todo: separate workerthread and scope logic, add scope type with lifetime

Cargo.toml

@@ -15,6 +15,9 @@ never-local = []
 [profile.bench]
 debug = true
 
+[profile.release]
+debug = true
+
 [dependencies]
 
 futures = "0.3"
@@ -39,4 +42,5 @@ parking_lot_core = "0.9.10"
 # derive_more = "1.0.0"
 
 [dev-dependencies]
+async-std = "1.13.0"
 tracing-test = "0.2.5"

src/lib.rs

@@ -4,6 +4,7 @@
     debug_closure_helpers,
     cell_update,
     cold_path,
+    fn_align,
     let_chains
 )]
 

src/praetor/mod.rs

@@ -3,7 +3,7 @@ mod util {
         cell::UnsafeCell,
         marker::PhantomData,
         mem::ManuallyDrop,
-        num::NonZero,
+        ops::{Deref, DerefMut},
         ptr::NonNull,
         sync::atomic::{AtomicPtr, Ordering},
     };
@@ -30,6 +30,56 @@ mod util {
         }
     }
 
+    #[repr(transparent)]
+    #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
+    pub struct SendPtr<T>(NonNull<T>);
+
+    impl<T> std::fmt::Pointer for SendPtr<T> {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            <NonNull<T> as core::fmt::Pointer>::fmt(&self.0, f)
+        }
+    }
+
+    unsafe impl<T> Send for SendPtr<T> {}
+
+    impl<T> Deref for SendPtr<T> {
+        type Target = NonNull<T>;
+
+        fn deref(&self) -> &Self::Target {
+            &self.0
+        }
+    }
+
+    impl<T> DerefMut for SendPtr<T> {
+        fn deref_mut(&mut self) -> &mut Self::Target {
+            &mut self.0
+        }
+    }
+
+    impl<T> SendPtr<T> {
+        pub const fn new(ptr: *mut T) -> Option<Self> {
+            match NonNull::new(ptr) {
+                Some(ptr) => Some(Self(ptr)),
+                None => None,
+            }
+        }
+        pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
+            unsafe { Self(NonNull::new_unchecked(ptr)) }
+        }
+
+        pub const fn new_const(ptr: *const T) -> Option<Self> {
+            Self::new(ptr.cast_mut())
+        }
+
+        pub const unsafe fn new_const_unchecked(ptr: *const T) -> Self {
+            Self::new_unchecked(ptr.cast_mut())
+        }
+    }
+
+    // Miri doesn't like tagging pointers that it doesn't know the alignment of.
+    // This includes function pointers, which aren't guaranteed to be aligned to
+    // anything, but generally have an alignment of 8, and can be specified to
+    // be aligned to `n` with `#[repr(align(n))]`.
     #[repr(transparent)]
     pub struct TaggedAtomicPtr<T, const BITS: usize>(AtomicPtr<()>, PhantomData<T>);
 
@@ -49,8 +99,12 @@ mod util {
 
         pub fn ptr(&self, order: Ordering) -> NonNull<T> {
             unsafe {
-                NonNull::new_unchecked(self.0.load(order) as _)
+                NonNull::new_unchecked(
-                    .map_addr(|addr| NonZero::new_unchecked(addr.get() & !Self::mask()))
+                    self.0
+                        .load(order)
+                        .map_addr(|addr| addr & !Self::mask())
+                        .cast(),
+                )
             }
         }
 
@@ -77,8 +131,8 @@ mod util {
             let mask = Self::mask();
             let old_ptr = self.0.load(failure);
 
-            let old = old_ptr.with_addr((old_ptr.addr() & !mask) | (old & mask));
+            let old = old_ptr.map_addr(|addr| (addr & !mask) | (old & mask));
-            let new = old_ptr.with_addr((old_ptr.addr() & !mask) | (new & mask));
+            let new = old_ptr.map_addr(|addr| (addr & !mask) | (new & mask));
 
             let result = cmpxchg(&self.0, old, new, success, failure);
 
