detach a string if the attached data is nil

Author: huandu

README.md

@@ -11,7 +11,7 @@ This `magicstring` package is designed to attach arbitrary data to a Go built-in
 
 ### Attach data and then read it
 
-Call `Attach` to attach data into a string and `Read` to read the attached data in the magic string.
+Call `Attach` to attach data to a string and `Read` to read the attached data in the magic string.
 
 ```go
 type T struct {

attachment.go

@@ -9,9 +9,15 @@ import (
 	"unsafe"
 )
 
-// Attach associates the data with a newly allocated string.
+// Attach associates a newly allocated string with data.
 // The value of the returned string is guaranteed to be identical to str.
+//
+// Calling `Attach(str, nil)` is equivalent to `Detach(str)`.
 func Attach(str string, data interface{}) string {
+	if data == nil {
+		return Detach(str)
+	}
+
 	sz := len(str)
 
 	if sz == 0 {

attachment_test.go

@@ -5,15 +5,17 @@ package magicstring
 
 import (
 	"fmt"
+	"reflect"
 	"testing"
+	"unsafe"
 
 	"github.com/huandu/go-assert"
 )
 
 func TestAttachData(t *testing.T) {
 	a := assert.New(t)
 	cases := []interface{}{
-		1, "abcd", true, nil, 1.2, complex(1, 2),
+		1, "abcd", true, 1.2, complex(1, 2),
 		struct{}{}, []byte("123"), &testStruct{},
 		map[string][]int{"foo": {1, 2, 3}},
 	}
@@ -30,6 +32,30 @@ func TestAttachData(t *testing.T) {
 	}
 }
 
+func TestAttachNilData(t *testing.T) {
+	a := assert.New(t)
+	s1 := "nil data"
+	s2 := Attach(s1, nil)
+	s3 := Attach(s1, 123)
+	s4 := Attach(s3, nil)
+	a.Equal(s1, s2)
+	a.Equal(s1, s3)
+	a.Equal(s1, s4)
+
+	// There is no magic attached to s2.
+	a.Assert(!Is(s2))
+
+	// The buffer in s1 and s2 must be identical.
+	data1 := (*reflect.StringHeader)(unsafe.Pointer(&s1)).Data
+	data2 := (*reflect.StringHeader)(unsafe.Pointer(&s2)).Data
+	a.Equal(data1, data2)
+
+	// The buffer in s3 and s4 must be different.
+	data3 := (*reflect.StringHeader)(unsafe.Pointer(&s3)).Data
+	data4 := (*reflect.StringHeader)(unsafe.Pointer(&s4)).Data
+	a.NotEqual(data3, data4)
+}
+
 func TestReadInvalidString(t *testing.T) {
 	a := assert.New(t)
 	s := "dummy"