don't create intermediate Vec's while serializing messages

hptp/src/msg.rs

@@ -17,7 +17,9 @@ pub struct DesError;
 
 pub trait SerDes: Sized {
     fn des(data: &[u8]) -> Result<Self, DesError>;
-    fn ser_into(self) -> Vec<u8>;
+
+    // `buf.len()` must be >= `MAX_SERIALIZED_SIZE`
+    fn ser_to(&self, buf: &mut [u8]) -> usize;
 }
 
 impl SerDes for UpMsg {
@@ -25,23 +27,27 @@ impl SerDes for UpMsg {
         Ok(UpMsg::Data(data.into()))
     }
 
-    fn ser_into(self) -> Vec<u8> {
+    fn ser_to(&self, buf: &mut [u8]) -> usize {
         match self {
-            UpMsg::Data(data) => data,
+            UpMsg::Data(data) => {
+                let len = data.len();
+                buf[..len].copy_from_slice(&data[..]);
+                len
+            }
         }
     }
 }
 
 impl SerDes for DownMsg {
     fn des(data: &[u8]) -> Result<Self, DesError> {
-        if data == [0] {
-            Ok(DownMsg::Ack)
-        } else {
-            Err(DesError)
+        match data.first() {
+            Some(0) => Ok(DownMsg::Ack),
+            _ => Err(DesError),
         }
     }
 
-    fn ser_into(self) -> Vec<u8> {
-        vec![0]
+    fn ser_to(&self, buf: &mut [u8]) -> usize {
+        buf[0] = 0;
+        1
     }
 }

hptp/src/peer.rs

@@ -55,9 +55,10 @@ impl<F, T> Peer<F, T> {
     where
         F: SerDes,
     {
-        let targ = self.targ.ok_or(SendError::NoTarget)?;
-        let data = msg.ser_into();
-        let _n_sent = self.sock.send_to(&data, targ).await?;
+        let mut buf = [0u8; msg::MAX_SERIALIZED_SIZE];
+        let len = msg.ser_to(&mut buf);
+        let who = self.targ.ok_or(SendError::NoTarget)?;
+        let _n_sent = self.sock.send_to(&buf[..len], who).await?;
         Ok(())
     }