fmt: normalize processing of format string

The old loop was a bit odd; change it to be more regular.
This also enables a diagnostic for Printf("%", 3): %!(NOVERB)

R=rsc, Kyle C
CC=golang-dev
https://golang.org/cl/3749044

src/pkg/fmt/fmt_test.go

@@ -381,11 +381,13 @@ var fmttests = []fmtTest{
 	{"%p", 27, "%!p(int=27)"}, // not a pointer at all
 
 	// erroneous things
+	{"%s %", "hello", "hello %!(NOVERB)"},
+	{"%s %.2", "hello", "hello %!(NOVERB)"},
 	{"%d", "hello", "%!d(string=hello)"},
 	{"no args", "hello", "no args%!(EXTRA string=hello)"},
 	{"%s", nil, "%!s(<nil>)"},
 	{"%T", nil, "<nil>"},
-	{"%-1", 100, "%!1(int=100)"},
+	{"%-1", 100, "%!(NOVERB)%!(EXTRA int=100)"},
 }
 
 func TestSprintf(t *testing.T) {
@@ -638,7 +640,7 @@ var startests = []starTest{
 	{"%.*d", args(nil, 42), "%!(BADPREC)42"},
 	{"%*d", args(5, "foo"), "%!d(string=  foo)"},
 	{"%*% %d", args(20, 5), "% 5"},
-	{"%*", args(4), "%!(BADWIDTH)%!*(int=4)"},
+	{"%*", args(4), "%!(NOVERB)"},
 	{"%*d", args(int32(4), 42), "%!(BADWIDTH)42"},
 }
 
@@ -655,7 +657,7 @@ func TestWidthAndPrecision(t *testing.T) {
 	for _, tt := range startests {
 		s := sprintf[len(tt.in)](tt.fmt, tt.in)
 		if s != tt.out {
-			t.Errorf("got %q expected %q", s, tt.out)
+			t.Errorf("%q: got %q expected %q", tt.fmt, s, tt.out)
 		}
 	}
 }

src/pkg/fmt/print.go

@@ -26,6 +26,7 @@ var (
 	bytesBytes      = []byte("[]byte{")
 	widthBytes      = []byte("%!(BADWIDTH)")
 	precBytes       = []byte("%!(BADPREC)")
+	noVerbBytes     = []byte("%!(NOVERB)")
 )
 
 // State represents the printer state passed to custom formatters.
@@ -821,19 +822,16 @@ func intFromArg(a []interface{}, end, i, fieldnum int) (num int, isInt bool, new
 }
 
 func (p *pp) doPrintf(format string, a []interface{}) {
-	end := len(format) - 1
+	end := len(format)
 	fieldnum := 0 // we process one field per non-trivial format
-	for i := 0; i <= end; {
+	for i := 0; i < end; {
 		c, w := utf8.DecodeRuneInString(format[i:])
-		if c != '%' || i == end {
-			if w == 1 {
-				p.buf.WriteByte(byte(c))
-			} else {
-				p.buf.WriteString(format[i : i+w])
-			}
+		if c != '%' {
+			p.buf.WriteRune(c)
 			i += w
 			continue
 		}
+		// Process one verb
 		i++
 		// flags and widths
 		p.fmt.clearflags()
@@ -855,7 +853,7 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 			}
 		}
 		// do we have width?
-		if format[i] == '*' {
+		if i < end && format[i] == '*' {
 			p.fmt.wid, p.fmt.widPresent, i, fieldnum = intFromArg(a, end, i, fieldnum)
 			if !p.fmt.widPresent {
 				p.buf.Write(widthBytes)
@@ -874,6 +872,10 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 				p.fmt.prec, p.fmt.precPresent, i = parsenum(format, i+1, end)
 			}
 		}
+		if i >= end {
+			p.buf.Write(noVerbBytes)
+			continue
+		}
 		c, w = utf8.DecodeRuneInString(format[i:])
 		i += w
 		// percent is special - absorbs no operand