fmt: make all errors begin with the string "%!", always.

Fixes #5730.

R=dsymonds, r, kamil.kisiel
CC=golang-dev
https://golang.org/cl/11998044

src/pkg/fmt/doc.go

@@ -156,8 +156,8 @@
 			Printf("%*s", 4.5, "hi"):  %!(BADWIDTH)hi
 			Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi
 		Invalid or invalid use of argument index: %!(BADINDEX)
-			Printf("%*[2]d", 7):       %d!(BADINDEX)
-			Printf("%.[2]d", 7):       %d!(BADINDEX)
+			Printf("%*[2]d", 7):       %!d(BADINDEX)
+			Printf("%.[2]d", 7):       %!d(BADINDEX)
 
 	All errors begin with the string "%!" followed sometimes
 	by a single character (the verb) and end with a parenthesized
@@ -169,9 +169,9 @@
 	through the fmt package.  For example, if a String method
 	calls panic("bad"), the resulting formatted message will look
 	like
-		%s(PANIC=bad)
+		%!s(PANIC=bad)
 
-	The %s just shows the print verb in use when the failure
+	The %!s just shows the print verb in use when the failure
 	occurred.
 
 	Scanning

src/pkg/fmt/fmt_test.go

@@ -563,17 +563,17 @@ var reorderTests = []struct {
 	{"%d %d %d %#[1]o %#o %#o", SE{11, 12, 13}, "11 12 13 013 014 015"},
 
 	// Erroneous cases.
-	{"%[d", SE{2, 1}, "%d!(BADINDEX)"},
+	{"%[d", SE{2, 1}, "%!d(BADINDEX)"},
 	{"%]d", SE{2, 1}, "%!](int=2)d%!(EXTRA int=1)"},
-	{"%[]d", SE{2, 1}, "%d!(BADINDEX)"},
-	{"%[-3]d", SE{2, 1}, "%d!(BADINDEX)"},
-	{"%[99]d", SE{2, 1}, "%d!(BADINDEX)"},
+	{"%[]d", SE{2, 1}, "%!d(BADINDEX)"},
+	{"%[-3]d", SE{2, 1}, "%!d(BADINDEX)"},
+	{"%[99]d", SE{2, 1}, "%!d(BADINDEX)"},
 	{"%[3]", SE{2, 1}, "%!(NOVERB)"},
-	{"%[1].2d", SE{5, 6}, "%d!(BADINDEX)"},
-	{"%[1]2d", SE{2, 1}, "%d!(BADINDEX)"},
-	{"%3.[2]d", SE{7}, "%d!(BADINDEX)"},
-	{"%.[2]d", SE{7}, "%d!(BADINDEX)"},
-	{"%d %d %d %#[1]o %#o %#o %#o", SE{11, 12, 13}, "11 12 13 013 014 015 %o!(MISSING)"},
+	{"%[1].2d", SE{5, 6}, "%!d(BADINDEX)"},
+	{"%[1]2d", SE{2, 1}, "%!d(BADINDEX)"},
+	{"%3.[2]d", SE{7}, "%!d(BADINDEX)"},
+	{"%.[2]d", SE{7}, "%!d(BADINDEX)"},
+	{"%d %d %d %#[1]o %#o %#o %#o", SE{11, 12, 13}, "11 12 13 013 014 015 %!o(MISSING)"},
 }
 
