perf(header): allocate once in HeaderValue::from for integers

benches/src/header_value.rs

@@ -44,3 +44,8 @@ fn from_shared_unchecked_long(b: &mut Bencher) {
         HeaderValue::from_maybe_shared_unchecked(bytes.clone());
     });
 }
+
+#[bench]
+fn from_u64(b: &mut Bencher) {
+    b.iter(|| HeaderValue::from(test::black_box(1024u64)));
+}

src/header/value.rs

@@ -1,8 +1,7 @@
-use bytes::{Bytes, BytesMut};
+use bytes::Bytes;
 
 use std::convert::TryFrom;
 use std::error::Error;
-use std::fmt::Write;
 use std::hash::{Hash, Hasher};
 use std::str::FromStr;
 use std::{cmp, fmt, str};
@@ -405,13 +404,11 @@ impl From<HeaderName> for HeaderValue {
 }
 
 macro_rules! from_integers {
-    ($($name:ident: $t:ident => $max_len:expr),*) => {$(
+    ($($name:ident: $t:ident),*) => {$(
         impl From<$t> for HeaderValue {
             fn from(num: $t) -> HeaderValue {
-                let mut buf = BytesMut::with_capacity($max_len);
-                let _ = buf.write_str(::itoa::Buffer::new().format(num));
                 HeaderValue {
-                    inner: buf.freeze(),
+                    inner: Bytes::copy_from_slice(::itoa::Buffer::new().format(num).as_bytes()),
                     is_sensitive: false,
                 }
             }
@@ -431,33 +428,15 @@ macro_rules! from_integers {
 }
 
 from_integers! {
-    // integer type => maximum decimal length
-
     // u8 purposely left off... HeaderValue::from(b'3') could be confusing
-    from_u16: u16 => 5,
-    from_i16: i16 => 6,
-    from_u32: u32 => 10,
-    from_i32: i32 => 11,
-    from_u64: u64 => 20,
-    from_i64: i64 => 20
-}
-
-#[cfg(target_pointer_width = "16")]
-from_integers! {
-    from_usize: usize => 5,
-    from_isize: isize => 6
-}
-
-#[cfg(target_pointer_width = "32")]
-from_integers! {
-    from_usize: usize => 10,
-    from_isize: isize => 11
-}
-
-#[cfg(target_pointer_width = "64")]
-from_integers! {
-    from_usize: usize => 20,
-    from_isize: isize => 20
+    from_u16: u16,
+    from_i16: i16,
+    from_u32: u32,
+    from_i32: i32,
+    from_u64: u64,
+    from_i64: i64,
+    from_usize: usize,
+    from_isize: isize
 }
 
 #[cfg(test)]