json: use @[required] to disallow parsing nulls (#23218)

doc/docs.md

@@ -4692,7 +4692,7 @@ struct User {
 	last_name string @[json: lastName]
 }
 
-data := '{ "name": "Frodo", "lastName": "Baggins", "age": 25 }'
+data := '{ "name": "Frodo", "lastName": "Baggins", "age": 25, "nullable": null }'
 user := json.decode(User, data) or {
 	eprintln('Failed to decode json, error: ${err}')
 	return

vlib/json/tests/json_test.v

@@ -516,12 +516,9 @@ fn test_encode_alias_field() {
 			a: 1
 		}
 	})
-	println(s)
 	assert s == '{"sub":{"a":1}}'
 }
 
-//
-
 struct APrice {}
 
 struct Association {
@@ -538,3 +535,55 @@ fn test_encoding_struct_with_pointers() {
 	}
 	assert json.encode(value) == '{"association":{"price":{}},"price":{}}'
 }
+
+struct ChildNullish {
+	name string
+}
+
+struct NullishStruct {
+	name     string
+	lastname string
+	age      int
+	salary   f32
+	child    ChildNullish
+}
+
+struct RequiredStruct {
+	name     string @[required]
+	lastname string
+}
+
+fn test_required() {
+	nullish_one := json.decode(NullishStruct, '{"name":"Peter", "lastname":null, "age":28,"salary":95000.5,"title":"worker"}')!
+	assert nullish_one.name == 'Peter'
+	assert nullish_one.lastname == ''
+	assert nullish_one.age == 28
+	assert nullish_one.salary == 95000.5
+	assert nullish_one.child.name == ''
+
+	nullish_two := json.decode(NullishStruct, '{"name":"Peter", "lastname": "Parker", "age":28,"salary":95000.5,"title":"worker"}')!
+	assert nullish_two.name == 'Peter'
+	assert nullish_two.lastname == 'Parker'
+	assert nullish_two.age == 28
+	assert nullish_two.salary == 95000.5
+	assert nullish_two.child.name == ''
+
+	nullish_third := json.decode(NullishStruct, '{"name":"Peter", "age":28,"salary":95000.5,"title":"worker", "child": {"name":"Nullish"}}')!
+	assert nullish_third.name == 'Peter'
+	assert nullish_third.lastname == ''
+	assert nullish_third.age == 28
+	assert nullish_third.salary == 95000.5
+	assert nullish_third.child.name == 'Nullish'
+
+	required_struct := json.decode(RequiredStruct, '{"name":"Peter", "lastname": "Parker"}')!
+	assert required_struct.name == 'Peter'
+	assert required_struct.lastname == 'Parker'
+
+	required_struct_err := json.decode(RequiredStruct, '{"name": null, "lastname": "Parker"}') or {
+		assert err.msg() == "type mismatch for field 'name', expecting `string` type, got: null"
+		RequiredStruct{
+			name:     'Peter'
+			lastname: 'Parker'
+		}
+	}
+}

vlib/v/gen/c/json.v

@@ -627,8 +627,12 @@ fn (mut g Gen) gen_sumtype_enc_dec(utyp ast.Type, sym ast.TypeSymbol, mut enc st
 	}
 }
 
-fn (mut g Gen) gen_prim_type_validation(name string, typ ast.Type, tmp string, ret_styp string, mut dec strings.Builder) {
-	none_check := if typ.has_flag(.option) { 'cJSON_IsNull(jsonroot_${tmp}) || ' } else { '' }
+fn (mut g Gen) gen_prim_type_validation(name string, typ ast.Type, tmp string, is_required bool, ret_styp string, mut dec strings.Builder) {
+	none_check := if !is_required {
+		'cJSON_IsNull(jsonroot_${tmp}) || '
+	} else {
+		''
+	}
 	type_check := if typ.is_int() || typ.is_float() {
 		'${none_check}cJSON_IsNumber(jsonroot_${tmp}) || (cJSON_IsString(jsonroot_${tmp}) && strlen(jsonroot_${tmp}->valuestring))'
 	} else if typ.is_string() {
@@ -724,7 +728,7 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 				if utyp.has_flag(.option) {
 					dec.writeln('\t\tres.state = 0;')
 				}
-				g.gen_prim_type_validation(field.name, field.typ, tmp, '${result_name}_${styp}', mut
+				g.gen_prim_type_validation(field.name, field.typ, tmp, is_required, '${result_name}_${styp}', mut
 					dec)
 				dec.writeln('\t\t${prefix}${op}${c_name(field.name)} = ${dec_name}(jsonroot_${tmp});')
 				if field.has_default_expr {
@@ -791,8 +795,8 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 					tmp := g.new_tmp_var()
 					gen_js_get(styp, tmp, name, mut dec, is_required)
 					dec.writeln('\tif (jsonroot_${tmp}) {')
-					g.gen_prim_type_validation(field.name, parent_type, tmp, '${result_name}_${styp}', mut
-						dec)
+					g.gen_prim_type_validation(field.name, parent_type, tmp, is_required,
+						'${result_name}_${styp}', mut dec)
 					dec.writeln('\t\t${prefix}${op}${c_name(field.name)} = ${parent_dec_name} (jsonroot_${tmp});')
 					if field.has_default_expr {
 						dec.writeln('\t} else {')
@@ -832,8 +836,8 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 				gen_js_get_opt(dec_name, field_type, styp, tmp, name, mut dec, is_required)
 				dec.writeln('\tif (jsonroot_${tmp}) {')
 				if is_js_prim(g.styp(field.typ.clear_option_and_result())) {
-					g.gen_prim_type_validation(field.name, field.typ, tmp, '${result_name}_${styp}', mut
-						dec)
+					g.gen_prim_type_validation(field.name, field.typ, tmp, is_required,
+						'${result_name}_${styp}', mut dec)
 				}
 				if field.typ.has_flag(.option) {
 					dec.writeln('\t\tvmemcpy(&${prefix}${op}${c_name(field.name)}, (${field_type}*)${tmp}.data, sizeof(${field_type}));')