3 files changed, 39 insertions, 10 deletions
diff --git a/src/unicode/utf8/example_test.go b/src/unicode/utf8/example_test.go
index 7b3e7ac742..5cd931d242 100644
--- a/src/unicode/utf8/example_test.go
+++ b/src/unicode/utf8/example_test.go
@@ -107,6 +107,26 @@ func ExampleEncodeRune() {
 	// 3
 }
 
+func ExampleEncodeRune_outOfRange() {
+	runes := []rune{
+		// Less than 0, out of range.
+		-1,
+		// Greater than 0x10FFFF, out of range.
+		0x110000,
+		// The Unicode replacement character.
+		utf8.RuneError,
+	}
+	for i, c := range runes {
+		buf := make([]byte, 3)
+		size := utf8.EncodeRune(buf, c)
+		fmt.Printf("%d: %d %[2]s %d\n", i, buf, size)
+	}
+	// Output:
+	// 0: [239 191 189] � 3
+	// 1: [239 191 189] � 3
+	// 2: [239 191 189] � 3
+}
+
 func ExampleFullRune() {
 	buf := []byte{228, 184, 150} // 世
 	fmt.Println(utf8.FullRune(buf))
diff --git a/src/unicode/utf8/utf8.go b/src/unicode/utf8/utf8.go
index ef0d740960..557e8a7770 100644
--- a/src/unicode/utf8/utf8.go
+++ b/src/unicode/utf8/utf8.go
@@ -337,6 +337,7 @@ func RuneLen(r rune) int {
 }
 
 // EncodeRune writes into p (which must be large enough) the UTF-8 encoding of the rune.
+// If the rune is out of range, it writes the encoding of RuneError.
 // It returns the number of bytes written.
 func EncodeRune(p []byte, r rune) int {
 	// Negative values are erroneous. Making it unsigned addresses the problem.
diff --git a/src/unicode/utf8/utf8_test.go b/src/unicode/utf8/utf8_test.go
index 359461bd05..eaf1b5ffee 100644
--- a/src/unicode/utf8/utf8_test.go
+++ b/src/unicode/utf8/utf8_test.go
@@ -597,16 +597,24 @@ func BenchmarkDecodeJapaneseRune(b *testing.B) {
 	}
 }
 
-func BenchmarkFullASCIIRune(b *testing.B) {
-	a := []byte{'a'}
-	for i := 0; i < b.N; i++ {
-		FullRune(a)
-	}
-}
+// boolSink is used to reference the return value of benchmarked
+// functions to avoid dead code elimination.
+var boolSink bool
 
-func BenchmarkFullJapaneseRune(b *testing.B) {
-	nihon := []byte("本")
-	for i := 0; i < b.N; i++ {
-		FullRune(nihon)
+func BenchmarkFullRune(b *testing.B) {
+	benchmarks := []struct {
+		name string
+		data []byte
+	}{
+		{"ASCII", []byte("a")},
+		{"Incomplete", []byte("\xf0\x90\x80")},
+		{"Japanese", []byte("本")},
+	}
+	for _, bm := range benchmarks {
+		b.Run(bm.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				boolSink = FullRune(bm.data)
+			}
+		})
 	}
 }