理解Golang的nil

type Message struct {
	A *Message
}

func (x *Message) GetA() *Message {
	if x != nil {
		return x.A
	}
	return nil
}
func TestNil(t *testing.T) {
	var s *Message
	var v interface{} = s

	fmt.Println(v == s)                        // #=> true
	fmt.Println(s == nil)                      // #=> true
	fmt.Println(v == nil)                      // #=> false
	fmt.Println(s.GetA().GetA().GetA() == nil) // #=> true
}

type Err struct {
	err string
}

func (e *Err) Error() string {
	return e.err
}
func returnErr() *Err {
	return nil
}
func TestErr(t *testing.T) {
	var err error
	err = returnErr()
	fmt.Println(err, err != nil) // #=> true
}

// nil is a predeclared identifier representing the zero value for a
// pointer, channel, func, interface, map, or slice type.
var nil Type // Type must be a pointer, channel, func, interface, map, or slice type

// Type is here for the purposes of documentation only. It is a stand-in
// for any Go type, but represents the same type for any given function
// invocation.
type Type int

type iface struct {
	tab  *itab
	data unsafe.Pointer
}
type eface struct {
	_type *_type
	data  unsafe.Pointer
}
type itab struct {
	inter *interfacetype
	_type *_type
	hash  uint32 // copy of _type.hash. Used for type switches.
	_     [4]byte
	fun   [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter.
}

type _type struct {
	size       uintptr
	ptrdata    uintptr // size of memory prefix holding all pointers
	hash       uint32
	tflag      tflag
	align      uint8
	fieldAlign uint8
	kind       uint8
	// function for comparing objects of this type
	// (ptr to object A, ptr to object B) -> ==?
	equal func(unsafe.Pointer, unsafe.Pointer) bool
	// gcdata stores the GC type data for the garbage collector.
	// If the KindGCProg bit is set in kind, gcdata is a GC program.
	// Otherwise it is a ptrmask bitmap. See mbitmap.go for details.
	gcdata    *byte
	str       nameOff
	ptrToThis typeOff
}

go tool compile -S -N -l nil_test.go

➜  nil git:(master) ✗ go tool compile -S -N -l nil_test.go                                            .
"".TestNil STEXT size=634 args=0x8 locals=0xe8 funcid=0x0 align=0x0
TEXT    "".TestNil(SB), ABIInter # func TestNil(t *testing.T) {
MOVQ    $0, "".s+32(SP)          # var s *Message : 将0赋值给[s栈32位置]
LEAQ    type.*"".Message(SB), DX # var v interface{} = s : 将Message类型有效地址赋值给DX寄存器
MOVQ    DX, "".v+96(SP)          # var v interface{} = s : 将DX寄存器内容(Message类型地址)赋值给[v栈96位置]
MOVQ    $0, "".v+104(SP)         # var v interface{} = s : 将0赋值给[v栈的104位置](给v赋值)
MOVQ    "".s+32(SP), SI          # fmt.Println(v == s) : 将[s栈32位置]赋值给SI寄存器(值为0)
CMPQ    "".v+104(SP), SI         # fmt.Println(v == s) : 对比[v栈104位置]和[s栈32位置值](都是0)
SETEQ   DL                       # fmt.Println(v == s) : 将对比值赋值给DL寄存器
CALL    fmt.Println(SB)          # fmt.Println(v == s) : 打印对比结果，true
CMPQ    "".s+32(SP), $0          # fmt.Println(s == nil) : 对比[s栈32位置]和0值(都是0)
CALL    fmt.Println(SB)          # fmt.Println(s == nil) : 打印对比结果，true
CMPQ    "".v+96(SP), $0          # fmt.Println(v == nil) : 对比[v栈96位置]和0值([v栈96位置]是有值的，因此结果是false)
CALL    fmt.Println(SB)          # fmt.Println(v == nil) : 打印对比结果，false
MOVQ    "".s+32(SP), AX          # fmt.Println(s.GetA().GetA().GetA() == nil) :
CALL    "".(*Message).GetA(SB)   # fmt.Println(s.GetA().GetA().GetA() == nil) : 汇编会将nil转成*Message类型