json: fix ptr field access (#17690)

vlib/json/json_encode_with_mut_test.v

@@ -0,0 +1,42 @@
+module main
+
+import json
+
+pub enum PlatformType {
+	unknown
+	osx
+	ubuntu
+	alpine
+}
+
+pub enum CPUType {
+	unknown
+	intel
+	arm
+	intel32
+	arm32
+}
+
+[heap]
+pub struct Node {
+pub:
+	name string = 'mymachine'
+pub mut:
+	platform    PlatformType
+	cputype     CPUType
+	done        map[string]string
+	environment map[string]string
+}
+
+pub fn (mut node Node) save() ! {
+	data := json.encode(node)
+	dump(data)
+}
+
+fn test_encode_with_mut_struct() {
+	mut n := Node{
+		platform: .osx
+		cputype: .unknown
+	}
+	n.save() or { panic(err) }
+}

vlib/v/gen/c/json.v

@@ -520,21 +520,25 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 			}
 		}
 		if field_sym.kind == .enum_ {
-			enc.writeln('\tcJSON_AddItemToObject(o, "${name}", json__encode_u64(${prefix_enc}.${c_name(field.name)}));\n')
+			enc.writeln('\tcJSON_AddItemToObject(o, "${name}", json__encode_u64(${prefix_enc}${op}${c_name(field.name)}));\n')
 		} else {
 			if field_sym.name == 'time.Time' {
 				// time struct requires special treatment
 				// it has to be encoded as a unix timestamp number
-				enc.writeln('\tcJSON_AddItemToObject(o, "${name}", json__encode_u64(${prefix_enc}.${c_name(field.name)}._v_unix));')
+				enc.writeln('\tcJSON_AddItemToObject(o, "${name}", json__encode_u64(${prefix_enc}${op}${c_name(field.name)}._v_unix));')
 			} else {
 				if !field.typ.is_real_pointer() {
 					enc.writeln('\tcJSON_AddItemToObject(o, "${name}", ${enc_name}(${prefix_enc}${op}${c_name(field.name)})); /*A*/')
 				} else {
 					arg_prefix := if field.typ.is_ptr() { '' } else { '*' }
 					sptr_value := '${prefix_enc}${op}${c_name(field.name)}'
+					if !field.typ.has_flag(.option) {
 						enc.writeln('\tif (${sptr_value} != 0) {')
 						enc.writeln('\t\tcJSON_AddItemToObject(o, "${name}", ${enc_name}(${arg_prefix}${sptr_value}));')
 						enc.writeln('\t}\n')
+					} else {
+						enc.writeln('\t\tcJSON_AddItemToObject(o, "${name}", ${enc_name}(${arg_prefix}${sptr_value}));')
+					}
 				}
 			}
 		}