@@ -130,7 +184,7 @@ mod util {
             let ptr = ptr.cast::<()>();
             loop {
                 let old = self.0.load(failure);
-                let new = ptr.with_addr((ptr.addr() & !mask) | (old.addr() & mask));
+                let new = ptr.map_addr(|addr| (addr & !mask) | (old.addr() & mask));
                 if self
                     .0
                     .compare_exchange_weak(old, new, success, failure)
@@ -145,7 +199,8 @@ mod util {
             let mask = Self::mask();
             loop {
                 let ptr = self.0.load(failure);
-                let new = ptr.with_addr((ptr.addr() & !mask) | (tag & mask));
+                let new = ptr.map_addr(|addr| (addr & !mask) | (tag & mask));
+
                 if self
                     .0
                     .compare_exchange_weak(ptr, new, success, failure)
@@ -160,8 +215,8 @@ mod util {
             let mask = Self::mask();
             let ptr = self.0.load(order);
             let tag = ptr.addr() & mask;
-            let addr = ptr.addr() & !mask;
+            let ptr = ptr.map_addr(|addr| addr & !mask);
-            let ptr = unsafe { NonNull::new_unchecked(ptr.with_addr(addr).cast()) };
+            let ptr = unsafe { NonNull::new_unchecked(ptr.cast()) };
             (ptr, tag)
         }
     }
@@ -188,6 +243,7 @@ mod job {
     use super::util::TaggedAtomicPtr;
 
     #[derive(Debug)]
+    #[repr(transparent)]
     pub struct SmallBox<T>(pub MaybeUninit<Box<T>>);
 
     impl<T: Display> Display for SmallBox<T> {
@@ -557,10 +613,7 @@ mod job {
     unsafe impl<T> Send for Job<T> {}
 
     impl<T> Job<T> {
-        pub fn new(
+        pub fn new(harness: unsafe fn(*const (), *const Job<T>), this: NonNull<()>) -> Job<T> {
-            harness: unsafe fn(*const (), *const Job<T>, &super::Scope),
-            this: NonNull<()>,
-        ) -> Job<T> {
             Self {
                 harness_and_state: TaggedAtomicPtr::new(
                     unsafe { mem::transmute(harness) },
@@ -702,17 +755,18 @@ mod job {
             }
         }
 
-        pub fn execute(&self, scope: &super::Scope) {
+        pub fn execute(job: NonNull<Self>) {
             // SAFETY: self is non-null
             unsafe {
-                let (ptr, state) = self.harness_and_state.ptr_and_tag(Ordering::Relaxed);
+                let this = job.as_ref();
+                let (ptr, state) = this.harness_and_state.ptr_and_tag(Ordering::Relaxed);
+
                 debug_assert_eq!(state, JobState::Pending as usize);
+                let harness: unsafe fn(*const (), *const Self) = mem::transmute(ptr.as_ptr());
 
-                let harness: unsafe fn(*const (), *const Self, scope: &super::Scope) =
+                let this = (*this.val_or_this.get()).this;
-                    mem::transmute(ptr.as_ptr());
-                let this = (*self.val_or_this.get()).this;
 
-                harness(this.as_ptr().cast(), (self as *const Self).cast(), scope);
+                harness(this.as_ptr().cast(), job.as_ptr());
             }
         }
 
@@ -764,34 +818,41 @@ mod job {
     #[allow(dead_code)]
     pub struct HeapJob<F> {
         f: F,
+        _phantom: PhantomPinned,
     }
 
     impl<F> HeapJob<F> {
         #[allow(dead_code)]
         pub fn new(f: F) -> Box<Self> {
-            Box::new(Self { f })
+            Box::new(Self {
+                f,
+                _phantom: PhantomPinned,
+            })
+        }
+        pub fn into_inner(self) -> F {
+            self.f
         }
         #[allow(dead_code)]
-        pub fn into_boxed_job<T>(self: Box<Self>) -> Box<Job<()>>
+        pub fn into_boxed_job<T>(self: Box<Self>) -> Pin<Box<Job<()>>>
         where
-            F: FnOnce(&super::Scope) -> T + Send,
+            F: FnOnce() -> T + Send,
             T: Send,
         {
-            unsafe fn harness<F, T>(this: *const (), job: *const Job<()>, scope: &super::Scope)
+            #[repr(align(8))]
+            unsafe fn harness<F, T>(this: *const (), job: *const Job<()>)
             where
-                F: FnOnce(&super::Scope) -> T + Send,
+                F: FnOnce() -> T + Send,
                 T: Sized + Send,
             {
                 let this = unsafe { Box::from_raw(this.cast::<HeapJob<F>>().cast_mut()) };
                 let f = this.f;
 
