json: support @[json_null] tag to enforce null, when encoding none option values (#23319)

vlib/json/tests/json_is_null_attr_test.v

@@ -0,0 +1,26 @@
+import json
+
+struct Bar {
+	name ?string @[json_null]
+}
+
+struct Foo {
+	name   ?string @[json_null]
+	age    ?int    @[json_null]
+	text   ?string
+	other  ?Bar
+	other2 ?Bar @[json_null]
+}
+
+fn test_main() {
+	assert json.encode(Foo{}) == '{"name":null,"age":null,"other2":null}'
+	assert json.encode(Foo{ name: '' }) == '{"name":"","age":null,"other2":null}'
+	assert json.encode(Foo{ age: 10 }) == '{"name":null,"age":10,"other2":null}'
+	assert json.encode(Foo{
+		age:    10
+		other2: Bar{
+			name: none
+		}
+	}) == '{"name":null,"age":10,"other2":{"name":null}}'
+	assert json.decode(Foo, json.encode(Foo{}))! == Foo{}
+}

vlib/v/gen/c/json.v

@@ -649,6 +649,7 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 		mut is_required := false
 		mut is_omit_empty := false
 		mut skip_embed := false
+		mut is_json_null := false
 
 		for attr in field.attrs {
 			match attr.name {
@@ -672,6 +673,9 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 				'omitempty' {
 					is_omit_empty = true
 				}
+				'json_null' {
+					is_json_null = true
+				}
 				else {}
 			}
 		}
@@ -952,7 +956,11 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 		}
 
 		if is_option {
-			enc.writeln('\t} // !none')
+			if is_json_null {
+				enc.writeln('\t} else {')
+				enc.writeln('\t\tcJSON_AddItemToObject(o, "${name}", cJSON_CreateNull());')
+			}
+			enc.writeln('\t}')
 		}
 	}
 }