7 files changed, 50 insertions, 22 deletions
diff --git a/src/runtime/crash_test.go b/src/runtime/crash_test.go
index 19c9cddf36..9a5fa61588 100644
--- a/src/runtime/crash_test.go
+++ b/src/runtime/crash_test.go
@@ -966,11 +966,11 @@ func TestPanicWhilePanicking(t *testing.T) {
 		Func string
 	}{
 		{
-			"panic while printing panic value: important error message",
+			"panic while printing panic value: important multi-line\n\terror message",
 			"ErrorPanic",
 		},
 		{
-			"panic while printing panic value: important stringer message",
+			"panic while printing panic value: important multi-line\n\tstringer message",
 			"StringerPanic",
 		},
 		{
@@ -986,7 +986,7 @@ func TestPanicWhilePanicking(t *testing.T) {
 			"CircularPanic",
 		},
 		{
-			"important string message",
+			"important multi-line\n\tstring message",
 			"StringPanic",
 		},
 		{
diff --git a/src/runtime/error.go b/src/runtime/error.go
index fe95f31005..406f36ca5f 100644
--- a/src/runtime/error.go
+++ b/src/runtime/error.go
@@ -211,11 +211,16 @@ type stringer interface {
 	String() string
 }
 
-// printany prints an argument passed to panic.
+// printpanicval prints an argument passed to panic.
 // If panic is called with a value that has a String or Error method,
 // it has already been converted into a string by preprintpanics.
-func printany(i any) {
-	switch v := i.(type) {
+//
+// To ensure that the traceback can be unambiguously parsed even when
+// the panic value contains "\ngoroutine" and other stack-like
+// strings, newlines in the string representation of v are replaced by
+// "\n\t".
+func printpanicval(v any) {
+	switch v := v.(type) {
 	case nil:
 		print("nil")
 	case bool:
@@ -251,19 +256,22 @@ func printany(i any) {
 	case complex128:
 		print(v)
 	case string:
-		print(v)
+		printindented(v)
 	default:
-		printanycustomtype(i)
+		printanycustomtype(v)
 	}
 }
 
+// Invariant: each newline in the string representation is followed by a tab.
 func printanycustomtype(i any) {
 	eface := efaceOf(&i)
 	typestring := toRType(eface._type).string()
 
 	switch eface._type.Kind_ {
 	case abi.String:
-		print(typestring, `("`, *(*string)(eface.data), `")`)
+		print(typestring, `("`)
+		printindented(*(*string)(eface.data))
+		print(`")`)
 	case abi.Bool:
 		print(typestring, "(", *(*bool)(eface.data), ")")
 	case abi.Int:
@@ -301,6 +309,21 @@ func printanycustomtype(i any) {
 	}
 }
 
+// printindented prints s, replacing "\n" with "\n\t".
+func printindented(s string) {
+	for {
+		i := bytealg.IndexByteString(s, '\n')
+		if i < 0 {
+			break
+		}
+		i += len("\n")
+		print(s[:i])
+		print("\t")
+		s = s[i:]
+	}
+	print(s)
+}
+
 // panicwrap generates a panic for a call to a wrapped value method
 // with a nil pointer receiver.
 //
diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index 51b57520c1..27fcf73ff4 100644
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -656,7 +656,7 @@ func printpanics(p *_panic) {
 		return
 	}
 	print("panic: ")
-	printany(p.arg)
+	printpanicval(p.arg)
 	if p.recovered {
 		print(" [recovered]")
 	}
@@ -718,20 +718,20 @@ func gopanic(e any) {
 	gp := getg()
 	if gp.m.curg != gp {
 		print("panic: ")
-		printany(e)
+		printpanicval(e)
 		print("\n")
 		throw("panic on system stack")
 	}
 
 	if gp.m.mallocing != 0 {
 		print("panic: ")
-		printany(e)
+		printpanicval(e)
 		print("\n")
 		throw("panic during malloc")
 	}
 	if gp.m.preemptoff != "" {
 		print("panic: ")
-		printany(e)
+		printpanicval(e)
 		print("\n")
 		print("preempt off reason: ")
 		print(gp.m.preemptoff)
@@ -740,7 +740,7 @@ func gopanic(e any) {
 	}
 	if gp.m.locks != 0 {
 		print("panic: ")
-		printany(e)
+		printpanicval(e)
 		print("\n")
 		throw("panic holding locks")
 	}
@@ -1015,7 +1015,9 @@ func throw(s string) {
 	// Everything throw does should be recursively nosplit so it
 	// can be called even when it's unsafe to grow the stack.
 	systemstack(func() {
-		print("fatal error: ", s, "\n")
+		print("fatal error: ")
+		printpanicval(s)
+		print("\n")
 	})
 