-                let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| f(scope)));
+                _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| f()));
 
-                let job = unsafe { &*job.cast::<Job<T>>() };
+                _ = unsafe { Box::from_raw(job.cast_mut()) };
-                job.complete(result);
             }
 
-            Box::new(Job::new(harness::<F, T>, unsafe {
+            Box::pin(Job::new(harness::<F, T>, unsafe {
                 NonNull::new_unchecked(Box::into_raw(self)).cast()
             }))
         }
@@ -816,18 +877,19 @@ mod job {
 
         pub fn as_job<T>(self: Pin<&Self>) -> Job<()>
         where
-            F: FnOnce(&super::Scope) -> T + Send,
+            F: FnOnce() -> T + Send,
             T: Send,
         {
-            unsafe fn harness<F, T>(this: *const (), job: *const Job<()>, scope: &super::Scope)
+            #[repr(align(8))]
+            unsafe fn harness<F, T>(this: *const (), job: *const Job<()>)
             where
-                F: FnOnce(&super::Scope) -> T + Send,
+                F: FnOnce() -> T + Send,
                 T: Sized + Send,
             {
                 let this = unsafe { &*this.cast::<StackJob<F>>() };
                 let f = unsafe { this.unwrap() };
 
-                let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| f(scope)));
+                let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| f()));
 
                 let job_ref = unsafe { &*job.cast::<Job<T>>() };
                 job_ref.complete(result);
@@ -843,34 +905,77 @@ mod job {
 use std::{
     cell::{Cell, UnsafeCell},
     collections::BTreeMap,
-    mem,
+    future::Future,
+    marker::PhantomData,
+    mem::{self, MaybeUninit},
     pin::{pin, Pin},
     ptr::NonNull,
     sync::{
-        atomic::{AtomicBool, Ordering},
+        atomic::{AtomicBool, AtomicUsize, Ordering},
         Arc, OnceLock, Weak,
     },
     time::Duration,
 };
 
+use async_task::Runnable;
 use crossbeam::utils::CachePadded;
 use job::*;
 use parking_lot::{Condvar, Mutex};
-use util::DropGuard;
+use util::{DropGuard, SendPtr};
 
-pub struct Scope {
+#[derive(Debug, Default)]
-    join_count: Cell<usize>,
+pub struct JobCounter {
+    jobs_pending: AtomicUsize,
+    waker: Mutex<Option<std::thread::Thread>>,
+}
+
+impl JobCounter {
+    pub fn increment(&self) {
+        self.jobs_pending.fetch_add(1, Ordering::Relaxed);
+    }
+
+    pub fn decrement(&self) {
+        if self.jobs_pending.fetch_sub(1, Ordering::SeqCst) == 1 {
+            if let Some(thread) = self.waker.lock().take() {
+                thread.unpark();
+            }
+        }
+    }
+
+    /// must only be called once
+    pub unsafe fn wait(&self) {
+        _ = self.waker.lock().insert(std::thread::current());
+
+        let count = self.jobs_pending.load(Ordering::SeqCst);
+        if count > 0 {
+            std::thread::park();
+        }
+    }
+}
+
+struct WorkerThread {
     context: Arc<Context>,
     index: usize,
     heartbeat: Arc<CachePadded<AtomicBool>>,
     queue: UnsafeCell<JobList>,
 }
 
-thread_local! {
+pub struct Scope<'scope> {
-    static SCOPE: UnsafeCell<Option<NonNull<Scope>>> = const { UnsafeCell::new(None) };
+    join_count: Cell<usize>,
+    context: Arc<Context>,
+    index: usize,
+    heartbeat: Arc<CachePadded<AtomicBool>>,
+    queue: UnsafeCell<JobList>,
+
+    job_counter: JobCounter,
+    _pd: PhantomData<&'scope ()>,
 }
 
