diff --git a/Grammar/python.gram b/Grammar/python.gram
index d2d6fc0d339..19c85accf8d 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -551,7 +551,7 @@ yield_expr[expr_ty]:
 
 arguments[expr_ty] (memo):
     | a=args [','] &')' { a }
-    | incorrect_arguments
+    | invalid_arguments
 args[expr_ty]:
     | a[asdl_expr_seq*]=','.(starred_expression | named_expression !'=')+ b=[',' k=kwargs {k}] { _PyPegen_collect_call_seqs(p, a, b, EXTRA) }
     | a=kwargs { _Py_Call(_PyPegen_dummy_name(p),
@@ -637,7 +637,7 @@ t_atom[expr_ty]:
 
 
 # From here on, there are rules for invalid syntax with specialised error messages
-incorrect_arguments:
+invalid_arguments:
     | args ',' '*' { RAISE_SYNTAX_ERROR("iterable argument unpacking follows keyword argument unpacking") }
     | a=expression for_if_clauses ',' [args | expression for_if_clauses] {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "Generator expression must be parenthesized") }
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-10-23-02-43-24.bpo-42123.64gJWC.rst b/Misc/NEWS.d/next/Core and Builtins/2020-10-23-02-43-24.bpo-42123.64gJWC.rst
new file mode 100644
index 00000000000..6461efd76f0
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-10-23-02-43-24.bpo-42123.64gJWC.rst	
@@ -0,0 +1,3 @@
+Run the parser two times. On the first run, disable all the rules that only
+generate better error messages to gain performance. If there's a parse
+failure, run the parser a second time with those enabled.
diff --git a/Parser/parser.c b/Parser/parser.c
index 09be03c633a..e438f06c9be 100644
--- a/Parser/parser.c
+++ b/Parser/parser.c
@@ -209,7 +209,7 @@ static KeywordToken *reserved_keywords[] = {
 #define t_primary_type 1140  // Left-recursive
 #define t_lookahead_type 1141
 #define t_atom_type 1142
-#define incorrect_arguments_type 1143
+#define invalid_arguments_type 1143
 #define invalid_kwarg_type 1144
 #define invalid_named_expression_type 1145
 #define invalid_assignment_type 1146
@@ -527,7 +527,7 @@ static expr_ty target_rule(Parser *p);
 static expr_ty t_primary_rule(Parser *p);
 static void *t_lookahead_rule(Parser *p);
 static expr_ty t_atom_rule(Parser *p);
-static void *incorrect_arguments_rule(Parser *p);
+static void *invalid_arguments_rule(Parser *p);
 static void *invalid_kwarg_rule(Parser *p);
 static void *invalid_named_expression_rule(Parser *p);
 static void *invalid_assignment_rule(Parser *p);
@@ -2218,7 +2218,7 @@ assignment_rule(Parser *p)
             return NULL;
         }
     }
-    { // invalid_assignment
+    if (p->call_invalid_rules) { // invalid_assignment
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -2891,7 +2891,7 @@ del_stmt_rule(Parser *p)
         D(fprintf(stderr, "%*c%s del_stmt[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "'del' del_targets &(';' | NEWLINE)"));
     }
-    { // invalid_del_stmt
+    if (p->call_invalid_rules) { // invalid_del_stmt
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -3242,7 +3242,7 @@ import_from_targets_rule(Parser *p)
         D(fprintf(stderr, "%*c%s import_from_targets[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "'*'"));
     }
-    { // invalid_import_from_targets
+    if (p->call_invalid_rules) { // invalid_import_from_targets
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -4035,7 +4035,7 @@ for_stmt_rule(Parser *p)
             return NULL;
         }
     }
-    { // invalid_for_target
+    if (p->call_invalid_rules) { // invalid_for_target
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -4336,7 +4336,7 @@ with_item_rule(Parser *p)
         D(fprintf(stderr, "%*c%s with_item[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "expression 'as' star_target &(',' | ')' | ':')"));
     }
-    { // invalid_with_item
+    if (p->call_invalid_rules) { // invalid_with_item
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -5071,7 +5071,7 @@ func_type_comment_rule(Parser *p)
         D(fprintf(stderr, "%*c%s func_type_comment[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "NEWLINE TYPE_COMMENT &(NEWLINE INDENT)"));
     }
-    { // invalid_double_type_comments
+    if (p->call_invalid_rules) { // invalid_double_type_comments
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -5126,7 +5126,7 @@ params_rule(Parser *p)
     }
     arguments_ty _res = NULL;
     int _mark = p->mark;
-    { // invalid_parameters
+    if (p->call_invalid_rules) { // invalid_parameters
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -5601,7 +5601,7 @@ star_etc_rule(Parser *p)
         D(fprintf(stderr, "%*c%s star_etc[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "kwds"));
     }
-    { // invalid_star_etc
+    if (p->call_invalid_rules) { // invalid_star_etc
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -6304,7 +6304,7 @@ block_rule(Parser *p)
         D(fprintf(stderr, "%*c%s block[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "simple_stmt"));
     }
-    { // invalid_block
+    if (p->call_invalid_rules) { // invalid_block
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -6798,7 +6798,7 @@ named_expression_rule(Parser *p)
