perf(client): don't make a copy of Headers each Request

src/client/mod.rs

@@ -14,7 +14,7 @@ use http;
 use tokio::reactor::Handle;
 pub use tokio_service::Service;
 
-use header::{Headers, Host};
+use header::{Host};
 use proto;
 use proto::request;
 use method::Method;
@@ -180,12 +180,14 @@ where C: Connect,
                 ))));
             }
         };
-        let host = Host::new(domain.host().expect("authority implies host").to_owned(), domain.port());
         let (mut head, body) = request::split(req);
-        let mut headers = Headers::new();
+        if !head.headers.has::<Host>() {
-        headers.set(host);
+            let host = Host::new(
-        headers.extend(head.headers.iter());
+                domain.host().expect("authority implies host").to_owned(),
-        head.headers = headers;
+                domain.port(),
+            );
+            head.headers.set_pos(0, host);
+        }
 
         use futures::Sink;
         use futures::sync::{mpsc, oneshot};

src/header/internals/vec_map.rs

@@ -23,6 +23,11 @@ impl<K: PartialEq, V> VecMap<K, V> {
         self.vec.push((key, value));
     }
 
+    pub fn insert_pos(&mut self, key: K, value: V, pos: usize) {
+        debug_assert!(!self.contains_key(&key));
+        self.vec.insert(pos, (key, value))
+    }
+
     #[inline]
     pub fn append(&mut self, key: K, value: V) {
         self.vec.push((key, value));

src/header/mod.rs

@@ -392,6 +392,14 @@ impl Headers {
                          Item::new_typed(value));
     }
 
+    pub(crate) fn set_pos<H: Header>(&mut self, pos: usize, value: H) {
+        self.data.insert_pos(
+            HeaderName(Ascii::new(Cow::Borrowed(header_name::<H>()))),
+            Item::new_typed(value),
+            pos,
+        );
+    }
+
     /// Get a reference to the header field's value, if it exists.
     pub fn get<H: Header>(&self) -> Option<&H> {
         self.data.get(&HeaderName(Ascii::new(Cow::Borrowed(header_name::<H>()))))