fmt: make %q work for integers, printing a quoted character literal.

R=rsc
CC=golang-dev
https://golang.org/cl/4556060

src/pkg/fmt/doc.go

@@ -25,6 +25,7 @@
 		%c	the character represented by the corresponding Unicode code point
 		%d	base 10
 		%o	base 8
+		%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
 		%U	Unicode format: U+1234; same as "U+%04X"

src/pkg/fmt/fmt_test.go

@@ -135,6 +135,17 @@ var fmttests = []struct {
 	{"%q", "\u263a", `"\u263a"`},
 	{"%q", "\U0010ffff", `"\U0010ffff"`},
 
+	// escaped characters
+	{"%q", 'x', `'x'`},
+	{"%q", 0, `'\x00'`},
+	{"%q", '\n', `'\n'`},
+	{"%q", '\u1234', `'\u1234'`},
+	{"%q", '\U00012345', `'\U00012345'`},
+	{"%q", int64(0x7FFFFFFF), `%!q(int64=2147483647)`},
+	{"%q", uint64(0xFFFFFFFF), `%!q(uint64=4294967295)`},
+	{"%q", '"', `'"'`},
+	{"%q", '\'', `'\''`},
+
 	// width
 	{"%5s", "abc", "  abc"},
 	{"%2s", "\u263a", " \u263a"},

src/pkg/fmt/format.go

@@ -296,6 +296,13 @@ func (f *fmt) fmt_q(s string) {
 	f.padString(quoted)
 }
 
+// fmt_qc formats the integer as a single-quoted, escaped Go character constant.
+// If the character is not valid Unicode, it will print '\ufffd'.
+func (f *fmt) fmt_qc(c int64) {
+	quoted := strconv.QuoteRune(int(c))
+	f.padString(quoted)
+}
+
 // floating-point
 
 func doPrec(f *fmt, def int) int {

src/pkg/fmt/print.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"os"
 	"reflect"
+	"unicode"
 	"utf8"
 )
 
@@ -332,6 +333,12 @@ func (p *pp) fmtInt64(v int64, verb int, value interface{}) {
 		p.fmt.integer(v, 10, signed, ldigits)
 	case 'o':
 		p.fmt.integer(v, 8, signed, ldigits)
+	case 'q':
+		if 0 <= v && v <= unicode.MaxRune {
+			p.fmt.fmt_qc(v)
+		} else {
+			p.badVerb(verb, value)
+		}
 	case 'x':
 		p.fmt.integer(v, 16, signed, ldigits)
 	case 'U':
@@ -385,6 +392,12 @@ func (p *pp) fmtUint64(v uint64, verb int, goSyntax bool, value interface{}) {
 		}
 	case 'o':
 		p.fmt.integer(int64(v), 8, unsigned, ldigits)
+	case 'q':
+		if 0 <= v && v <= unicode.MaxRune {
+			p.fmt.fmt_qc(int64(v))
+		} else {
+			p.badVerb(verb, value)
+		}
 	case 'x':
 		p.fmt.integer(int64(v), 16, unsigned, ldigits)
 	case 'X':