         D(fprintf(stderr, "%*c%s named_expression[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "expression !':='"));
     }
-    { // invalid_named_expression
+    if (p->call_invalid_rules) { // invalid_named_expression
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -7192,7 +7192,7 @@ lambda_params_rule(Parser *p)
     }
     arguments_ty _res = NULL;
     int _mark = p->mark;
-    { // invalid_lambda_parameters
+    if (p->call_invalid_rules) { // invalid_lambda_parameters
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -7669,7 +7669,7 @@ lambda_star_etc_rule(Parser *p)
         D(fprintf(stderr, "%*c%s lambda_star_etc[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "lambda_kwds"));
     }
-    { // invalid_lambda_star_etc
+    if (p->call_invalid_rules) { // invalid_lambda_star_etc
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11163,7 +11163,7 @@ listcomp_rule(Parser *p)
             return NULL;
         }
     }
-    { // invalid_comprehension
+    if (p->call_invalid_rules) { // invalid_comprehension
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11294,7 +11294,7 @@ group_rule(Parser *p)
         D(fprintf(stderr, "%*c%s group[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "'(' (yield_expr | named_expression) ')'"));
     }
-    { // invalid_group
+    if (p->call_invalid_rules) { // invalid_group
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11388,7 +11388,7 @@ genexp_rule(Parser *p)
             return NULL;
         }
     }
-    { // invalid_comprehension
+    if (p->call_invalid_rules) { // invalid_comprehension
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11547,7 +11547,7 @@ setcomp_rule(Parser *p)
             return NULL;
         }
     }
-    { // invalid_comprehension
+    if (p->call_invalid_rules) { // invalid_comprehension
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11699,7 +11699,7 @@ dictcomp_rule(Parser *p)
         D(fprintf(stderr, "%*c%s dictcomp[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "'{' kvpair for_if_clauses '}'"));
     }
-    { // invalid_dict_comprehension
+    if (p->call_invalid_rules) { // invalid_dict_comprehension
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -12023,7 +12023,7 @@ for_if_clause_rule(Parser *p)
             return NULL;
         }
     }
-    { // invalid_for_target
+    if (p->call_invalid_rules) { // invalid_for_target
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -12149,7 +12149,7 @@ yield_expr_rule(Parser *p)
     return _res;
 }
 
-// arguments: args ','? &')' | incorrect_arguments
+// arguments: args ','? &')' | invalid_arguments
 static expr_ty
 arguments_rule(Parser *p)
 {
@@ -12194,24 +12194,24 @@ arguments_rule(Parser *p)
         D(fprintf(stderr, "%*c%s arguments[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "args ','? &')'"));
     }
-    { // incorrect_arguments
+    if (p->call_invalid_rules) { // invalid_arguments
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
         }
-        D(fprintf(stderr, "%*c> arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "incorrect_arguments"));
-        void *incorrect_arguments_var;
+        D(fprintf(stderr, "%*c> arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "invalid_arguments"));
+        void *invalid_arguments_var;
         if (
-            (incorrect_arguments_var = incorrect_arguments_rule(p))  // incorrect_arguments
+            (invalid_arguments_var = invalid_arguments_rule(p))  // invalid_arguments
         )
         {
-            D(fprintf(stderr, "%*c+ arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "incorrect_arguments"));
-            _res = incorrect_arguments_var;
+            D(fprintf(stderr, "%*c+ arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "invalid_arguments"));
+            _res = invalid_arguments_var;
             goto done;
         }
         p->mark = _mark;
         D(fprintf(stderr, "%*c%s arguments[%d-%d]: %s failed!\n", p->level, ' ',
-                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "incorrect_arguments"));
+                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "invalid_arguments"));
     }
     _res = NULL;
   done:
@@ -12548,7 +12548,7 @@ kwarg_or_starred_rule(Parser *p)
         D(fprintf(stderr, "%*c%s kwarg_or_starred[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "starred_expression"));
     }
-    { // invalid_kwarg
+    if (p->call_invalid_rules) { // invalid_kwarg
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -12668,7 +12668,7 @@ kwarg_or_double_starred_rule(Parser *p)
         D(fprintf(stderr, "%*c%s kwarg_or_double_starred[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "'**' expression"));
     }
-    { // invalid_kwarg
+    if (p->call_invalid_rules) { // invalid_kwarg
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -14380,14 +14380,14 @@ t_atom_rule(Parser *p)
     return _res;
 }
 
-// incorrect_arguments:
+// invalid_arguments:
 //     | args ',' '*'
 //     | expression for_if_clauses ',' [args | expression for_if_clauses]
 //     | args for_if_clauses
