptr as *mut _),
@@ -403,7 +403,7 @@ impl<Fut> FuturesUnordered<Fut> {
         // The `ReadyToRunQueue` stub is never inserted into the `head_all`
         // list, and its pointer value will remain valid for the lifetime of
         // this `FuturesUnordered`, so we can make use of its value here.
-        &*self.ready_to_run_queue.stub as *const _ as *mut _
+        Arc::as_ptr(&self.ready_to_run_queue.stub) as *mut _
     }
 }
 

futures-util/src/stream/futures_unordered/ready_to_run_queue.rs

@@ -83,7 +83,7 @@ impl<Fut> ReadyToRunQueue<Fut> {
     }
 
     pub(super) fn stub(&self) -> *const Task<Fut> {
-        &*self.stub
+        Arc::as_ptr(&self.stub)
     }
 
     // Clear the queue of tasks.

futures-util/src/stream/futures_unordered/task.rs

@@ -62,7 +62,7 @@ impl<Fut> ArcWake for Task<Fut> {
         // still.
         let prev = arc_self.queued.swap(true, SeqCst);
         if !prev {
-            inner.enqueue(&**arc_self);
+            inner.enqueue(Arc::as_ptr(arc_self));
             inner.waker.wake();
         }
     }