 	fatalthrow(throwTypeRuntime)
@@ -1034,7 +1036,9 @@ func fatal(s string) {
 	// Everything fatal does should be recursively nosplit so it
 	// can be called even when it's unsafe to grow the stack.
 	systemstack(func() {
-		print("fatal error: ", s, "\n")
+		print("fatal error: ")
+		printpanicval(s)
+		print("\n")
 	})
 
 	fatalthrow(throwTypeUser)
diff --git a/src/runtime/panic_test.go b/src/runtime/panic_test.go
index b8a300f6b1..994abfdd45 100644
--- a/src/runtime/panic_test.go
+++ b/src/runtime/panic_test.go
@@ -27,7 +27,7 @@ func TestPanicWithDirectlyPrintableCustomTypes(t *testing.T) {
 		{"panicCustomInt16", `panic: main.MyInt16(93)`},
 		{"panicCustomInt32", `panic: main.MyInt32(93)`},
 		{"panicCustomInt64", `panic: main.MyInt64(93)`},
-		{"panicCustomString", `panic: main.MyString("Panic")`},
+		{"panicCustomString", `panic: main.MyString("Panic` + "\n\t" + `line two")`},
 		{"panicCustomUint", `panic: main.MyUint(93)`},
 		{"panicCustomUint8", `panic: main.MyUint8(93)`},
 		{"panicCustomUint16", `panic: main.MyUint16(93)`},
diff --git a/src/runtime/testdata/testprog/crash.go b/src/runtime/testdata/testprog/crash.go
index 38c8f6a2fa..bdc395f652 100644
--- a/src/runtime/testdata/testprog/crash.go
+++ b/src/runtime/testdata/testprog/crash.go
@@ -77,7 +77,7 @@ func DoublePanic() {
 type exampleError struct{}
 
 func (e exampleError) Error() string {
-	panic("important error message")
+	panic("important multi-line\nerror message")
 }
 
 func ErrorPanic() {
@@ -97,7 +97,7 @@ func DoubleErrorPanic() {
 type exampleStringer struct{}
 
 func (s exampleStringer) String() string {
-	panic("important stringer message")
+	panic("important multi-line\nstringer message")
 }
 
 func StringerPanic() {
@@ -115,7 +115,7 @@ func DoubleStringerPanic() {
 }
 
 func StringPanic() {
-	panic("important string message")
+	panic("important multi-line\nstring message")
 }
 
 func NilPanic() {
diff --git a/src/runtime/testdata/testprog/panicprint.go b/src/runtime/testdata/testprog/panicprint.go
index c8deabe2ab..4ce958ba3d 100644
--- a/src/runtime/testdata/testprog/panicprint.go
+++ b/src/runtime/testdata/testprog/panicprint.go
@@ -31,7 +31,7 @@ func panicCustomComplex128() {
 }
 
 func panicCustomString() {
-	panic(MyString("Panic"))
+	panic(MyString("Panic\nline two"))
 }
 
 func panicCustomBool() {
diff --git a/src/testing/testing_test.go b/src/testing/testing_test.go
index d3822dfd57..4a9303952e 100644
--- a/src/testing/testing_test.go
+++ b/src/testing/testing_test.go
@@ -762,7 +762,8 @@ func parseRunningTests(out []byte) (runningTests []string, ok bool) {
 	inRunningTests := false
 	for _, line := range strings.Split(string(out), "\n") {
 		if inRunningTests {
-			if trimmed, ok := strings.CutPrefix(line, "\t"); ok {
+			// Package testing adds one tab, the panic printer adds another.
+			if trimmed, ok := strings.CutPrefix(line, "\t\t"); ok {
 				if name, _, ok := strings.Cut(trimmed, " "); ok {
 					runningTests = append(runningTests, name)
 					continue