fmt: restore padding for %x on byte slices and strings

Also improve the documentation. A prior fix in this release
changed the properties for empty strings and slices, incorrectly.
Previous behavior is now restored and better documented.

Add lots of tests.

The behavior is that when using a string-like format (%s %q %x %X)
a byte slice is equivalent to a string, and printed as a unit. The padding
applies to the entire object. (The space and sharp flags apply
elementwise.)

Fixes #11422.
Fixes #10430.

Change-Id: I758f0521caf71630437e43990ec6d6c9a92655e3
Reviewed-on: https://go-review.googlesource.com/11600Reviewed-by: Russ Cox <rsc@golang.org>

src/fmt/doc.go

@@ -40,7 +40,7 @@
 		%F	synonym for %f
 		%g	%e for large exponents, %f otherwise
 		%G	%E for large exponents, %F otherwise
-	String and slice of bytes:
+	String and slice of bytes (treated equivalently with these verbs):
 		%s	the uninterpreted bytes of the string or slice
 		%q	a double-quoted string safely escaped with Go syntax
 		%x	base 16, lower-case, two characters per byte
@@ -164,6 +164,9 @@
 	of strings, and %6.2f will control formatting for each element
 	of a floating-point array.
 
+	However, when printing a byte slice with a string-like verb
+	(%s %q %x %X), it is treated identically to a string, as a single item.
+
 	To avoid recursion in cases such as
 		type X string
 		func (x X) String() string { return Sprintf("<%s>", x) }

src/fmt/fmt_test.go

@@ -452,10 +452,30 @@ var fmtTests = []struct {
 	{"%q", []string{"a", "b"}, `["a" "b"]`},
 	{"% 02x", []byte{1}, "01"},
 	{"% 02x", []byte{1, 2, 3}, "01 02 03"},
-	// Special care for empty slices.
+	// Padding with byte slices.
 	{"%x", []byte{}, ""},
-	{"%02x", []byte{}, ""},
-	{"% 02x", []byte{}, ""},
+	{"%02x", []byte{}, "00"},
+	{"% 02x", []byte{}, "00"},
+	{"%08x", []byte{0xab}, "000000ab"},
+	{"% 08x", []byte{0xab}, "000000ab"},
+	{"%08x", []byte{0xab, 0xcd}, "0000abcd"},
+	{"% 08x", []byte{0xab, 0xcd}, "000ab cd"},
+	{"%8x", []byte{0xab}, "      ab"},
+	{"% 8x", []byte{0xab}, "      ab"},
+	{"%8x", []byte{0xab, 0xcd}, "    abcd"},
+	{"% 8x", []byte{0xab, 0xcd}, "   ab cd"},
+	// Same for strings
+	{"%x", "", ""},
+	{"%02x", "", "00"},
+	{"% 02x", "", "00"},
+	{"%08x", "\xab", "000000ab"},
+	{"% 08x", "\xab", "000000ab"},
+	{"%08x", "\xab\xcd", "0000abcd"},
+	{"% 08x", "\xab\xcd", "000ab cd"},
+	{"%8x", "\xab", "      ab"},
+	{"% 8x", "\xab", "      ab"},
+	{"%8x", "\xab\xcd", "    abcd"},
+	{"% 8x", "\xab\xcd", "   ab cd"},
 
 	// renamings
 	{"%v", renamedBool(true), "true"},

src/fmt/format.go

@@ -346,7 +346,7 @@ func (f *fmt) fmt_sbx(s string, b []byte, digits string) {
 		}
 		buf = append(buf, digits[c>>4], digits[c&0xF])
 	}
-	f.buf.Write(buf)
+	f.pad(buf)
 }
 
 // fmt_sx formats a string as a hexadecimal encoding of its bytes.