diff --git a/src/runtime/src/rpc.rs b/src/runtime/src/rpc.rs
index ff7aa86..6440cee 100644
--- a/src/runtime/src/rpc.rs
+++ b/src/runtime/src/rpc.rs
@@ -1,4 +1,5 @@
 use core::str;
+use core::future::Future;
 use cslice::{CSlice, CMutSlice};
 use log::debug;
 
@@ -13,9 +14,10 @@ use crate::proto_async;
 use self::tag::{Tag, TagIterator, split_tag};
 
 #[async_recursion(?Send)]
-async unsafe fn recv_value(stream: &TcpStream, tag: Tag<'async_recursion>, data: &mut *mut (),
-                           alloc: &dyn Fn(usize) -> *mut ())
-                          -> Result<(), smoltcp::Error>
+async unsafe fn recv_value<F>(stream: &TcpStream, tag: Tag<'async_recursion>, data: &mut *mut (),
+                              alloc: &(impl Fn(usize) -> F + 'async_recursion))
+                             -> Result<(), smoltcp::Error>
+                          where F: Future<Output=*mut ()>
 {
     macro_rules! consume_value {
         ($ty:ty, |$ptr:ident| $map:expr) => ({
@@ -45,7 +47,7 @@ async unsafe fn recv_value(stream: &TcpStream, tag: Tag<'async_recursion>, data:
         Tag::String | Tag::Bytes | Tag::ByteArray => {
             consume_value!(CMutSlice<u8>, |ptr| {
                 let length = proto_async::read_i32(stream).await? as usize;
-                *ptr = CMutSlice::new(alloc(length) as *mut u8, length);
+                *ptr = CMutSlice::new(alloc(length).await as *mut u8, length);
                 proto_async::read_chunk(stream, (*ptr).as_mut()).await?;
                 Ok(())
             })
@@ -64,7 +66,7 @@ async unsafe fn recv_value(stream: &TcpStream, tag: Tag<'async_recursion>, data:
                 (*ptr).length = proto_async::read_i32(stream).await? as u32;
 
                 let tag = it.clone().next().expect("truncated tag");
-                (*ptr).elements = alloc(tag.size() * (*ptr).length as usize);
+                (*ptr).elements = alloc(tag.size() * (*ptr).length as usize).await;
 
                 let mut data = (*ptr).elements;
                 for _ in 0..(*ptr).length as usize {
@@ -85,9 +87,10 @@ async unsafe fn recv_value(stream: &TcpStream, tag: Tag<'async_recursion>, data:
     }
 }
 
-pub async fn recv_return(stream: &TcpStream, tag_bytes: &[u8], data: *mut (),
-                         alloc: &dyn Fn(usize) -> *mut ())
-                        -> Result<(), smoltcp::Error>
+pub async fn recv_return<F>(stream: &TcpStream, tag_bytes: &[u8], data: *mut (),
+                            alloc: &impl Fn(usize) -> F)
+                           -> Result<(), smoltcp::Error>
+                        where F: Future<Output=*mut ()>
 {
     let mut it = TagIterator::new(tag_bytes);
     debug!("recv ...->{}", it);