Avoid a slice copy in send_relay_cell().

Convert slice to array in the parent function instead of copying.

tor-proto/src/circuit.rs

@@ -731,16 +731,12 @@ impl ClientCircImpl {
         // If the cell counted towards our sendme window, decrement
         // that window, and maybe remember the authentication tag.
         if c_t_w {
-            // XXXX I wish I didn't have to copy the tag.
             // TODO: I'd like to use get_hops_mut here, but the borrow checker
             // won't let me.
-            assert!(tag.len() == 20); // XXXX risky
-            let mut tag_copy = [0u8; 20];
-            (&mut tag_copy[..]).copy_from_slice(&tag[..]);
             // This blocks if the send window is empty.
             self.hops[Into::<usize>::into(hop)]
                 .sendwindow
-                .take(&tag_copy)
+                .take(&tag)
                 .await?;
         }
         self.send_msg(msg).await

tor-proto/src/crypto/cell.rs

@@ -8,6 +8,7 @@
 //!
 
 use crate::{Error, Result};
+use std::convert::TryInto;
 use tor_cell::chancell::RawCellBody;
 
 use generic_array::GenericArray;
@@ -159,7 +160,7 @@ impl OutboundClientCrypt {
     ///
     /// On success, returns a reference to tag that should be expected
     /// for an authenticated SENDME sent in response to this cell.
-    pub fn encrypt(&mut self, cell: &mut RelayCellBody, hop: HopNum) -> Result<&[u8]> {
+    pub fn encrypt(&mut self, cell: &mut RelayCellBody, hop: HopNum) -> Result<&[u8; 20]> {
         let hop: usize = hop.into();
         if hop >= self.layers.len() {
             return Err(Error::NoSuchHop);
@@ -171,7 +172,7 @@ impl OutboundClientCrypt {
         for layer in layers {
             layer.encrypt_outbound(cell);
         }
-        Ok(tag)
+        Ok(tag.try_into().expect("wrong SENDME digest size"))
     }
 
     /// Add a new layer to this OutboundClientCrypt