fmt: implement precs for %q.

Also fix a bug: precision was in terms of bytes; should be runes.
Fixes #1652.

R=rsc, bradfitzgo, r2, bradfitzwork
CC=golang-dev
https://golang.org/cl/4280086

src/pkg/fmt/fmt_test.go

@@ -139,7 +139,17 @@ var fmttests = []struct {
 	{"%5s", "abc", "  abc"},
 	{"%2s", "\u263a", " \u263a"},
 	{"%-5s", "abc", "abc  "},
+	{"%-8q", "abc", `"abc"   `},
 	{"%05s", "abc", "00abc"},
+	{"%08q", "abc", `000"abc"`},
+	{"%5s", "abcdefghijklmnopqrstuvwxyz", "abcdefghijklmnopqrstuvwxyz"},
+	{"%.5s", "abcdefghijklmnopqrstuvwxyz", "abcde"},
+	{"%.5s", "日本語日本語", "日本語日本"},
+	{"%.5s", []byte("日本語日本語"), "日本語日本"},
+	{"%.5q", "abcdefghijklmnopqrstuvwxyz", `"abcde"`},
+	{"%.3q", "日本語日本語", `"\u65e5\u672c\u8a9e"`},
+	{"%.3q", []byte("日本語日本語"), `"\u65e5\u672c\u8a9e"`},
+	{"%10.1q", "日本語日本語", `  "\u65e5"`},
 
 	// integers
 	{"%d", 12345, "12345"},

src/pkg/fmt/format.go

@@ -235,13 +235,24 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 	f.pad(buf[i:])
 }
 
-// fmt_s formats a string.
-func (f *fmt) fmt_s(s string) {
-	if f.precPresent {
-		if f.prec < len(s) {
-			s = s[0:f.prec]
+// truncate truncates the string to the specified precision, if present.
+func (f *fmt) truncate(s string) string {
+	if f.precPresent && f.prec < utf8.RuneCountInString(s) {
+		n := f.prec
+		for i := range s {
+			if n == 0 {
+				s = s[:i]
+				break
+			}
+			n--
 		}
 	}
+	return s
+}
+
+// fmt_s formats a string.
+func (f *fmt) fmt_s(s string) {
+	s = f.truncate(s)
 	f.padString(s)
 }
 
@@ -275,6 +286,7 @@ func (f *fmt) fmt_sX(s string) {
 
 // fmt_q formats a string as a double-quoted, escaped Go string constant.
 func (f *fmt) fmt_q(s string) {
+	s = f.truncate(s)
 	var quoted string
 	if f.sharp && strconv.CanBackquote(s) {
 		quoted = "`" + s + "`"