net/url: improve URL.String performance

URL.String performs memory allocation many times, but it can improve
performance by allocating memory that it clearly knows it needs.
This CL achieves 24.6% speedup, 18.3% memory reduction, and 46.7% fewer memory
allocations on existing benchmarks.

          │ string_old.txt │           string_new2.txt           │
          │     sec/op     │   sec/op     vs base                │
String-16      3.622µ ± 5%   2.730µ ± 2%  -24.63% (p=0.000 n=10)

          │ string_old.txt │           string_new2.txt            │
          │      B/op      │     B/op      vs base                │
String-16     1.406Ki ± 0%   1.148Ki ± 0%  -18.33% (p=0.000 n=10)

          │ string_old.txt │          string_new2.txt           │
          │   allocs/op    │ allocs/op   vs base                │
String-16       60.00 ± 0%   32.00 ± 0%  -46.67% (p=0.000 n=10)

Change-Id: I70199be952eddc44134945077e52740e8921088f
Reviewed-on: https://go-review.googlesource.com/c/go/+/581155
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Joedian Reid <joedian@google.com>

src/net/url/url.go

@@ -814,6 +814,22 @@ func validOptionalPort(port string) bool {
 //   - if u.Fragment is empty, #fragment is omitted.
 func (u *URL) String() string {
 	var buf strings.Builder
+
+	n := len(u.Scheme)
+	if u.Opaque != "" {
+		n += len(u.Opaque)
+	} else {
+		if !u.OmitHost && (u.Scheme != "" || u.Host != "" || u.User != nil) {
+			username := u.User.Username()
+			password, _ := u.User.Password()
+			n += len(username) + len(password) + len(u.Host)
+		}
+		n += len(u.Path)
+	}
+	n += len(u.RawQuery) + len(u.RawFragment)
+	n += len(":" + "//" + "//" + ":" + "@" + "/" + "./" + "?" + "#")
+	buf.Grow(n)
+
 	if u.Scheme != "" {
 		buf.WriteString(u.Scheme)
 		buf.WriteByte(':')