-impl Scope {
+thread_local! {
+    static SCOPE: UnsafeCell<Option<NonNull<Scope<'static>>>> = const { UnsafeCell::new(None) };
+}
+
+impl<'scope> Scope<'scope> {
     /// locks shared context
     #[allow(dead_code)]
     fn new() -> Self {
@@ -888,20 +993,33 @@ impl Scope {
             heartbeat,
             join_count: Cell::new(0),
             queue: UnsafeCell::new(JobList::new()),
+            job_counter: JobCounter::default(),
+            _pd: PhantomData,
+        }
+    }
+
+    unsafe fn drop_in_place_and_dealloc(this: NonNull<Scope>) {
+        unsafe {
+            let ptr = this.as_ptr();
+            ptr.drop_in_place();
+
+            _ = Box::<MaybeUninit<Self>>::from_raw(ptr.cast());
         }
     }
 
     fn with_in<T, F: FnOnce(&Scope) -> T>(ctx: &Arc<Context>, f: F) -> T {
-        let mut guard = Option::<DropGuard<Box<dyn FnOnce()>>>::None;
+        let mut _guard = Option::<DropGuard<Box<dyn FnOnce()>>>::None;
 
         let scope = match Self::current_ref() {
             Some(scope) if Arc::ptr_eq(&scope.context, ctx) => scope,
             Some(_) => {
                 let old = unsafe { Self::unset_current().unwrap().as_ptr() };
-                guard = Some(DropGuard::new(Box::new(move || unsafe {
+                _guard = Some(DropGuard::new(Box::new(move || unsafe {
-                    _ = Box::from_raw(Self::unset_current().unwrap().as_ptr());
+                    Self::drop_in_place_and_dealloc(Self::unset_current().unwrap());
+
                     Self::set_current(old.cast_const());
                 })));
+
                 let current = Box::into_raw(Box::new(Self::new_in(ctx.clone())));
                 unsafe {
                     Self::set_current(current.cast_const());
@@ -911,8 +1029,8 @@ impl Scope {
             None => {
                 let current = Box::into_raw(Box::new(Self::new_in(ctx.clone())));
 
-                guard = Some(DropGuard::new(Box::new(|| unsafe {
+                _guard = Some(DropGuard::new(Box::new(|| unsafe {
-                    _ = Box::from_raw(Self::unset_current().unwrap().as_ptr());
+                    Self::drop_in_place_and_dealloc(Self::unset_current().unwrap());
                 })));
 
                 unsafe {
@@ -924,7 +1042,6 @@ impl Scope {
         };
 
         let t = f(scope);
-        drop(guard);
         t
     }
 
@@ -932,22 +1049,22 @@ impl Scope {
         Self::with_in(Context::global(), f)
     }
 
-    unsafe fn set_current(scope: *const Scope) {
+    unsafe fn set_current(scope: *const Scope<'static>) {
         SCOPE.with(|ptr| unsafe {
             _ = (&mut *ptr.get()).insert(NonNull::new_unchecked(scope.cast_mut()));
         })
     }
 
-    unsafe fn unset_current() -> Option<NonNull<Scope>> {
+    unsafe fn unset_current() -> Option<NonNull<Scope<'static>>> {
         SCOPE.with(|ptr| unsafe { (&mut *ptr.get()).take() })
     }
 
     #[allow(dead_code)]
-    fn current() -> Option<NonNull<Scope>> {
+    fn current() -> Option<NonNull<Scope<'scope>>> {
         SCOPE.with(|ptr| unsafe { *ptr.get() })
     }
 
-    fn current_ref<'a>() -> Option<&'a Scope> {
+    fn current_ref<'a>() -> Option<&'a Scope<'scope>> {
         SCOPE.with(|ptr| unsafe { (&*ptr.get()).map(|ptr| ptr.as_ref()) })
     }
 
@@ -970,6 +1087,91 @@ impl Scope {
         unsafe { self.queue.as_mut_unchecked().pop_front() }
     }
 
