cgen: fix json encode struct with optional field (#16866)

vlib/json/json_encode_struct_with_optional_field_test.v

@@ -0,0 +1,20 @@
+import json
+
+struct Foo {
+	name string
+	num  ?int
+}
+
+fn test_json_encode_struct_with_optional_field() {
+	f1 := Foo{
+		name: 'hello'
+	}
+	ret1 := json.encode(f1)
+	println(ret1)
+	assert ret1 == '{"name":"hello","num":null}'
+
+	f2 := Foo{'hello', 22}
+	ret2 := json.encode(f2)
+	println(ret2)
+	assert ret2 == '{"name":"hello","num":22}'
+}

vlib/v/gen/c/cgen.v

@@ -1061,7 +1061,11 @@ fn (mut g Gen) optional_type_name(t ast.Type) (string, string) {
 }
 
 fn (mut g Gen) result_type_name(t ast.Type) (string, string) {
-	base := g.base_type(t)
+	mut base := g.base_type(t)
+	if t.has_flag(.optional) {
+		g.register_optional(t)
+		base = '_option_' + base
+	}
 	mut styp := ''
 	sym := g.table.sym(t)
 	if sym.language == .c && sym.kind == .struct_ {

vlib/v/gen/c/json.v

@@ -26,7 +26,7 @@ import strings
 fn (mut g Gen) gen_json_for_type(typ ast.Type) {
 	utyp := g.unwrap_generic(typ).set_nr_muls(0)
 	sym := g.table.sym(utyp)
-	if is_js_prim(sym.name) || sym.kind == .enum_ {
+	if (is_js_prim(sym.name) && !utyp.has_flag(.optional)) || sym.kind == .enum_ {
 		return
 	}
 	g.json_types << utyp
@@ -54,10 +54,14 @@ fn (mut g Gen) gen_jsons() {
 		mut init_styp := '${styp} res'
 		if sym.kind == .struct_ {
 			init_styp += ' = '
-			init_styp += g.expr_string(ast.Expr(ast.StructInit{
+			g.set_current_pos_as_last_stmt_pos()
+			pos := g.out.len
+			g.write(init_styp)
+			g.expr(ast.Expr(ast.StructInit{
 				typ: utyp
 				typ_str: styp
 			}))
+			init_styp = g.out.cut_to(pos).trim_space()
 		}
 
 		dec.writeln('
@@ -144,6 +148,8 @@ ${enc_fn_dec} {
 				verror('json: ${sym.name} is not a sumtype')
 			}
 			g.gen_sumtype_enc_dec(sym, mut enc, mut dec)
+		} else if utyp.has_flag(.optional) {
+			g.gen_option_enc_dec(utyp, mut enc, mut dec)
 		} else {
 			enc.writeln('\to = cJSON_CreateObject();')
 			// Structs. Range through fields
@@ -162,6 +168,15 @@ ${enc_fn_dec} {
 	}
 }
 
+fn (mut g Gen) gen_option_enc_dec(typ ast.Type, mut enc strings.Builder, mut dec strings.Builder) {
+	enc.writeln('\tif (val.state == 2) {')
+	enc.writeln('\t\treturn cJSON_CreateNull();')
+	enc.writeln('\t}')
+	type_str := g.typ(typ.clear_flag(.optional))
+	encode_name := js_enc_name(type_str)
+	enc.writeln('\to = ${encode_name}(*(${type_str}*)val.data);')
+}
+
 [inline]
 fn (mut g Gen) gen_sumtype_enc_dec(sym ast.TypeSymbol, mut enc strings.Builder, mut dec strings.Builder) {
 	info := sym.info as ast.SumType