text/template/parse: use strings.Builder in String methods

As mentioned in godoc, strings.Builder is more efficient for
concatenating and building strings.
Running a simple bench test on VariableNode.String() gives:

benchmark                     old ns/op     new ns/op     delta
BenchmarkParseLarge-8         25676831      24453285      -4.77%
BenchmarkVariableString-8     296           115           -61.15%

benchmark                     old allocs     new allocs     delta
BenchmarkVariableString-8     8              3              -62.50%

benchmark                     old bytes     new bytes     delta
BenchmarkVariableString-8     112           72            -35.71%

Change-Id: I13c9340080738fcad1edeed859d33ba608e4b05a
Reviewed-on: https://go-review.googlesource.com/c/go/+/198078
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/src/text/template/parse/parse_test.go b/src/text/template/parse/parse_test.go
index 6932cf232e..371de5d67c 100644
--- a/src/text/template/parse/parse_test.go
+++ b/src/text/template/parse/parse_test.go
@@ -553,3 +553,19 @@ func BenchmarkParseLarge(b *testing.B) {
 		}
 	}
 }
+
+var sink string
+
+func BenchmarkVariableString(b *testing.B) {
+	v := &VariableNode{
+		Ident: []string{"$", "A", "BB", "CCC", "THIS_IS_THE_VARIABLE_BEING_PROCESSED"},
+	}
+	b.ResetTimer()
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		sink = v.String()
+	}
+	if sink == "" {
+		b.Fatal("Benchmark was not run")
+	}
+}