json: fix codegen for struct default expr (fix #23216) (#23233)

vlib/json/tests/json_decode_struct_with_default_test.v

@@ -0,0 +1,34 @@
+import json
+
+pub struct Response {
+pub:
+	results []Result = []Result{len: 0} @[json: list]
+	tags    []string = []string{len: 0} @[json: tags]
+	kind    string   @[json: result_type]
+}
+
+pub struct Result {
+pub:
+	id            int      @[json: defid]
+	author        string   @[json: author]
+	definition    string   @[json: definition]
+	link          string   @[json: permalink]
+	thumbs_down   int      @[json: thumbs_down]
+	thumbs_up     int      @[json: thumbs_up]
+	word          string   @[json: word]
+	date          string   @[json: written_on]
+	audio_samples []string @[json: sound_urls]
+	example       string   @[json: example]
+}
+
+pub fn define(word string) !&Response {
+	resp := '{"list":[{"defid":3439287}]}'
+	response := json.decode(Response, resp) or { return err }
+	return &response
+}
+
+fn test_main() {
+	response := define('')!
+	assert response.results.len > 0
+	assert response.results[0].id == 3439287
+}

vlib/v/gen/c/json.v

@@ -850,7 +850,14 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 				}
 				if field.has_default_expr {
 					dec.writeln('\t} else {')
-					dec.writeln('\t\t${prefix}${op}${c_name(field.name)} = ${g.expr_string(field.default_expr)};')
+					default_str := g.expr_string(field.default_expr)
+					lines := default_str.count('\n')
+					if lines > 1 {
+						dec.writeln(default_str.all_before_last('\n'))
+						dec.writeln('\t\t${prefix}${op}${c_name(field.name)} = ${default_str.all_after_last('\n')};')
+					} else {
+						dec.writeln('\t\t${prefix}${op}${c_name(field.name)} = ${default_str};')
+					}
 				}
 				dec.writeln('\t}')
 			}