runtime: avoid heap allocs for plain errors

testdata/recover.go

@@ -313,12 +313,29 @@ func recoverMustPanic(name string, f func()) {
 	f()
 }
 
+func recoverRuntimeErrorValue(f func()) {
+	defer func() {
+		r := recover()
+		err, ok := r.(runtime.Error)
+		if ok {
+			println("  recovered runtime error:", err.Error())
+		} else {
+			println("  failed runtime error:", r)
+		}
+	}()
+	f()
+}
+
 // Test recovering from nil map assignment and closed channel send.
 func recoverNilMapAndChan() {
 	recoverMustPanic("nil map", func() {
 		var m map[string]int
 		m["x"] = 1
 	})
+	recoverRuntimeErrorValue(func() {
+		var m map[string]int
+		m["x"] = 1
+	})
 	recoverMustPanic("closed chan", func() {
 		ch := make(chan int)
 		close(ch)

testdata/recover.txt

@@ -52,6 +52,7 @@ outer recovered: repanic value
 
 # recover from nil map and closed channel
   recovered: nil map
+  recovered runtime error: assignment to entry in nil map
   recovered: closed chan
   recovered: close nil chan