compiler: alignof [0]func() = 1

In the go protobuf code, a pattern is used to statically prevent
comparable structs by embedding:

```
type DoNotCompare [0]func()

type message struct {
  DoNotCompare
  data *uint32
}
```

Previously, sizezof(message{}) is 2 words large, but it only needs to be
1 byte.  Making it be 1 byte allows protobufs to compile slightly more
(though not all the way).

Author: skabbes

compiler/sizes.go

@@ -21,6 +21,12 @@ func (s *stdSizes) Alignof(T types.Type) int64 {
 	// of alignment of the elements and fields, respectively.
 	switch t := T.Underlying().(type) {
 	case *types.Array:
+		if t.Len() == 0 {
+			// 0-sized arrays, always have 0 size.
+			// And from the spec, should have an alignment of _at least_ 1
+			return 1
+		}
+
 		// spec: "For a variable x of array type: unsafe.Alignof(x)
 		// is the same as unsafe.Alignof(x[0]), but at least 1."
 		return s.Alignof(t.Elem())

testdata/reflect.go

@@ -52,6 +52,12 @@ func main() {
 	println("\nvalues of interfaces")
 	var zeroSlice []byte
 	var zeroFunc func()
+	// by embedding a 0-array func type in your struct, it is not comparable
+	type doNotCompare [0]func()
+	type notComparable struct {
+		doNotCompare
+		data *int32
+	}
 	var zeroMap map[string]int
 	var zeroChan chan int
 	n := 42
@@ -170,6 +176,10 @@ func main() {
 	assertSize(reflect.TypeOf(new(int)).Size() == unsafe.Sizeof(new(int)), "*int")
 	assertSize(reflect.TypeOf(zeroFunc).Size() == unsafe.Sizeof(zeroFunc), "func()")
 
+	// make sure embedding a zero-sized "not comparable" struct does not add size to a struct
+	assertSize(reflect.TypeOf(doNotCompare{}).Size() == unsafe.Sizeof(doNotCompare{}), "[0]func()")
+	assertSize(unsafe.Sizeof(notComparable{}) == unsafe.Sizeof((*int32)(nil)), "struct{[0]func(); *int32}")
+
 	// Test that offset is correctly calculated.
 	// This doesn't just test reflect but also (indirectly) that unsafe.Alignof
 	// works correctly.