fix: add Send and Sync bounds for Buffered (#18)

Author: loichyan

CHANGELOG.md

@@ -43,6 +43,12 @@ follows <https://www.conventionalcommits.org/en/v1.0.0/> to track changes.
 
 [#17]: https://github.com/loichyan/dynify/pull/17
 
+### Fixed
+
+- Add `Send` and `Sync` bounds for `Buffered` ([#18])
+
+[#18]: https://github.com/loichyan/dynify/pull/18
+
 ## [0.1.1] - 2025-08-28
 
 The major update since the previous release is the introduction of the

examples/async_sendable.rs

@@ -1,6 +1,7 @@
 use std::future::Future;
+use std::mem::MaybeUninit;
 
-use dynify::PinDynify;
+use dynify::Dynify;
 
 #[trait_variant::make(Send)]
 #[dynify::dynify]
@@ -9,7 +10,12 @@ trait Client {
 }
 
 async fn make_request(client: &(dyn Sync + DynClient)) {
-    client.request("http://magic/coffee/shop").pin_boxed().await;
+    let mut stack = [MaybeUninit::<u8>::uninit(); 16];
+    let mut heap = Vec::<MaybeUninit<u8>>::new();
+    client
+        .request("http://magic/coffee/shop")
+        .init2(&mut stack, &mut heap)
+        .await;
 }
 
 fn poll_future(fut: impl Send + Future<Output = ()>) {

src/container.rs

@@ -109,7 +109,7 @@ pub unsafe trait PinEmplace<T: ?Sized>: Emplace<T> {
 ///
 /// **Tips**: `Buffered<T: Future>` implements `Future`, so you can simply write
 /// `async_hello().init(&mut stack).await` in practice.
-pub struct Buffered<'a, T: ?Sized>(NonNull<T>, PhantomData<&'a mut [u8]>);
+pub struct Buffered<'a, T: ?Sized>(NonNull<T>, PhantomData<&'a mut T>);
 impl<'a, T: ?Sized> Buffered<'a, T> {
     /// Constructs a new instance with the provided pointer.
     ///
@@ -214,6 +214,11 @@ impl<'a> Buffered<'a, dyn Any + Send + Sync> {
     }
 }
 
+// SAFETY: Since we hold a exclusive reference to `T`, it's okay to inherit the
+// `Send` and `Sync` bounds.
+unsafe impl<T: ?Sized + Send> Send for Buffered<'_, T> {}
+unsafe impl<T: ?Sized + Sync> Sync for Buffered<'_, T> {}
+
 // Pretend `Buffered` owns the value of `T` rather than just a pointer to it.
 // This, along with the `Buffered::project*` APIs, makes it easy to obtain a
 // pinned reference to `T` in safe Rust. But the downside is that this prevents
@@ -275,7 +280,7 @@ impl fmt::Display for OutOfCapacity {
     }
 }
 
-unsafe impl<'a, T: ?Sized, const N: usize> Emplace<T> for &'a mut MaybeUninit<[u8; N]> {
+unsafe impl<'a, T: 'a + ?Sized, const N: usize> Emplace<T> for &'a mut MaybeUninit<[u8; N]> {
     type Ptr = Buffered<'a, T>;
     type Err = OutOfCapacity;
 
@@ -287,7 +292,7 @@ unsafe impl<'a, T: ?Sized, const N: usize> Emplace<T> for &'a mut MaybeUninit<[u
         uninit_slice.emplace(constructor)
     }
 }
-unsafe impl<'a, T: ?Sized, const N: usize> Emplace<T> for &'a mut [MaybeUninit<u8>; N] {
+unsafe impl<'a, T: 'a + ?Sized, const N: usize> Emplace<T> for &'a mut [MaybeUninit<u8>; N] {
     type Ptr = Buffered<'a, T>;
     type Err = OutOfCapacity;
 
@@ -298,7 +303,7 @@ unsafe impl<'a, T: ?Sized, const N: usize> Emplace<T> for &'a mut [MaybeUninit<u
         self.as_mut_slice().emplace(constructor)
     }
 }
-unsafe impl<'a, T: ?Sized> Emplace<T> for &'a mut [MaybeUninit<u8>] {
+unsafe impl<'a, T: 'a + ?Sized> Emplace<T> for &'a mut [MaybeUninit<u8>] {
     type Ptr = Buffered<'a, T>;
     type Err = OutOfCapacity;
 
@@ -393,7 +398,7 @@ mod __alloc {
 
     // TODO: pinned vector?
     #[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
-    unsafe impl<'a, T: ?Sized> Emplace<T> for &'a mut Vec<MaybeUninit<u8>> {
+    unsafe impl<'a, T: 'a + ?Sized> Emplace<T> for &'a mut Vec<MaybeUninit<u8>> {
         type Ptr = Buffered<'a, T>;
         type Err = Infallible;
 
@@ -446,7 +451,7 @@ mod __smallvec {
     unsafe impl<'a, A, T> Emplace<T> for &'a mut SmallVec<A>
     where
         A: Array<Item = MaybeUninit<u8>>,
-        T: ?Sized,
+        T: 'a + ?Sized,
     {
         type Ptr = Buffered<'a, T>;
         type Err = Infallible;

src/container_tests.rs

@@ -120,6 +120,24 @@ fn unpin_buffered() {
     let _: &mut Buffered<usize> = Pin::into_inner(val);
 }
 
+#[test]
+fn send_buffered() {
+    let mut stack = newstk::<16>();
+    let init = from_closure(|slot| slot.write(123));
+    let val: Buffered<usize> = init.init(&mut stack);
+    fn ensure_send(_: impl Send) {}
+    ensure_send(val);
+}
+
+#[test]
+fn sync_buffered() {
+    let mut stack = newstk::<16>();
+    let init = from_closure(|slot| slot.write(123));
+    let val: Buffered<usize> = init.init(&mut stack);
+    fn ensure_sync(_: impl Sync) {}
+    ensure_sync(val);
+}
+
 #[test]
 fn project_buffered() {
     let mut stack = newstk::<16>();

src/dynify.md

@@ -121,7 +121,8 @@ illustrated below, you can combine `#[dynify]` with
 [trait-variant](https://crates.io/crates/trait-variant) to achieve this:
 
 ```rust
-# use dynify::{PinDynify, dynify};
+# use dynify::{Dynify, dynify};
+# use std::mem::MaybeUninit;
 // You can also use `#(make(SendClient: Send))`. However, in this case, you can
 // no longer specify a name in `#[dynify]` because `#[trait_variant::make]`
 // will generate two traits, which leads to conflicting trait definitions.
@@ -133,8 +134,10 @@ trait Client {
 fn run_client(
     client: &(dyn DynClient + Sync),
 ) -> impl '_ + std::future::Future<Output = ()> + Send {
+    let mut stack = [MaybeUninit::<u8>::uninit(); 16];
+    let mut heap = Vec::<MaybeUninit<u8>>::new();
     async move {
-        client.request("http://magic/request").pin_boxed().await;
+        client.request("http://magic/request").init2(&mut stack, &mut heap).await;
     }
 }
 ```