proof of concept: it's just work with GATs

examples/demo_async.rs

@@ -1,4 +1,6 @@
-use async_trait::async_trait;
+#![feature(associated_type_defaults, generic_associated_types, type_alias_impl_trait)]
+
+use futures::Future;
 use messagebus::{derive::Message, error, AsyncHandler, Bus, Handler, Message, TypeTagged};
 use thiserror::Error;
 
@@ -37,117 +39,166 @@ struct MsgI32(i32);
 #[message(clone)]
 struct MsgI16(i16);
 
-#[async_trait]
 impl AsyncHandler<MsgF32> for TmpReceiver {
     type Error = Error;
     type Response = ();
 
-    async fn handle(&self, msg: MsgF32, bus: &Bus) -> Result<Self::Response, Self::Error> {
+    type AsyncHandleFuture<'a> = impl Future<Output = Result<Self::Response, Self::Error>> + Send + Sync + 'a;
+    type AsyncSyncFuture<'a> = impl Future<Output = Result<(), Self::Error>> + Send + Sync + 'a;
+
+    fn handle(&self, msg: MsgF32, bus: &Bus) -> Self::AsyncHandleFuture<'_> {
+        let bus = bus.clone();
+
+        async move {
             bus.send(MsgU16(1)).await?;
 
             println!("TmpReceiver ---> {:?} {}", msg, msg.type_tag());
 
             Ok(())
         }
+    }
 
-    async fn sync(&self, _bus: &Bus) -> Result<(), Self::Error> {
+    fn sync(&self, _bus: &Bus) -> Self::AsyncSyncFuture<'_> {
+        async move {
             println!("TmpReceiver f32: sync");
 
             Ok(())
         }
     }
+}
 
-#[async_trait]
 impl AsyncHandler<MsgU16> for TmpReceiver {
     type Error = Error;
     type Response = ();
 
-    async fn handle(&self, msg: MsgU16, bus: &Bus) -> Result<Self::Response, Self::Error> {
+    type AsyncHandleFuture<'a> = impl Future<Output = Result<Self::Response, Self::Error>> + Send + Sync + 'a;
+    type AsyncSyncFuture<'a> = impl Future<Output = Result<(), Self::Error>> + Send + Sync + 'a;
+
+    fn handle(&self, msg: MsgU16, bus: &Bus) -> Self::AsyncHandleFuture<'_> {
+        let bus = bus.clone();
+
+        async move {
             bus.send(MsgU32(2)).await?;
             println!("TmpReceiver ---> {:?}", msg);
 
             Ok(())
         }
+    }
 
-    async fn sync(&self, _bus: &Bus) -> Result<(), Self::Error> {
+    fn sync(&self, _bus: &Bus) -> Self::AsyncSyncFuture<'_> {
+        async move {
             println!("TmpReceiver u16: sync");
 
             Ok(())
         }
     }
+}
 
-#[async_trait]
 impl AsyncHandler<MsgU32> for TmpReceiver {
     type Error = Error;
     type Response = ();
 
-    async fn handle(&self, msg: MsgU32, bus: &Bus) -> Result<Self::Response, Self::Error> {
+    type AsyncHandleFuture<'a> = impl Future<Output = Result<Self::Response, Self::Error>> + Send + Sync + 'a;
+    type AsyncSyncFuture<'a> = impl Future<Output = Result<(), Self::Error>> + Send + Sync + 'a;
+
+    fn handle(&self, msg: MsgU32, bus: &Bus) -> Self::AsyncHandleFuture<'_> {
+        let bus = bus.clone();
+
+        async move {
             bus.send(MsgI32(3)).await?;
             println!("TmpReceiver ---> {:?}", msg);
 
             Ok(())
         }
-    async fn sync(&self, _bus: &Bus) -> Result<(), Self::Error> {
+    }
+
+    fn sync(&self, _bus: &Bus) -> Self::AsyncSyncFuture<'_> {
+        async move {
             println!("TmpReceiver u32: sync");
 
             Ok(())
         }
     }
+}
 
-#[async_trait]
 impl AsyncHandler<MsgI32> for TmpReceiver {
     type Error = Error;
     type Response = ();
 
-    async fn handle(&self, msg: MsgI32, bus: &Bus) -> Result<Self::Response, Self::Error> {
+    type AsyncHandleFuture<'a> = impl Future<Output = Result<Self::Response, Self::Error>> + Send + Sync + 'a;
+    type AsyncSyncFuture<'a> = impl Future<Output = Result<(), Self::Error>> + Send + Sync + 'a;
+
+    fn handle(&self, msg: MsgI32, bus: &Bus) -> Self::AsyncHandleFuture<'_> {
+        let bus = bus.clone();
+
+        async move {
             bus.send(MsgI16(4)).await?;
             println!("TmpReceiver ---> {:?}", msg);
 
