Fix encoding/decoding fixed byte arrays

gen.go

@@ -369,6 +369,7 @@ func emitCborMarshalSliceField(w io.Writer, f Field) error {
 	}
 	e := f.Type.Elem()
 
+	// Note: this re-slices the slice to deal with arrays.
 	if e.Kind() == reflect.Uint8 {
 		return doTemplate(w, f, `
 	if len({{ .Name }}) > cbg.ByteArrayMaxLen {
@@ -377,7 +378,7 @@ func emitCborMarshalSliceField(w io.Writer, f Field) error {
 
 	{{ MajorType "w" "cbg.MajByteString" (print "len(" .Name ")" ) }}
 
-	if _, err := w.Write({{ .Name }}); err != nil {
+	if _, err := w.Write({{ .Name }}[:]); err != nil {
 		return err
 	}
 `)
@@ -858,8 +859,18 @@ func emitCborUnmarshalSliceField(w io.Writer, f Field) error {
 	if maj != cbg.MajByteString {
 		return fmt.Errorf("expected byte array")
 	}
-	{{ .Name }} = make([]byte, extra)
-	if _, err := io.ReadFull(br, {{ .Name }}); err != nil {
+	{{if .IsArray}}
+	if extra != {{ .Len }} {
+		return fmt.Errorf("expected array to have {{ .Len }} elements")
+	}
+
+	{{ .Name }} = {{ .TypeName }}{}
+	{{else}}
+	if extra > 0 {
+		{{ .Name }} = make({{ .TypeName }}, extra)
+	}
+	{{end}}
+	if _, err := io.ReadFull(br, {{ .Name }}[:]); err != nil {
 		return err
 	}
 `)

testgen/main.go

@@ -11,6 +11,7 @@ func main() {
 		types.SimpleTypeOne{},
 		types.SimpleTypeTwo{},
 		types.DeferredContainer{},
+		types.FixedArrays{},
 	); err != nil {
 		panic(err)
 	}

testing/roundtrip_test.go

@@ -105,3 +105,9 @@ func TestNilValueDeferredUnmarshaling(t *testing.T) {
 		t.Fatal("shouldnt be nil!")
 	}
 }
+
+func TestFixedArrays(t *testing.T) {
+	zero := &FixedArrays{}
+	recepticle := &FixedArrays{}
+	testValueRoundtrip(t, zero, recepticle)
+}

testing/types.go

@@ -46,3 +46,9 @@ type DeferredContainer struct {
 	Deferred *cbg.Deferred
 	Value    uint64
 }
+
+type FixedArrays struct {
+	Bytes  [20]byte
+	Uint8  [20]uint8
+	Uint64 [20]uint64
+}