Use BytesMut for messages

Benchmarks indicate that malloc accounts for a significant amount of the
runtime of queries. The message buffer accounts for ~half of that (the
other being channels), and this change should eliminate it.

postgres-derive/src/tosql.rs

@@ -59,7 +59,7 @@ pub fn expand_derive_tosql(input: DeriveInput) -> Result<TokenStream, Error> {
         impl postgres_types::ToSql for #ident {
             fn to_sql(&self,
                       _type: &postgres_types::Type,
-                      buf: &mut std::vec::Vec<u8>)
+                      buf: &mut postgres_types::private::BytesMut)
                       -> std::result::Result<postgres_types::IsNull,
                                              std::boxed::Box<std::error::Error +
                                                              std::marker::Sync +

postgres-protocol/src/lib.rs

@@ -13,6 +13,7 @@
 #![warn(missing_docs, rust_2018_idioms, clippy::all)]
 
 use byteorder::{BigEndian, ByteOrder};
+use bytes::{BufMut, BytesMut};
 use std::io;
 
 pub mod authentication;
@@ -30,14 +31,37 @@ pub enum IsNull {
     No,
 }
 
-#[inline]
-fn write_nullable<F, E>(serializer: F, buf: &mut Vec<u8>) -> Result<(), E>
+// https://github.com/tokio-rs/bytes/issues/170
+struct B<'a>(&'a mut BytesMut);
+
+impl<'a> BufMut for B<'a> {
+    #[inline]
+    fn remaining_mut(&self) -> usize {
+        usize::max_value() - self.0.len()
+    }
+
+    #[inline]
+    unsafe fn advance_mut(&mut self, cnt: usize) {
+        self.0.advance_mut(cnt);
+    }
+
+    #[inline]
+    unsafe fn bytes_mut(&mut self) -> &mut [u8] {
+        if !self.0.has_remaining_mut() {
+            self.0.reserve(64);
+        }
+
+        self.0.bytes_mut()
+    }
+}
+
+fn write_nullable<F, E>(serializer: F, buf: &mut BytesMut) -> Result<(), E>
 where
-    F: FnOnce(&mut Vec<u8>) -> Result<IsNull, E>,
+    F: FnOnce(&mut BytesMut) -> Result<IsNull, E>,
     E: From<io::Error>,
 {
     let base = buf.len();
-    buf.extend_from_slice(&[0; 4]);
+    B(buf).put_i32_be(0);
     let size = match serializer(buf)? {
         IsNull::No => i32::from_usize(buf.len() - base - 4)?,
         IsNull::Yes => -1,