builtin: add a proper str() method to IError (#9379)

vlib/builtin/option.v

@@ -16,6 +16,14 @@ pub:
 	code int
 }
 
+pub fn (err IError) str() string {
+	return match err {
+		None__ { 'none' }
+		Error { err.msg }
+		else { '$err.type_name(): $err.msg' }
+	}
+}
+
 const none__ = IError(&None__{})
 
 struct None__ {

vlib/v/gen/c/auto_str_methods.v

@@ -151,7 +151,7 @@ fn (mut g Gen) gen_str_for_option(typ table.Type, styp string, str_fn_name strin
 		g.auto_str_funcs.writeln('\t\tres = ${parent_str_fn_name}(*($sym.cname*)it.data);')
 	}
 	g.auto_str_funcs.writeln('\t} else {')
-	g.auto_str_funcs.writeln('\t\tres = _STR("error: %.*s\\000", 2, indent_IError_str(it.err, indent_count));')
+	g.auto_str_funcs.writeln('\t\tres = _STR("error: %.*s\\000", 2, IError_str(it.err));')
 	g.auto_str_funcs.writeln('\t}')
 	g.auto_str_funcs.writeln('\treturn _STR("Option(%.*s\\000)", 2, res);')
 	g.auto_str_funcs.writeln('}')

vlib/v/tests/str_gen_test.v

@@ -296,12 +296,7 @@ fn create_option_err() ?string {
 }
 
 fn test_option_err() {
-	assert '$create_option_err()' == "
+	assert '$create_option_err()' == "Option(error: this is an error)"
-Option(error: IError(Error{
-    msg: 'this is an error'
-    code: 0
-}))
-".trim_space()
 }
 
 fn create_option_none() ?string {
@@ -309,7 +304,7 @@ fn create_option_none() ?string {
 }
 
 fn test_option_none() {
-	assert '$create_option_none()' == 'Option(error: IError(none))'
+	assert '$create_option_none()' == 'Option(error: none)'
 }
 
 fn create_option_string() ?string {