fmt: include failing method name in panic message

Fixes #25707

Change-Id: Idfa379db8cc0e105ea68455ec0b4a0dbc1b3f485
Reviewed-on: https://go-review.googlesource.com/c/153827
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/fmt/fmt_test.go

@@ -1683,18 +1683,18 @@ var panictests = []struct {
 }{
 	// String
 	{"%s", (*PanicS)(nil), "<nil>"}, // nil pointer special case
-	{"%s", PanicS{io.ErrUnexpectedEOF}, "%!s(PANIC=unexpected EOF)"},
-	{"%s", PanicS{3}, "%!s(PANIC=3)"},
+	{"%s", PanicS{io.ErrUnexpectedEOF}, "%!s(PANIC=String method: unexpected EOF)"},
+	{"%s", PanicS{3}, "%!s(PANIC=String method: 3)"},
 	// GoString
 	{"%#v", (*PanicGo)(nil), "<nil>"}, // nil pointer special case
-	{"%#v", PanicGo{io.ErrUnexpectedEOF}, "%!v(PANIC=unexpected EOF)"},
-	{"%#v", PanicGo{3}, "%!v(PANIC=3)"},
+	{"%#v", PanicGo{io.ErrUnexpectedEOF}, "%!v(PANIC=GoString method: unexpected EOF)"},
+	{"%#v", PanicGo{3}, "%!v(PANIC=GoString method: 3)"},
 	// Issue 18282. catchPanic should not clear fmtFlags permanently.
-	{"%#v", []interface{}{PanicGo{3}, PanicGo{3}}, "[]interface {}{%!v(PANIC=3), %!v(PANIC=3)}"},
+	{"%#v", []interface{}{PanicGo{3}, PanicGo{3}}, "[]interface {}{%!v(PANIC=GoString method: 3), %!v(PANIC=GoString method: 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=Format method: unexpected EOF)"},
+	{"%s", PanicF{3}, "%!s(PANIC=Format method: 3)"},
 }
 
 func TestPanics(t *testing.T) {

src/fmt/print.go

@@ -538,7 +538,7 @@ func (p *pp) fmtPointer(value reflect.Value, verb rune) {
 	}
 }
 
-func (p *pp) catchPanic(arg interface{}, verb rune) {
+func (p *pp) catchPanic(arg interface{}, verb rune, method string) {
 	if err := recover(); err != nil {
 		// If it's a nil pointer, just say "<nil>". The likeliest causes are a
 		// Stringer that fails to guard against nil or a nil pointer for a
@@ -561,6 +561,8 @@ func (p *pp) catchPanic(arg interface{}, verb rune) {
 		p.buf.WriteString(percentBangString)
 		p.buf.WriteRune(verb)
 		p.buf.WriteString(panicString)
+		p.buf.WriteString(method)
+		p.buf.WriteString(" method: ")
 		p.panicking = true
 		p.printArg(err, 'v')
 		p.panicking = false
@@ -577,7 +579,7 @@ func (p *pp) handleMethods(verb rune) (handled bool) {
 	// Is it a Formatter?
 	if formatter, ok := p.arg.(Formatter); ok {
 		handled = true
-		defer p.catchPanic(p.arg, verb)
+		defer p.catchPanic(p.arg, verb, "Format")
 		formatter.Format(p, verb)
 		return
 	}
@@ -586,7 +588,7 @@ func (p *pp) handleMethods(verb rune) (handled bool) {
 	if p.fmt.sharpV {
 		if stringer, ok := p.arg.(GoStringer); ok {
 			handled = true
-			defer p.catchPanic(p.arg, verb)
+			defer p.catchPanic(p.arg, verb, "GoString")
 			// Print the result of GoString unadorned.
 			p.fmt.fmtS(stringer.GoString())
 			return
@@ -604,13 +606,13 @@ func (p *pp) handleMethods(verb rune) (handled bool) {
 			switch v := p.arg.(type) {
 			case error:
 				handled = true
-				defer p.catchPanic(p.arg, verb)
+				defer p.catchPanic(p.arg, verb, "Error")
 				p.fmtString(v.Error(), verb)
 				return
 
 			case Stringer:
 				handled = true
-				defer p.catchPanic(p.arg, verb)
+				defer p.catchPanic(p.arg, verb, "String")
 				p.fmtString(v.String(), verb)
 				return
 			}