+    fn complete_jobs(&self) {
+        while let Some(job) = self.pop_front() {
+            unsafe {
+                job.as_ref().set_pending();
+            }
+            self.execute(job);
+        }
+
+        unsafe {
+            self.job_counter.wait();
+        }
+    }
+
+    pub fn spawn<F>(&self, f: F)
+    where
+        F: FnOnce(&Scope<'scope>) + Send + 'scope,
+    {
+        self.job_counter.increment();
+
+        let this = SendPtr::new_const(self).unwrap();
+
+        let job = HeapJob::new(move || unsafe {
+            f(this.as_ref());
+            this.as_ref().job_counter.decrement();
+        })
+        .into_boxed_job();
+
+        self.push_front(Pin::as_ref(&job));
+        mem::forget(job);
+    }
+
+    pub fn spawn_future<T, F>(&self, future: F) -> async_task::Task<T>
+    where
+        F: Future<Output = T> + Send + 'scope,
+        T: Send + 'scope,
+    {
+        self.job_counter.increment();
+
+        let this = SendPtr::new_const(&self.job_counter).unwrap();
+
+        let future = async move {
+            let _guard = DropGuard::new(move || unsafe {
+                this.as_ref().decrement();
+            });
+            future.await
+        };
+
+        let this = SendPtr::new_const(self).unwrap();
+        let schedule = move |runnable: Runnable| {
+            #[repr(align(8))]
+            unsafe fn harness<T>(this: *const (), job: *const Job<T>) {
+                let runnable = Runnable::<()>::from_raw(NonNull::new_unchecked(this.cast_mut()));
+                runnable.run();
+
+                drop(Box::from_raw(job.cast_mut()));
+            }
+
+            let job = Box::pin(Job::<T>::new(harness::<T>, runnable.into_raw()));
+
+            unsafe {
+                this.as_ref().push_front(job.as_ref());
+            }
+            mem::forget(job);
+        };
+
+        let (runnable, task) = unsafe { async_task::spawn_unchecked(future, schedule) };
+
+        runnable.schedule();
+
+        task
+    }
+
+    #[allow(dead_code)]
+    fn spawn_async<'a, T, Fut, Fn>(&'a self, f: Fn) -> async_task::Task<T>
+    where
+        Fn: FnOnce(&Scope) -> Fut + Send + 'static,
+        Fut: Future<Output = T> + Send + 'static,
+        T: Send + 'static,
+    {
+        let this = SendPtr::new_const(self).unwrap();
+        let future = async move { f(unsafe { this.as_ref() }).await };
+
+        self.spawn_future(future)
+    }
+
     #[inline]
     pub fn join<A, B, RA, RB>(&self, a: A, b: B) -> (RA, RB)
     where
@@ -1020,7 +1222,9 @@ impl Scope {
         A: FnOnce(&Self) -> RA + Send,
         B: FnOnce(&Self) -> RB + Send,
     {
-        let a = pin!(StackJob::new(move |scope: &Scope| {
+        let this = SendPtr::new_const(self).unwrap();
+        let a = pin!(StackJob::new(move || unsafe {
+            let scope = this.as_ref();
             scope.tick();
 
             a(scope)
@@ -1036,14 +1240,14 @@ impl Scope {
                 job.unlink();
             }
 
-            unsafe { a.unwrap()(self) }
+            unsafe { a.unwrap()() }
         } else {
             match self.wait_until::<RA>(unsafe {
                 mem::transmute::<Pin<&Job<()>>, Pin<&Job<RA>>>(job.as_ref())
             }) {
                 Some(Ok(t)) => t,
                 Some(Err(payload)) => std::panic::resume_unwind(payload),
-                None => unsafe { a.unwrap()(self) },
+                None => unsafe { a.unwrap()() },
             }
         };
 
@@ -1059,9 +1263,9 @@ impl Scope {
     }
 
     #[inline]
-    fn execute(&self, job: &Job) {
+    fn execute(&self, job: NonNull<Job>) {
         self.tick();
-        job.execute(self);
+        Job::execute(job);
     }
 
     #[cold]
@@ -1088,9 +1292,7 @@ impl Scope {
             if ptr.as_ptr() == &*job as *const _ as *mut _ {
                 return None;
             } else {
-                unsafe {
+                self.execute(ptr);
-                    self.execute(ptr.as_ref());
-                }
             }
         }
 
@@ -1111,9 +1313,7 @@ impl Scope {
                 break;
             };
 
-            unsafe {
+            self.execute(job);
-                self.execute(job.as_ref());
-            }
         }
         // while job isn't done, run other jobs.
         Some(job.wait())
@@ -1232,8 +1432,10 @@ fn worker(ctx: Arc<Context>, barrier: Arc<std::sync::Barrier>) {
     unsafe {
         Scope::set_current(Box::into_raw(Box::new(Scope::new_in(ctx.clone()))).cast_const());
     }
-    let _guard =
+
-        DropGuard::new(|| unsafe { drop(Box::from_raw(Scope::unset_current().unwrap().as_ptr())) });
+    let _guard = DropGuard::new(|| unsafe {
+        Scope::drop_in_place_and_dealloc(Scope::unset_current().unwrap());
+    });
 
     let scope = Scope::current_ref().unwrap();
 
