preserve nil maps and slices in generated Marshal* methods

genmethod.go

@@ -206,7 +206,15 @@ func (m *marshalMethod) marshalConversions(from, to Var, format string) (s []Sta
 	for _, f := range m.mtyp.Fields {
 		accessFrom := Dotted{Receiver: from, Name: f.name}
 		accessTo := Dotted{Receiver: to, Name: f.name}
-		s = append(s, m.convert(accessFrom, accessTo, f.origTyp, f.typ)...)
+		conversion := m.convert(accessFrom, accessTo, f.origTyp, f.typ)
+		if underlyingMap(f.origTyp) != nil || underlyingSlice(f.origTyp) != nil {
+			// Preserve nil maps and slices.
+			conversion = []Statement{If{
+				Condition: NotEqual{Lhs: accessFrom, Rhs: NIL},
+				Body:      conversion,
+			}}
+		}
+		s = append(s, conversion...)
 	}
 	return s
 }

internal/tests/mapconv/output.go

@@ -13,14 +13,18 @@ func (x *X) MarshalJSON() ([]byte, error) {
 		Named namedMap2
 	}
 	var enc XJSON
+	if x.Map != nil {
 		enc.Map = make(map[replacedString]replacedInt, len(x.Map))
 		for k, v := range x.Map {
 			enc.Map[replacedString(k)] = replacedInt(v)
 		}
+	}
+	if x.Named != nil {
 		enc.Named = make(namedMap2, len(x.Named))
 		for k, v := range x.Named {
 			enc.Named[replacedString(k)] = replacedInt(v)
 		}
+	}
 	return json.Marshal(&enc)
 }
 
@@ -58,14 +62,18 @@ func (x *X) MarshalYAML() (interface{}, error) {
 		Named namedMap2
 	}
 	var enc XYAML
+	if x.Map != nil {
 		enc.Map = make(map[replacedString]replacedInt, len(x.Map))
 		for k, v := range x.Map {
 			enc.Map[replacedString(k)] = replacedInt(v)
 		}
+	}
+	if x.Named != nil {
 		enc.Named = make(namedMap2, len(x.Named))
 		for k, v := range x.Named {
 			enc.Named[replacedString(k)] = replacedInt(v)
 		}
+	}
 	return &enc, nil
 }
 

internal/tests/sliceconv/output.go

@@ -13,14 +13,18 @@ func (x *X) MarshalJSON() ([]byte, error) {
 		Named namedSlice2
 	}
 	var enc XJSON
+	if x.Slice != nil {
 		enc.Slice = make([]replacedInt, len(x.Slice))
 		for k, v := range x.Slice {
 			enc.Slice[k] = replacedInt(v)
 		}
+	}
+	if x.Named != nil {
 		enc.Named = make(namedSlice2, len(x.Named))
 		for k, v := range x.Named {
 			enc.Named[k] = replacedInt(v)
 		}
+	}
 	return json.Marshal(&enc)
 }
 
@@ -58,14 +62,18 @@ func (x *X) MarshalYAML() (interface{}, error) {
 		Named namedSlice2
 	}
 	var enc XYAML
+	if x.Slice != nil {
 		enc.Slice = make([]replacedInt, len(x.Slice))
 		for k, v := range x.Slice {
 			enc.Slice[k] = replacedInt(v)
 		}
+	}
+	if x.Named != nil {
 		enc.Named = make(namedSlice2, len(x.Named))
 		for k, v := range x.Named {
 			enc.Named[k] = replacedInt(v)
 		}
+	}
 	return &enc, nil
 }