fmt: format 0b, 0o prefixes in %#b and %O

This CL modifies fmt's printer to implement %#b and %O
to emit leading 0b and 0o prefixes on binary and octal.
(%#o is already taken and emits "0377"; %O emits "0o377".)

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

For #19308.
For #12711.
Vet update is #29986.

Change-Id: I7c38a4484c48a03abe9f6d45c7d981c7c314f583
Reviewed-on: https://go-review.googlesource.com/c/160246
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

4 files changed, 31 insertions, 12 deletions

diff --git a/src/fmt/doc.go b/src/fmt/doc.go
index c349f8e321..e0edff456c 100644
--- a/src/fmt/doc.go
+++ b/src/fmt/doc.go
@@ -26,6 +26,7 @@
 		%c	the character represented by the corresponding Unicode code point
 		%d	base 10
 		%o	base 8
+		%O	base 8 with 0o prefix
 		%q	a single-quoted character literal safely escaped with Go syntax.
 		%x	base 16, with lower-case letters for a-f
 		%X	base 16, with upper-case letters for A-F
@@ -113,8 +114,8 @@
 		+	always print a sign for numeric values;
 			guarantee ASCII-only output for %q (%+q)
 		-	pad with spaces on the right rather than the left (left-justify the field)
-		#	alternate format: add leading 0 for octal (%#o), 0x for hex (%#x);
-			0X for hex (%#X); suppress 0x for %p (%#p);
+		#	alternate format: add leading 0b for binary (%#b), 0 for octal (%#o),
+			0x or 0X for hex (%#x or %#X); suppress 0x for %p (%#p);
 			for %q, print a raw (backquoted) string if strconv.CanBackquote
 			returns true;
 			always print a decimal point for %e, %E, %f, %F, %g and %G;
diff --git a/src/fmt/fmt_test.go b/src/fmt/fmt_test.go
index 2d10c7a841..bbaf40a619 100644
--- a/src/fmt/fmt_test.go
+++ b/src/fmt/fmt_test.go
@@ -354,11 +354,17 @@ var fmtTests = []struct {
 	{"%+d", -12345, "-12345"},
 	{"%b", 7, "111"},
 	{"%b", -6, "-110"},
+	{"%#b", 7, "0b111"},
+	{"%#b", -6, "-0b110"},
 	{"%b", ^uint32(0), "11111111111111111111111111111111"},
 	{"%b", ^uint64(0), "1111111111111111111111111111111111111111111111111111111111111111"},
 	{"%b", int64(-1 << 63), zeroFill("-1", 63, "")},
 	{"%o", 01234, "1234"},
+	{"%o", -01234, "-1234"},
 	{"%#o", 01234, "01234"},
+	{"%#o", -01234, "-01234"},
+	{"%O", 01234, "0o1234"},
+	{"%O", -01234, "-0o1234"},
 	{"%o", ^uint32(0), "37777777777"},
 	{"%o", ^uint64(0), "1777777777777777777777"},
 	{"%#X", 0, "0X0"},
diff --git a/src/fmt/format.go b/src/fmt/format.go
index 6d93908095..24e7e9551a 100644
--- a/src/fmt/format.go
+++ b/src/fmt/format.go
@@ -191,7 +191,7 @@ func (f *fmt) fmtUnicode(u uint64) {
 }
 
 // fmtInteger formats signed and unsigned integers.
-func (f *fmt) fmtInteger(u uint64, base int, isSigned bool, digits string) {
+func (f *fmt) fmtInteger(u uint64, base int, isSigned bool, verb rune, digits string) {
 	negative := isSigned && int64(u) < 0
 	if negative {
 		u = -u
@@ -275,6 +275,12 @@ func (f *fmt) fmtInteger(u uint64, base int, isSigned bool, digits string) {
 	// Various prefixes: 0x, -, etc.
 	if f.sharp {
 		switch base {
+		case 2:
+			// Add a leading 0b.
+			i--
+			buf[i] = 'b'
+			i--
+			buf[i] = '0'
 		case 8:
 			if buf[i] != '0' {
 				i--
@@ -288,6 +294,12 @@ func (f *fmt) fmtInteger(u uint64, base int, isSigned bool, digits string) {
 			buf[i] = '0'
 		}
 	}
+	if verb == 'O' {
+		i--
+		buf[i] = 'o'
+		i--
+		buf[i] = '0'
+	}
 
 	if negative {
 		i--
diff --git a/src/fmt/print.go b/src/fmt/print.go
index 9976b8d263..121c7c59e4 100644
--- a/src/fmt/print.go
+++ b/src/fmt/print.go
@@ -363,7 +363,7 @@ func (p *pp) fmtBool(v bool, verb rune) {
 func (p *pp) fmt0x64(v uint64, leading0x bool) {
 	sharp := p.fmt.sharp
 	p.fmt.sharp = leading0x
-	p.fmt.fmtInteger(v, 16, unsigned, ldigits)
+	p.fmt.fmtInteger(v, 16, unsigned, 'v', ldigits)
 	p.fmt.sharp = sharp
 }
 
@@ -374,18 +374,18 @@ func (p *pp) fmtInteger(v uint64, isSigned bool, verb rune) {
 		if p.fmt.sharpV && !isSigned {
 			p.fmt0x64(v, true)
 		} else {
-			p.fmt.fmtInteger(v, 10, isSigned, ldigits)
+			p.fmt.fmtInteger(v, 10, isSigned, verb, ldigits)
 		}
 	case 'd':
-		p.fmt.fmtInteger(v, 10, isSigned, ldigits)
+		p.fmt.fmtInteger(v, 10, isSigned, verb, ldigits)
 	case 'b':
-		p.fmt.fmtInteger(v, 2, isSigned, ldigits)
-	case 'o':
-		p.fmt.fmtInteger(v, 8, isSigned, ldigits)
+		p.fmt.fmtInteger(v, 2, isSigned, verb, ldigits)
+	case 'o', 'O':
+		p.fmt.fmtInteger(v, 8, isSigned, verb, ldigits)
 	case 'x':
-		p.fmt.fmtInteger(v, 16, isSigned, ldigits)
+		p.fmt.fmtInteger(v, 16, isSigned, verb, ldigits)
 	case 'X':
-		p.fmt.fmtInteger(v, 16, isSigned, udigits)
+		p.fmt.fmtInteger(v, 16, isSigned, verb, udigits)
 	case 'c':
 		p.fmt.fmtC(v)
 	case 'q':
@@ -483,7 +483,7 @@ func (p *pp) fmtBytes(v []byte, verb rune, typeString string) {
 				if i > 0 {
 					p.buf.WriteByte(' ')
 				}
-				p.fmt.fmtInteger(uint64(c), 10, unsigned, ldigits)
+				p.fmt.fmtInteger(uint64(c), 10, unsigned, verb, ldigits)
 			}
 			p.buf.WriteByte(']')
 		}