net/url: use bytes.Buffer in (*URL).String

BenchmarkString before:

        11990 ns/op            1621 B/op         73 allocs/op

Using bytes.Buffer:

        8774 ns/op            1994 B/op         40 allocs/op

I also tried making a version of escape() that writes directly to the
bytes.Buffer, but it only saved 1 alloc/op and increased CPU time by
about 10%. Didn't seem worth the extra code path.

R=bradfitz
CC=golang-dev
https://golang.org/cl/7182050

src/pkg/net/url/url.go

@@ -434,32 +434,35 @@ func parseAuthority(authority string) (user *Userinfo, host string, err error) {
 
 // String reassembles the URL into a valid URL string.
 func (u *URL) String() string {
-	// TODO: Rewrite to use bytes.Buffer
-	result := ""
+	var buf bytes.Buffer
 	if u.Scheme != "" {
-		result += u.Scheme + ":"
+		buf.WriteString(u.Scheme)
+		buf.WriteByte(':')
 	}
 	if u.Opaque != "" {
-		result += u.Opaque
+		buf.WriteString(u.Opaque)
 	} else {
 		if u.Scheme != "" || u.Host != "" || u.User != nil {
-			result += "//"
+			buf.WriteString("//")
 			if u := u.User; u != nil {
-				result += u.String() + "@"
+				buf.WriteString(u.String())
+				buf.WriteByte('@')
 			}
 			if h := u.Host; h != "" {
-				result += u.Host
+				buf.WriteString(h)
 			}
 		}
-		result += escape(u.Path, encodePath)
+		buf.WriteString(escape(u.Path, encodePath))
 	}
 	if u.RawQuery != "" {
-		result += "?" + u.RawQuery
+		buf.WriteByte('?')
+		buf.WriteString(u.RawQuery)
 	}
 	if u.Fragment != "" {
-		result += "#" + escape(u.Fragment, encodeFragment)
+		buf.WriteByte('#')
+		buf.WriteString(escape(u.Fragment, encodeFragment))
 	}
-	return result
+	return buf.String()
 }
 
 // Values maps a string key to a list of values.

src/pkg/net/url/url_test.go

@@ -280,6 +280,30 @@ func DoTest(t *testing.T, parse func(string) (*URL, error), name string, tests [
 	}
 }
 
+func BenchmarkString(b *testing.B) {
+	b.StopTimer()
+	b.ReportAllocs()
+	for _, tt := range urltests {
+		u, err := Parse(tt.in)
+		if err != nil {
+			b.Errorf("Parse(%q) returned error %s", tt.in, err)
+			continue
+		}
+		if tt.roundtrip == "" {
+			continue
+		}
+		b.StartTimer()
+		var g string
+		for i := 0; i < b.N; i++ {
+			g = u.String()
+		}
+		b.StopTimer()
+		if w := tt.roundtrip; g != w {
+			b.Errorf("Parse(%q).String() == %q, want %q", tt.in, g, w)
+		}
+	}
+}
+
 func TestParse(t *testing.T) {
 	DoTest(t, Parse, "Parse", urltests)
 }