replace reflect.interfaceHeader with strongly typed runtime functions

src/reflect/type.go

@@ -318,7 +318,7 @@ func (t Type) Size() uintptr {
 	case Slice:
 		return unsafe.Sizeof(SliceHeader{})
 	case Interface:
-		return unsafe.Sizeof(interfaceHeader{})
+		return unsafe.Sizeof(interface{}(nil))
 	case Array:
 		return t.Elem().Size() * uintptr(t.Len())
 	case Struct:
@@ -364,7 +364,7 @@ func (t Type) Align() int {
 	case Slice:
 		return int(unsafe.Alignof(SliceHeader{}))
 	case Interface:
-		return int(unsafe.Alignof(interfaceHeader{}))
+		return int(unsafe.Alignof(interface{}(nil)))
 	case Struct:
 		numField := t.NumField()
 		alignment := 1

src/reflect/value.go

@@ -35,20 +35,22 @@ func Indirect(v Value) Value {
 	return v.Elem()
 }
 
+//go:linkname composeInterface runtime.composeInterface
+func composeInterface(Type, unsafe.Pointer) interface{}
+
+//go:linkname decomposeInterface runtime.decomposeInterface
+func decomposeInterface(i interface{}) (Type, unsafe.Pointer)
+
 func ValueOf(i interface{}) Value {
-	v := (*interfaceHeader)(unsafe.Pointer(&i))
+	typecode, value := decomposeInterface(i)
 	return Value{
-		typecode: v.typecode,
-		value:    v.value,
+		typecode: typecode,
+		value:    value,
 		flags:    valueFlagExported,
 	}
 }
 
 func (v Value) Interface() interface{} {
-	i := interfaceHeader{
-		typecode: v.typecode,
-		value:    v.value,
-	}
 	if v.isIndirect() && v.Type().Size() <= unsafe.Sizeof(uintptr(0)) {
 		// Value was indirect but must be put back directly in the interface
 		// value.
@@ -56,9 +58,9 @@ func (v Value) Interface() interface{} {
 		for j := v.Type().Size(); j != 0; j-- {
 			value = (value << 8) | uintptr(*(*uint8)(unsafe.Pointer(uintptr(v.value) + j - 1)))
 		}
-		i.value = unsafe.Pointer(value)
+		v.value = unsafe.Pointer(value)
 	}
-	return *(*interface{})(unsafe.Pointer(&i))
+	return composeInterface(v.typecode, v.value)
 }
 
 func (v Value) Type() Type {
@@ -94,8 +96,8 @@ func (v Value) IsNil() bool {
 		if v.value == nil {
 			return true
 		}
-		itf := (*interfaceHeader)(v.value)
-		return itf.value == nil
+		_, val := decomposeInterface(*(*interface{})(v.value))
+		return val == nil
 	default:
 		panic(&ValueError{"IsNil"})
 	}
@@ -657,12 +659,6 @@ type funcHeader struct {
 	Code    unsafe.Pointer
 }
 
-// This is the same thing as an interface{}.
-type interfaceHeader struct {
-	typecode Type
-	value    unsafe.Pointer
-}
-
 type SliceHeader struct {
 	Data uintptr
 	Len  uintptr

src/runtime/interface.go

@@ -12,6 +12,16 @@ type _interface struct {
 	value    unsafe.Pointer
 }
 
+//go:inline
+func composeInterface(typecode uintptr, value unsafe.Pointer) _interface {
+	return _interface{typecode, value}
+}
+
+//go:inline
+func decomposeInterface(i _interface) (uintptr, unsafe.Pointer) {
+	return i.typecode, i.value
+}
+
 // Return true iff both interfaces are equal.
 func interfaceEqual(x, y _interface) bool {
 	if x.typecode != y.typecode {