put nil preserve logic behind a field tag

gen.go

@@ -95,8 +95,9 @@ type Field struct {
 	Pkg     string
 	Const   *string
 
-	OmitEmpty bool
-	IterLabel string
+	OmitEmpty   bool
+	PreserveNil bool
+	IterLabel   string
 
 	MaxLen int
 }
@@ -226,16 +227,18 @@ func ParseTypeInfo(i interface{}) (*GenTypeInfo, error) {
 		}
 
 		_, omitempty := tags["omitempty"]
+		_, preservenil := tags["preservenil"]
 
 		out.Fields = append(out.Fields, Field{
-			Name:      f.Name,
-			MapKey:    mapk,
-			Pointer:   pointer,
-			Type:      ft,
-			Pkg:       pkg,
-			OmitEmpty: omitempty,
-			MaxLen:    usrMaxLen,
-			Const:     constval,
+			Name:        f.Name,
+			MapKey:      mapk,
+			Pointer:     pointer,
+			Type:        ft,
+			Pkg:         pkg,
+			OmitEmpty:   omitempty,
+			PreserveNil: preservenil,
+			MaxLen:      usrMaxLen,
+			Const:       constval,
 		})
 	}
 
@@ -259,6 +262,8 @@ func tagparse(v string) (map[string]string, error) {
 			out[strings.TrimSpace(parts[0])] = strings.TrimSpace(parts[1])
 		} else if elem == "omitempty" {
 			out["omitempty"] = "true"
+		} else if elem == "preservenil" {
+			out["preservenil"] = "true"
 		} else if elem == "ignore" || elem == "-" {
 			out["ignore"] = "true"
 		} else {
@@ -528,18 +533,22 @@ func emitCborMarshalSliceField(w io.Writer, f Field) error {
 		return xerrors.Errorf("Byte array in field {{ .Name }} was too long")
 	}
 
+	{{ if .PreserveNil }}
 	if {{ .Name }} == nil {
 		_, err := w.Write(cbg.CborNull)
 		if err != nil {
 			return err
 		}
 	} else {
+	{{ end }}
 		{{ MajorType "cw" "cbg.MajByteString" (print "len(" .Name ")" ) }}
 
 		if _, err := cw.Write({{ .Name }}[:]); err != nil {
 			return err
 		}
+	{{ if .PreserveNil }}
 	}
+	{{ end }}
 `)
 	}
 
@@ -554,12 +563,14 @@ func emitCborMarshalSliceField(w io.Writer, f Field) error {
 		return xerrors.Errorf("Slice value in field {{ .Name }} was too long")
 	}
 
+	{{ if .PreserveNil }}
 	if {{ .Name }} == nil {
 		_, err := w.Write(cbg.CborNull)
 		if err != nil {
 			return err
 		}
 	} else {
+	{{ end }}
 		{{ MajorType "cw" "cbg.MajArray" ( print "len(" .Name ")" ) }}
 		for _, v := range {{ .Name }} {`)
 	if err != nil {
@@ -588,7 +599,15 @@ func emitCborMarshalSliceField(w io.Writer, f Field) error {
 	}
 
 	// array end
-	fmt.Fprintf(w, "\t\t}\n\t}\n")
+	if err := doTemplate(w, f, `
+	{{ if .PreserveNil }}
+		}
+	{{ end }}
+	}
+`); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -1099,6 +1118,7 @@ func emitCborUnmarshalSliceField(w io.Writer, f Field) error {
 	}
 
 	err := doTemplate(w, f, `
+	{{ if .PreserveNil }}
 	{
 		b, err := cr.ReadByte()
 		if err != nil {
@@ -1109,6 +1129,7 @@ func emitCborUnmarshalSliceField(w io.Writer, f Field) error {
 				return err
 			}
 
+	{{ end }}
 			maj, extra, err = {{ ReadHeader "cr" }}
 			if err != nil {
 				return err
@@ -1126,12 +1147,21 @@ func emitCborUnmarshalSliceField(w io.Writer, f Field) error {
 			if maj != cbg.MajByteString {
 				return fmt.Errorf("expected byte array")
 			}
+	{{ if .PreserveNil }}
 			{{ .Name }} = make({{ .TypeName }}, extra)
+	{{ else }}
+			if extra > 0 {
+				{{ .Name }} = make({{ .TypeName }}, extra)
+			}
+	{{ end }}
 			if _, err := io.ReadFull(cr, {{ .Name }}[:]); err != nil {
 				return err
 			}
+
+	{{ if .PreserveNil }}
 		}
 	}
+	{{ end }}
 `)
 	}
 
@@ -1147,7 +1177,13 @@ func emitCborUnmarshalSliceField(w io.Writer, f Field) error {
 	if maj != cbg.MajArray {
 		return fmt.Errorf("expected cbor array")
 	}
+	{{ if .PreserveNil }}
 	{{ .Name }} = make({{ .TypeName }}, extra)
+	{{ else }}
+	if extra > 0 {
+		{{ .Name }} = make({{ .TypeName }}, extra)
+	}
+	{{ end }}
 	for {{ .IterLabel }} := 0; {{ .IterLabel }} < int(extra); {{ .IterLabel }}++ {
 `)
 	if err != nil {
@@ -1228,8 +1264,10 @@ func emitCborUnmarshalSliceField(w io.Writer, f Field) error {
 	}
 
 	if err := doTemplate(w, f, `
+	{{ if .PreserveNil }}
 				}
 			}
+	{{ end }}
 		}
 	}
 	`); err != nil {

testgen/main.go

@@ -28,6 +28,7 @@ func main() {
 		types.TestConstField{},
 		types.TestCanonicalFieldOrder{},
 		types.MapStringString{},
+		types.TestSliceNilPreserve{},
 	); err != nil {
 		panic(err)
 	}