             Ok(())
         }
+    }
 
-    async fn sync(&self, _bus: &Bus) -> Result<(), Self::Error> {
+    fn sync(&self, _bus: &Bus) -> Self::AsyncSyncFuture<'_> {
+        async move {
             println!("TmpReceiver i32: sync");
 
             Ok(())
         }
     }
+}
 
-#[async_trait]
 impl AsyncHandler<MsgI16> for TmpReceiver {
     type Error = Error;
     type Response = ();
 
-    async fn handle(&self, msg: MsgI16, _bus: &Bus) -> Result<Self::Response, Self::Error> {
+    type AsyncHandleFuture<'a> = impl Future<Output = Result<Self::Response, Self::Error>> + Send + Sync + 'a;
+    type AsyncSyncFuture<'a> = impl Future<Output = Result<(), Self::Error>> + Send + Sync + 'a;
+
+    fn handle(&self, msg: MsgI16, _bus: &Bus) -> Self::AsyncHandleFuture<'_> {
+        async move {
             println!("TmpReceiver ---> {:?}", msg);
 
             Ok(())
         }
-    async fn sync(&self, _bus: &Bus) -> Result<(), Self::Error> {
+    }
+
+    fn sync(&self, _bus: &Bus) -> Self::AsyncSyncFuture<'_> {
+        async move {
             println!("TmpReceiver i16: sync");
 
             Ok(())
         }
     }
+}
+
 
-#[async_trait]
 impl AsyncHandler<MsgI32> for TmpReceiver2 {
     type Error = Error;
     type Response = ();
 
-    async fn handle(&self, msg: MsgI32, bus: &Bus) -> Result<Self::Response, Self::Error> {
+    type AsyncHandleFuture<'a> = impl Future<Output = Result<Self::Response, Self::Error>> + Send + Sync + 'a;
+    type AsyncSyncFuture<'a> = impl Future<Output = Result<(), Self::Error>> + Send + Sync + 'a;
+
+    fn handle(&self, msg: MsgI32, bus: &Bus) -> Self::AsyncHandleFuture<'_> {
+        let bus = bus.clone();
+
+        async move {
             println!("TmpReceiver2: ---> {:?}", msg);
 
             bus.send(MsgI16(5)).await?;
 
             Ok(())
         }
-    async fn sync(&self, _bus: &Bus) -> Result<(), Self::Error> {
+    }
+    fn sync(&self, _bus: &Bus) -> Self::AsyncSyncFuture<'_> {
+        async move {
             println!("TmpReceiver2: i32: sync");
 
             Ok(())
         }
     }
+}
 
 impl Handler<MsgI16> for TmpReceiver2 {
     type Error = Error;

src/handler.rs

@@ -2,6 +2,7 @@ use core::iter::FromIterator;
 
 use crate::{error::StdSyncSendError, Bus, Message};
 use async_trait::async_trait;
+use futures::Future;
 
 pub trait Handler<M: Message>: Send + Sync {
     type Error: StdSyncSendError;
@@ -13,15 +14,15 @@ pub trait Handler<M: Message>: Send + Sync {
     }
 }
 
-#[async_trait]
 pub trait AsyncHandler<M: Message>: Send + Sync {
     type Error: StdSyncSendError;
     type Response: Message;
 
-    async fn handle(&self, msg: M, bus: &Bus) -> Result<Self::Response, Self::Error>;
+    type AsyncHandleFuture<'a>: Future<Output = Result<Self::Response, Self::Error>> + Send + Sync + 'a;
-    async fn sync(&self, _bus: &Bus) -> Result<(), Self::Error> {
+    type AsyncSyncFuture<'a>: Future<Output = Result<(), Self::Error>> + Send + Sync + 'a;
-        Ok(())
+
-    }
+    fn handle(&self, msg: M, bus: &Bus) -> Self::AsyncHandleFuture<'_>;
+    fn sync(&self, _bus: &Bus) -> Self::AsyncSyncFuture<'_>;
 }
 
 pub trait SynchronizedHandler<M: Message>: Send {

src/lib.rs

@@ -1,3 +1,5 @@
+#![feature(associated_type_defaults, generic_associated_types, type_alias_impl_trait)]
+
 mod builder;
 mod envelop;
 pub mod error;