fmt: allow # and x together for strings

Silly and small but easy to be consistent.
To make it worthwhile, I eliminated an allocation when using
%x on a byte slice.

Fixes #4149.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6574046

src/pkg/fmt/fmt_test.go

@@ -127,6 +127,10 @@ var fmttests = []struct {
 	{"%s", []byte("abc"), "abc"},
 	{"%x", []byte("abc"), "616263"},
 	{"% x", []byte("abc\xff"), "61 62 63 ff"},
+	{"%#x", []byte("abc\xff"), "0x610x620x630xff"},
+	{"%#X", []byte("abc\xff"), "0X610X620X630XFF"},
+	{"%# x", []byte("abc\xff"), "0x61 0x62 0x63 0xff"},
+	{"%# X", []byte("abc\xff"), "0X61 0X62 0X63 0XFF"},
 	{"% X", []byte("abc\xff"), "61 62 63 FF"},
 	{"%x", []byte("xyz"), "78797a"},
 	{"%X", []byte("xyz"), "78797A"},
@@ -350,10 +354,12 @@ var fmttests = []struct {
 	{"%+v", B{1, 2}, `{I:<1> j:2}`},
 	{"%+v", C{1, B{2, 3}}, `{i:1 B:{I:<2> j:3}}`},
 
-	// q on Stringable items
+	// other formats on Stringable items
 	{"%s", I(23), `<23>`},
 	{"%q", I(23), `"<23>"`},
 	{"%x", I(23), `3c32333e`},
+	{"%#x", I(23), `0x3c0x320x330x3e`},
+	{"%# x", I(23), `0x3c 0x32 0x33 0x3e`},
 	{"%d", I(23), `23`}, // Stringer applies only to string formats.
 
 	// go syntax

src/pkg/fmt/format.go

@@ -285,18 +285,41 @@ func (f *fmt) fmt_s(s string) {
 	f.padString(s)
 }
 
+// fmt_sbx formats a string or byte slice as a hexadecimal encoding of its bytes.
+func (f *fmt) fmt_sbx(s string, b []byte, digits string) {
+	n := len(b)
+	if b == nil {
+		n = len(s)
+	}
+	x := digits[10] - 'a' + 'x'
+	// TODO: Avoid buffer by pre-padding.
+	var buf []byte
+	for i := 0; i < n; i++ {
+		if i > 0 && f.space {
+			buf = append(buf, ' ')
+		}
+		if f.sharp {
+			buf = append(buf, '0', x)
+		}
+		var c byte
+		if b == nil {
+			c = s[i]
+		} else {
+			c = b[i]
+		}
+		buf = append(buf, digits[c>>4], digits[c&0xF])
+	}
+	f.pad(buf)
+}
+
 // fmt_sx formats a string as a hexadecimal encoding of its bytes.
 func (f *fmt) fmt_sx(s, digits string) {
-	// TODO: Avoid buffer by pre-padding.
-	var b []byte
-	for i := 0; i < len(s); i++ {
-		if i > 0 && f.space {
-			b = append(b, ' ')
-		}
-		v := s[i]
-		b = append(b, digits[v>>4], digits[v&0xF])
-	}
-	f.pad(b)
+	f.fmt_sbx(s, nil, digits)
+}
+
+// fmt_bx formats a byte slice as a hexadecimal encoding of its bytes.
+func (f *fmt) fmt_bx(b []byte, digits string) {
+	f.fmt_sbx("", b, digits)
 }
 
 // fmt_q formats a string as a double-quoted, escaped Go string constant.

src/pkg/fmt/print.go

@@ -569,16 +569,15 @@ func (p *pp) fmtBytes(v []byte, verb rune, goSyntax bool, depth int) {
 		}
 		return
 	}
-	s := string(v)
 	switch verb {
 	case 's':
-		p.fmt.fmt_s(s)
+		p.fmt.fmt_s(string(v))
 	case 'x':
-		p.fmt.fmt_sx(s, ldigits)
+		p.fmt.fmt_bx(v, ldigits)
 	case 'X':
-		p.fmt.fmt_sx(s, udigits)
+		p.fmt.fmt_bx(v, udigits)
 	case 'q':
-		p.fmt.fmt_q(s)
+		p.fmt.fmt_q(string(v))
 	default:
 		p.badVerb(verb)
 	}