math/big: add %#b and %O integer formats

Matching fmt, %#b now prints an 0b prefix,
and %O prints octal with an 0o prefix.

See golang.org/design/19308-number-literals for background.

For #19308.
For #12711.

Change-Id: I139c5a9a1dfae15415621601edfa13c6a5f19cfc
Reviewed-on: https://go-review.googlesource.com/c/160250Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/intconv.go

@@ -50,8 +50,9 @@ func writeMultiple(s fmt.State, text string, count int) {
 var _ fmt.Formatter = intOne // *Int must implement fmt.Formatter
 
 // Format implements fmt.Formatter. It accepts the formats
-// 'b' (binary), 'o' (octal), 'd' (decimal), 'x' (lowercase
-// hexadecimal), and 'X' (uppercase hexadecimal).
+// 'b' (binary), 'o' (octal with 0 prefix), 'O' (octal with 0o prefix),
+// 'd' (decimal), 'x' (lowercase hexadecimal), and
+// 'X' (uppercase hexadecimal).
 // Also supported are the full suite of package fmt's format
 // flags for integral types, including '+' and ' ' for sign
 // control, '#' for leading zero in octal and for hexadecimal,
@@ -66,7 +67,7 @@ func (x *Int) Format(s fmt.State, ch rune) {
 	switch ch {
 	case 'b':
 		base = 2
-	case 'o':
+	case 'o', 'O':
 		base = 8
 	case 'd', 's', 'v':
 		base = 10
@@ -98,6 +99,8 @@ func (x *Int) Format(s fmt.State, ch rune) {
 	prefix := ""
 	if s.Flag('#') {
 		switch ch {
+		case 'b': // binary
+			prefix = "0b"
 		case 'o': // octal
 			prefix = "0"
 		case 'x': // hexadecimal
@@ -106,6 +109,9 @@ func (x *Int) Format(s fmt.State, ch rune) {
 			prefix = "0X"
 		}
 	}
+	if ch == 'O' {
+		prefix = "0o"
+	}
 
 	digits := x.abs.utoa(base)
 	if ch == 'X' {

src/math/big/intconv_test.go

@@ -214,8 +214,12 @@ var formatTests = []struct {
 	{"10", "%y", "%!y(big.Int=10)"},
 	{"-10", "%y", "%!y(big.Int=-10)"},
 
-	{"10", "%#b", "1010"},
+	{"10", "%#b", "0b1010"},
 	{"10", "%#o", "012"},
+	{"10", "%O", "0o12"},
+	{"-10", "%#b", "-0b1010"},
+	{"-10", "%#o", "-012"},
+	{"-10", "%O", "-0o12"},
 	{"10", "%#d", "10"},
 	{"10", "%#v", "10"},
 	{"10", "%#x", "0xa"},