fmt: give correct error for % at end of string when scanning

Previously it said, "bad verb %% for ...", which is not only wrong,
it's ironic as the fix is to use %% rather than % at the end of the
string. Diagnose the case where a simple % is at EOF.

If there's anything after the percent, the error is already good
but this CL also puts quotes around the verb designation ('%d' etc.)
to make it even clearer, especially when there is a space involved.

Fixes #12315.

Change-Id: I31d30659965e940d0bd9ce92a475aab3e2369ef0
Reviewed-on: https://go-review.googlesource.com/17150Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/scan.go

@@ -538,7 +538,7 @@ func (s *ss) okVerb(verb rune, okVerbs, typ string) bool {
 			return true
 		}
 	}
-	s.errorString("bad verb %" + string(verb) + " for " + typ)
+	s.errorString("bad verb '%" + string(verb) + "' for " + typ)
 	return false
 }
 
@@ -1078,6 +1078,10 @@ func (s *ss) advance(format string) (i int) {
 	for i < len(format) {
 		fmtc, w := utf8.DecodeRuneInString(format[i:])
 		if fmtc == '%' {
+			// % at end of string is an error.
+			if i+w == len(format) {
+				s.errorString("missing verb: % at end of format string")
+			}
 			// %% acts like a real percent
 			nextc, _ := utf8.DecodeRuneInString(format[i+w:]) // will not match % if string is empty
 			if nextc != '%' {
@@ -1179,7 +1183,7 @@ func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err erro
 		}
 
 		if numProcessed >= len(a) { // out of operands
-			s.errorString("too few operands for format %" + format[i-w:])
+			s.errorString("too few operands for format '%" + format[i-w:] + "'")
 			break
 		}
 		arg := a[numProcessed]

src/fmt/scan_test.go

@@ -293,6 +293,7 @@ var scanfTests = []ScanfTest{
 	// Interesting formats
 	{"here is\tthe value:%d", "here is   the\tvalue:118\n", &intVal, 118},
 	{"%% %%:%d", "% %:119\n", &intVal, 119},
+	{"%d%%", "42%", &intVal, 42}, // %% at end of string.
 
 	// Corner cases
 	{"%x", "FFFFFFFF\n", &uint32Val, uint32(0xFFFFFFFF)},
@@ -358,6 +359,8 @@ var multiTests = []ScanfMultiTest{
 	{"%d %d", "23 18 27", args(&i, &j, &k), args(23, 18), "too many operands"},
 	{"%c", "\u0100", args(&int8Val), nil, "overflow"},
 	{"X%d", "10X", args(&intVal), nil, "input does not match format"},
+	{"%d%", "42%", args(&intVal), args(42), "missing verb: % at end of format string"},
+	{"%d% ", "42%", args(&intVal), args(42), "too few operands for format '% '"}, // Slightly odd error, but correct.
 
 	// Bad UTF-8: should see every byte.
 	{"%c%c%c", "\xc2X\xc2", args(&r1, &r2, &r3), args(utf8.RuneError, 'X', utf8.RuneError), ""},