reflect: add SetBytes()

src/reflect/value.go

@@ -982,7 +982,13 @@ func (v Value) SetString(x string) {
 }
 
 func (v Value) SetBytes(x []byte) {
-	panic("unimplemented: (reflect.Value).SetBytes()")
+	v.checkAddressable()
+	if v.typecode.Kind() != Slice || v.typecode.elem().Kind() != Uint8 {
+		panic("reflect.Value.SetBytes called on not []byte")
+	}
+
+	// copy the header contents over
+	*(*sliceHeader)(v.value) = *(*sliceHeader)(unsafe.Pointer(&x))
 }
 
 func (v Value) SetCap(n int) {

src/reflect/value_test.go

@@ -321,6 +321,20 @@ func TestNilType(t *testing.T) {
 	}
 }
 
+func TestSetBytes(t *testing.T) {
+	var b []byte
+	refb := ValueOf(&b).Elem()
+	s := []byte("hello")
+	refb.SetBytes(s)
+	s[0] = 'b'
+
+	refbSlice := refb.Interface().([]byte)
+
+	if len(refbSlice) != len(s) || b[0] != s[0] || refbSlice[0] != s[0] {
+		t.Errorf("SetBytes(): reflection slice mismatch")
+	}
+}
+
 func equal[T comparable](a, b []T) bool {
 	if len(a) != len(b) {
 		return false