regexp/syntax: make Op an fmt.Stringer

Using stringer.

Fixes #22684.

Change-Id: I62fbde5dcb337cf269686615616bd39a27491ac1
Reviewed-on: https://go-review.googlesource.com/95355
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/regexp/syntax/op_string.go

+// Code generated by "stringer -type Op -trimprefix Op"; DO NOT EDIT.
+
+package syntax
+
+import "strconv"
+
+const (
+	_Op_name_0 = "NoMatchEmptyMatchLiteralCharClassAnyCharNotNLAnyCharBeginLineEndLineBeginTextEndTextWordBoundaryNoWordBoundaryCaptureStarPlusQuestRepeatConcatAlternate"
+	_Op_name_1 = "opPseudo"
+)
+
+var (
+	_Op_index_0 = [...]uint8{0, 7, 17, 24, 33, 45, 52, 61, 68, 77, 84, 96, 110, 117, 121, 125, 130, 136, 142, 151}
+)
+
+func (i Op) String() string {
+	switch {
+	case 1 <= i && i <= 19:
+		i -= 1
+		return _Op_name_0[_Op_index_0[i]:_Op_index_0[i+1]]
+	case i == 128:
+		return _Op_name_1
+	default:
+		return "Op(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+}

src/regexp/syntax/regexp.go

@@ -27,6 +27,8 @@ type Regexp struct {
 	Name     string     // capturing name, for OpCapture
 }
 
+//go:generate stringer -type Op -trimprefix Op
+
 // An Op is a single regular expression operator.
 type Op uint8