strconv: faster Unquote in common case

Also reject literal newline in " and ' quoted strings.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5139045

src/pkg/strconv/quote.go

@@ -288,6 +288,22 @@ func Unquote(s string) (t string, err os.Error) {
 	if quote != '"' && quote != '\'' {
 		return "", os.EINVAL
 	}
+	if strings.Index(s, "\n") >= 0 {
+		return "", os.EINVAL
+	}
+
+	// Is it trivial?  Avoid allocation.
+	if strings.Index(s, `\`) < 0 && strings.IndexRune(s, int(quote)) < 0 {
+		switch quote {
+		case '"':
+			return s, nil
+		case '\'':
+			r, size := utf8.DecodeRuneInString(s)
+			if size == len(s) && (r != utf8.RuneError || size != 1) {
+				return s, nil
+			}
+		}
+	}
 
 	var buf bytes.Buffer
 	for len(s) > 0 {

src/pkg/strconv/quote_test.go

@@ -168,6 +168,7 @@ var unquotetests = []unQuoteTest{
 	{"`\\xFF`", `\xFF`},
 	{"`\\377`", `\377`},
 	{"`\\`", `\`},
+	{"`\n`", "\n"},
 	{"`	`", `	`},
 	{"` `", ` `},
 }
@@ -189,6 +190,9 @@ var misquoted = []string{
 	"`\"",
 	`"\'"`,
 	`'\"'`,
+	"\"\n\"",
+	"\"\\n\n\"",
+	"'\n'",
 }
 
 func TestUnquote(t *testing.T) {
@@ -211,3 +215,15 @@ func TestUnquote(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkUnquoteEasy(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Unquote(`"Give me a rock, paper and scissors and I will move the world."`)
+	}
+}
+
+func BenchmarkUnquoteHard(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Unquote(`"\x47ive me a \x72ock, \x70aper and \x73cissors and \x49 will move the world."`)
+	}
+}