strings: add Builder

This is like a write-only subset of bytes.Buffer with an
allocation-free String method.

Fixes #18990.

Change-Id: Icdf7240f4309a52924dc3af04a39ecd737a210f4
Reviewed-on: https://go-review.googlesource.com/74931
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/strings/example_test.go b/src/strings/example_test.go
index f7a78b4385..607e4a0a70 100644
--- a/src/strings/example_test.go
+++ b/src/strings/example_test.go
@@ -351,3 +351,14 @@ func ExampleTrimRightFunc() {
 	}))
 	// Output: ¡¡¡Hello, Gophers
 }
+
+func ExampleBuilder() {
+	var b strings.Builder
+	for i := 3; i >= 1; i-- {
+		fmt.Fprintf(&b, "%d...", i)
+	}
+	b.WriteString("ignition")
+	fmt.Println(b.String())
+
+	// Output: 3...2...1...ignition
+}