fmt: print byte stringers correctly

type T byte
func (T) String() string { return "X" }

fmt.Sprintf("%s", []T{97, 98, 99, 100}) == "abcd"
fmt.Sprintf("%x", []T{97, 98, 99, 100}) == "61626364"
fmt.Sprintf("%v", []T{97, 98, 99, 100}) == "[X X X X]"

This change makes the last case print correctly.
Before, it would have been "[97 98 99 100]".

Fixes #8360.

LGTM=r
R=r, dan.kortschak
CC=golang-codereviews
https://golang.org/cl/129330043

src/pkg/fmt/fmt_test.go

@@ -108,6 +108,20 @@ func (p *P) String() string {
 var barray = [5]renamedUint8{1, 2, 3, 4, 5}
 var bslice = barray[:]
 
+type byteStringer byte
+
+func (byteStringer) String() string { return "X" }
+
+var byteStringerSlice = []byteStringer{97, 98, 99, 100}
+
+type byteFormatter byte
+
+func (byteFormatter) Format(f State, _ rune) {
+	Fprint(f, "X")
+}
+
+var byteFormatterSlice = []byteFormatter{97, 98, 99, 100}
+
 var b byte
 
 var fmtTests = []struct {
@@ -629,6 +643,21 @@ var fmtTests = []struct {
 	{"%+010.2f", -104.66 + 440.51i, "(-000104.66+000440.51i)"},
 	{"%+010.2f", +104.66 - 440.51i, "(+000104.66-000440.51i)"},
 	{"%+010.2f", -104.66 - 440.51i, "(-000104.66-000440.51i)"},
+
+	// []T where type T is a byte with a Stringer method.
+	{"%v", byteStringerSlice, "[X X X X]"},
+	{"%s", byteStringerSlice, "abcd"},
+	{"%q", byteStringerSlice, "\"abcd\""},
+	{"%x", byteStringerSlice, "61626364"},
+	{"%#v", byteStringerSlice, "[]fmt_test.byteStringer{0x61, 0x62, 0x63, 0x64}"},
+
+	// And the same for Formatter.
+	{"%v", byteFormatterSlice, "[X X X X]"},
+	{"%s", byteFormatterSlice, "abcd"},
+	{"%q", byteFormatterSlice, "\"abcd\""},
+	{"%x", byteFormatterSlice, "61626364"},
+	// This next case seems wrong, but the docs say the Formatter wins here.
+	{"%#v", byteFormatterSlice, "[]fmt_test.byteFormatter{X, X, X, X}"},
 }
 
 // zeroFill generates zero-filled strings of the specified width. The length

src/pkg/fmt/print.go

@@ -832,6 +832,8 @@ func (p *pp) printValue(value reflect.Value, verb rune, plus, goSyntax bool, dep
 	return p.printReflectValue(value, verb, plus, goSyntax, depth)
 }
 
+var byteType = reflect.TypeOf(byte(0))
+
 // printReflectValue is the fallback for both printArg and printValue.
 // It uses reflect to print the value.
 func (p *pp) printReflectValue(value reflect.Value, verb rune, plus, goSyntax bool, depth int) (wasString bool) {
@@ -925,8 +927,12 @@ BigSwitch:
 			wasString = p.printValue(value, verb, plus, goSyntax, depth+1)
 		}
 	case reflect.Array, reflect.Slice:
-		// Byte slices are special.
-		if typ := f.Type(); typ.Elem().Kind() == reflect.Uint8 {
+		// Byte slices are special:
+		// - Handle []byte (== []uint8) with fmtBytes.
+		// - Handle []T, where T is a named byte type, with fmtBytes only
+		//   for the s, q, an x verbs. For other verbs, T might be a
+		//   Stringer, so we use printValue to print each element.
+		if typ := f.Type(); typ.Elem().Kind() == reflect.Uint8 && (typ.Elem() == byteType || verb == 's' || verb == 'q' || verb == 'x') {
 			var bytes []byte
 			if f.Kind() == reflect.Slice {
 				bytes = f.Bytes()