text/template: replace bytes.Buffer with strings.Builder

After Go.1.10+ strings.Builder is known as more efficient in
concatenating and building strings than bytes.Buffer.

In this CL,
there is a minor logic fix for getting advantage of strings.builder.

name               old time/op    new time/op    delta
DefinedTemplate-8     543ns ± 3%     512ns ± 2%   -5.73%  (p=0.000 n=8+8)

name               old alloc/op   new alloc/op   delta
DefinedTemplate-8      192B ± 0%      160B ± 0%  -16.67%  (p=0.000 n=8+8)

name               old allocs/op  new allocs/op  delta
DefinedTemplate-8      5.00 ± 0%      5.00 ± 0%     ~     (all equal)

Change-Id: Icda0054d146e6c5e32ed8a4d13221bb6850d31b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/175261
Run-TryBot: Emmanuel Odeke <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

src/text/template/exec.go

@@ -5,7 +5,6 @@
 package template
 
 import (
- "bytes"
  "fmt"
  "internal/fmtsort"
  "io"
@@ -230,21 +229,19 @@ func (t *Template) DefinedTemplates() string {
  if t.common == nil {
  return ""
  }
- var b bytes.Buffer
+ var b strings.Builder
  for name, tmpl := range t.tmpl {
  if tmpl.Tree == nil || tmpl.Root == nil {
  continue
  }
- if b.Len() > 0 {
+ if b.Len() == 0 {
+ b.WriteString("; defined templates are: ")
+ } else {
  b.WriteString(", ")
  }
  fmt.Fprintf(&b, "%q", name)
  }
- var s string
- if b.Len() > 0 {
- s = "; defined templates are: " + b.String()
- }
- return s
+ return b.String()
 }
 
 // Walk functions step through the major pieces of the template structure,