@@ -1242,9 +1444,7 @@ fn worker(ctx: Arc<Context>, barrier: Arc<std::sync::Barrier>) {
     let mut job = ctx.shared.lock().jobs.pop_first();
     loop {
         if let Some((_, job)) = job {
-            unsafe {
+            scope.execute(job);
-                scope.execute(job.as_ref());
-            }
         }
 
         let mut guard = ctx.shared.lock();

src/praetor/tests.rs

@@ -1,4 +1,4 @@
-use std::pin::Pin;
+use std::{mem::MaybeUninit, pin::Pin};
 
 use super::{util::TaggedAtomicPtr, *};
 
@@ -315,3 +315,134 @@ fn value_boxed_drop() {
     }
     assert_eq!(dropped, 2);
 }
+
+#[test]
+fn job_list_pop_front_empty() {
+    let mut list = JobList::new();
+
+    assert_eq!(list.pop_front(), None);
+    assert_eq!(list.pop_front(), None);
+    assert_eq!(list.pop_front(), None);
+    assert_eq!(list.pop_front(), None);
+}
+
+#[test]
+fn job_list_pop_back_empty() {
+    let mut list = JobList::new();
+
+    assert_eq!(list.pop_back(), None);
+    assert_eq!(list.pop_back(), None);
+    assert_eq!(list.pop_back(), None);
+    assert_eq!(list.pop_back(), None);
+}
+
+#[test]
+fn job_list_pop_back_emptied() {
+    let mut list = JobList::new();
+    let a = pin!(Job::<()>::empty());
+
+    unsafe {
+        list.push_front(a.as_ref());
+    }
+
+    assert_eq!(list.pop_back(), Some(pin_ptr(&a)));
+    assert_eq!(list.pop_back(), None);
+    assert_eq!(list.pop_back(), None);
+    assert_eq!(list.pop_back(), None);
+}
+
+#[test]
+fn job_list_pop_front_emptied() {
+    let mut list = JobList::new();
+    let a = pin!(Job::<()>::empty());
+
+    unsafe {
+        list.push_front(a.as_ref());
+    }
+
+    assert_eq!(list.pop_front(), Some(pin_ptr(&a)));
+    assert_eq!(list.pop_front(), None);
+    assert_eq!(list.pop_front(), None);
+    assert_eq!(list.pop_front(), None);
+}
+
+#[test]
+fn spawn() {
+    let pool = ThreadPool::new();
+
+    let mut x = 0;
+    pool.scope(|s| {
+        s.spawn(|_| {
+            x += 1;
+        });
+    });
+
+    eprintln!("x: {x}");
+}
+
+#[test]
+fn spawn_borrow() {
+    let pool = ThreadPool::new();
+
+    pool.scope(|s| {
+        let mut x = 0;
+        s.spawn(|_| {
+            x += 1;
+            assert_eq!(x, 1);
+        });
+    });
+}
+
+#[test]
+fn spawn_future() {
+    let pool = ThreadPool::new();
+
+    let task = pool.scope(|s| {
+        let task = s.spawn_future(async {
+            eprintln!("executing future");
+            3 + 1
+        });
+
+        // let task = s.spawn_future(async {
+        //     eprintln!("task: {}", task.await);
+        // });
+        task
+    });
+    let x = async_std::task::block_on(task);
+    eprintln!("x: {x}");
+}
+
+#[test]
+fn join() {
+    let pool = ThreadPool::new();
+
+    let x = pool.scope(|s| {
+        let (a, b) = s.join(|_| 3, |_| 4);
+
+        a + b
+    });
+
+    eprintln!("x: {x}");
+}
+
+#[test]
+fn rebox() {
+    struct A(u32);
+
+    impl Drop for A {
+        fn drop(&mut self) {
+            eprintln!("drop");
+        }
+    }
+
+    let ptr = Box::into_raw(Box::new(A(5)));
+
+    let x = unsafe { &*ptr.offset(mem::offset_of!(A, 0) as isize).cast::<u32>() };
+    let y = *x;
+
+    unsafe {
+        ptr.drop_in_place();
+        _ = Box::<MaybeUninit<A>>::from_raw(ptr.cast());
+    }
+    assert_eq!(y, 5);
+}