 func TestReorder(t *testing.T) {
@@ -877,16 +877,16 @@ var panictests = []struct {
 }{
 	// String
 	{"%s", (*Panic)(nil), "<nil>"}, // nil pointer special case
-	{"%s", Panic{io.ErrUnexpectedEOF}, "%s!(PANIC=unexpected EOF)"},
-	{"%s", Panic{3}, "%s!(PANIC=3)"},
+	{"%s", Panic{io.ErrUnexpectedEOF}, "%!s(PANIC=unexpected EOF)"},
+	{"%s", Panic{3}, "%!s(PANIC=3)"},
 	// GoString
 	{"%#v", (*Panic)(nil), "<nil>"}, // nil pointer special case
-	{"%#v", Panic{io.ErrUnexpectedEOF}, "%v!(PANIC=unexpected EOF)"},
-	{"%#v", Panic{3}, "%v!(PANIC=3)"},
+	{"%#v", Panic{io.ErrUnexpectedEOF}, "%!v(PANIC=unexpected EOF)"},
+	{"%#v", Panic{3}, "%!v(PANIC=3)"},
 	// Format
 	{"%s", (*PanicF)(nil), "<nil>"}, // nil pointer special case
-	{"%s", PanicF{io.ErrUnexpectedEOF}, "%s!(PANIC=unexpected EOF)"},
-	{"%s", PanicF{3}, "%s!(PANIC=3)"},
+	{"%s", PanicF{io.ErrUnexpectedEOF}, "%!s(PANIC=unexpected EOF)"},
+	{"%s", PanicF{3}, "%!s(PANIC=3)"},
 }
 
 func TestPanics(t *testing.T) {

src/pkg/fmt/print.go

@@ -16,20 +16,21 @@ import (
 // Some constants in the form of bytes, to avoid string overhead.
 // Needlessly fastidious, I suppose.
 var (
-	commaSpaceBytes = []byte(", ")
-	nilAngleBytes   = []byte("<nil>")
-	nilParenBytes   = []byte("(nil)")
-	nilBytes        = []byte("nil")
-	mapBytes        = []byte("map[")
-	missingBytes    = []byte("!(MISSING)")
-	badIndexBytes   = []byte("!(BADINDEX)")
-	panicBytes      = []byte("!(PANIC=")
-	extraBytes      = []byte("%!(EXTRA ")
-	irparenBytes    = []byte("i)")
-	bytesBytes      = []byte("[]byte{")
-	badWidthBytes   = []byte("%!(BADWIDTH)")
-	badPrecBytes    = []byte("%!(BADPREC)")
-	noVerbBytes     = []byte("%!(NOVERB)")
+	commaSpaceBytes  = []byte(", ")
+	nilAngleBytes    = []byte("<nil>")
+	nilParenBytes    = []byte("(nil)")
+	nilBytes         = []byte("nil")
+	mapBytes         = []byte("map[")
+	percentBangBytes = []byte("%!")
+	missingBytes     = []byte("(MISSING)")
+	badIndexBytes    = []byte("(BADINDEX)")
+	panicBytes       = []byte("(PANIC=")
+	extraBytes       = []byte("%!(EXTRA ")
+	irparenBytes     = []byte("i)")
+	bytesBytes       = []byte("[]byte{")
+	badWidthBytes    = []byte("%!(BADWIDTH)")
+	badPrecBytes     = []byte("%!(BADPREC)")
+	noVerbBytes      = []byte("%!(NOVERB)")
 )
 
 // State represents the printer state passed to custom formatters.
@@ -660,7 +661,7 @@ func (p *pp) catchPanic(arg interface{}, verb rune) {
 			// Nested panics; the recursion in printArg cannot succeed.
 			panic(err)
 		}
-		p.buf.WriteByte('%')
+		p.buf.Write(percentBangBytes)
 		p.add(verb)
 		p.buf.Write(panicBytes)
 		p.panicking = true
@@ -1165,12 +1166,12 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 			continue
 		}
 		if !p.goodArgNum {
-			p.buf.WriteByte('%')
+			p.buf.Write(percentBangBytes)
 			p.add(c)
 			p.buf.Write(badIndexBytes)
 			continue
 		} else if argNum >= len(a) { // out of operands
-			p.buf.WriteByte('%')
+			p.buf.Write(percentBangBytes)
 			p.add(c)
 			p.buf.Write(missingBytes)
 			continue