size: fix empty struct collection expressions

empty_struct_test.go

@@ -0,0 +1,69 @@
+package main
+
+import "testing"
+
+func TestEmptyStructCollections(t *testing.T) {
+	filename, err := generate(t, emptyStructCollections)
+	if err != nil {
+		t.Fatalf("generate failed: %v", err)
+	}
+	goExec(t, filename, false)
+	goExec(t, filename, true)
+}
+
+var emptyStructCollections = `package main
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+
+	"github.com/tinylib/msgp/msgp"
+)
+
+type Empty struct{}
+type ZeroArray [0]Empty
+type NestedArray [2][1]Empty
+
+type Collection struct {
+	Named []Empty
+	Anonymous []struct{}
+	Array [2]Empty
+}
+
+func main() {
+	input := Collection{
+		Named: []Empty{{}, {}},
+		Anonymous: []struct{}{{}, {}, {}},
+	}
+	data, err := input.MarshalMsg(nil)
+	if err != nil {
+		panic(err)
+	}
+	if size := input.Msgsize(); size < len(data) {
+		panic(fmt.Sprintf("Msgsize %d is smaller than encoded length %d", size, len(data)))
+	}
+	var decoded Collection
+	if rest, err := decoded.UnmarshalMsg(data); err != nil || len(rest) != 0 {
+		panic(fmt.Sprintf("UnmarshalMsg: rest %x, error %v", rest, err))
+	}
+	if !reflect.DeepEqual(input, decoded) {
+		panic(fmt.Sprintf("UnmarshalMsg got %#v, want %#v", decoded, input))
+	}
+
+	var stream bytes.Buffer
+	if err := msgp.Encode(&stream, &input); err != nil {
+		panic(err)
+	}
+	if !bytes.Equal(stream.Bytes(), data) {
+		panic("stream encoding differs from MarshalMsg")
+	}
+	var streamed Collection
+	if err := msgp.Decode(&stream, &streamed); err != nil {
+		panic(err)
+	}
+	if !reflect.DeepEqual(input, streamed) {
+		panic(fmt.Sprintf("DecodeMsg got %#v, want %#v", streamed, input))
+	}
+}
+`

gen/size.go

@@ -324,6 +324,9 @@ func fixedsizeExpr(e Elem) (string, bool) {
 			strbody = msgp.AppendString(strbody[:0], f.FieldTag)
 			hdrlen += len(strbody)
 		}
+		if str == "" {
+			return strconv.Itoa(hdrlen), true
+		}
 		return fmt.Sprintf("%d + %s", hdrlen, str), true
 	}
 	return "", false