flag: use strings.Builder instead of concatenating strings

There is a single function in the flag package whose implementation
uses string concatenation instead of the recommended strings.Builder.
The function was last touched before strings.Builder was introduced
in Go 1.10, which explains the old style code. This PR updates
the implementation.

Fixes #45392

Change-Id: Id2d8f1788765a0c4faaeb1e6870914f72b3c8442
GitHub-Last-Rev: 0e12fe304593afc627fc4f1597670efd354809b0
GitHub-Pull-Request: golang/go#45393
Reviewed-on: https://go-review.googlesource.com/c/go/+/307329
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>

diff --git a/src/flag/flag.go b/src/flag/flag.go
index f7598a6758f2237d007047e799214708136583cd..885a4c8369663f99b9b492bb65d4e768b66d51f6 100644 (file)
--- a/src/flag/flag.go
+++ b/src/flag/flag.go
@@ -508,31 +508,33 @@ func UnquoteUsage(flag *Flag) (name string, usage string) {
 // documentation for the global function PrintDefaults for more information.
 func (f *FlagSet) PrintDefaults() {
        f.VisitAll(func(flag *Flag) {
-               s := fmt.Sprintf("  -%s", flag.Name) // Two spaces before -; see next two comments.
+               var b strings.Builder
+               fmt.Fprintf(&b, "  -%s", flag.Name) // Two spaces before -; see next two comments.
                name, usage := UnquoteUsage(flag)
                if len(name) > 0 {
-                       s += " " + name
+                       b.WriteString(" ")
+                       b.WriteString(name)
                }
                // Boolean flags of one ASCII letter are so common we
                // treat them specially, putting their usage on the same line.
-               if len(s) <= 4 { // space, space, '-', 'x'.
-                       s += "\t"
+               if b.Len() <= 4 { // space, space, '-', 'x'.
+                       b.WriteString("\t")
                } else {
                        // Four spaces before the tab triggers good alignment
                        // for both 4- and 8-space tab stops.
-                       s += "\n    \t"
+                       b.WriteString("\n    \t")
                }
-               s += strings.ReplaceAll(usage, "\n", "\n    \t")
+               b.WriteString(strings.ReplaceAll(usage, "\n", "\n    \t"))
 
                if !isZeroValue(flag, flag.DefValue) {
                        if _, ok := flag.Value.(*stringValue); ok {
                                // put quotes on the value
-                               s += fmt.Sprintf(" (default %q)", flag.DefValue)
+                               fmt.Fprintf(&b, " (default %q)", flag.DefValue)
                        } else {
-                               s += fmt.Sprintf(" (default %v)", flag.DefValue)
+                               fmt.Fprintf(&b, " (default %v)", flag.DefValue)
                        }
                }
-               fmt.Fprint(f.Output(), s, "\n")
+               fmt.Fprint(f.Output(), b.String(), "\n")
        })
 }