 //     | args ',' expression for_if_clauses
 //     | args ',' args
 static void *
-incorrect_arguments_rule(Parser *p)
+invalid_arguments_rule(Parser *p)
 {
     D(p->level++);
     if (p->error_indicator) {
@@ -14401,7 +14401,7 @@ incorrect_arguments_rule(Parser *p)
             D(p->level--);
             return NULL;
         }
-        D(fprintf(stderr, "%*c> incorrect_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "args ',' '*'"));
+        D(fprintf(stderr, "%*c> invalid_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "args ',' '*'"));
         Token * _literal;
         Token * _literal_1;
         expr_ty args_var;
@@ -14413,7 +14413,7 @@ incorrect_arguments_rule(Parser *p)
             (_literal_1 = _PyPegen_expect_token(p, 16))  // token='*'
         )
         {
-            D(fprintf(stderr, "%*c+ incorrect_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "args ',' '*'"));
+            D(fprintf(stderr, "%*c+ invalid_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "args ',' '*'"));
             _res = RAISE_SYNTAX_ERROR ( "iterable argument unpacking follows keyword argument unpacking" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
@@ -14423,7 +14423,7 @@ incorrect_arguments_rule(Parser *p)
             goto done;
         }
         p->mark = _mark;
-        D(fprintf(stderr, "%*c%s incorrect_arguments[%d-%d]: %s failed!\n", p->level, ' ',
+        D(fprintf(stderr, "%*c%s invalid_arguments[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "args ',' '*'"));
     }
     { // expression for_if_clauses ',' [args | expression for_if_clauses]
@@ -14431,7 +14431,7 @@ incorrect_arguments_rule(Parser *p)
             D(p->level--);
             return NULL;
         }
-        D(fprintf(stderr, "%*c> incorrect_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "expression for_if_clauses ',' [args | expression for_if_clauses]"));
+        D(fprintf(stderr, "%*c> invalid_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "expression for_if_clauses ',' [args | expression for_if_clauses]"));
         Token * _literal;
         void *_opt_var;
         UNUSED(_opt_var); // Silence compiler warnings
@@ -14447,7 +14447,7 @@ incorrect_arguments_rule(Parser *p)
             (_opt_var = _tmp_127_rule(p), 1)  // [args | expression for_if_clauses]
         )
         {
-            D(fprintf(stderr, "%*c+ incorrect_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "expression for_if_clauses ',' [args | expression for_if_clauses]"));
+            D(fprintf(stderr, "%*c+ invalid_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "expression for_if_clauses ',' [args | expression for_if_clauses]"));
             _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "Generator expression must be parenthesized" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
@@ -14457,7 +14457,7 @@ incorrect_arguments_rule(Parser *p)
             goto done;
         }
         p->mark = _mark;
-        D(fprintf(stderr, "%*c%s incorrect_arguments[%d-%d]: %s failed!\n", p->level, ' ',
+        D(fprintf(stderr, "%*c%s invalid_arguments[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "expression for_if_clauses ',' [args | expression for_if_clauses]"));
     }
     { // args for_if_clauses
@@ -14465,7 +14465,7 @@ incorrect_arguments_rule(Parser *p)
             D(p->level--);
             return NULL;
         }
-        D(fprintf(stderr, "%*c> incorrect_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "args for_if_clauses"));
+        D(fprintf(stderr, "%*c> invalid_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "args for_if_clauses"));
         expr_ty a;
         asdl_comprehension_seq* for_if_clauses_var;
         if (
@@ -14474,7 +14474,7 @@ incorrect_arguments_rule(Parser *p)
             (for_if_clauses_var = for_if_clauses_rule(p))  // for_if_clauses
         )
         {
-            D(fprintf(stderr, "%*c+ incorrect_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "args for_if_clauses"));
+            D(fprintf(stderr, "%*c+ invalid_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "args for_if_clauses"));
             _res = _PyPegen_nonparen_genexp_in_call ( p , a );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
@@ -14484,7 +14484,7 @@ incorrect_arguments_rule(Parser *p)
             goto done;
         }
         p->mark = _mark;
-        D(fprintf(stderr, "%*c%s incorrect_arguments[%d-%d]: %s failed!\n", p->level, ' ',
+        D(fprintf(stderr, "%*c%s invalid_arguments[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "args for_if_clauses"));
     }
     { // args ',' expression for_if_clauses
@@ -14492,7 +14492,7 @@ incorrect_arguments_rule(Parser *p)
             D(p->level--);
             return NULL;
         }
-        D(fprintf(stderr, "%*c> incorrect_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "args ',' expression for_if_clauses"));
+        D(fprintf(stderr, "%*c> invalid_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "args ',' expression for_if_clauses"));
         Token * _literal;
         expr_ty a;
         expr_ty args_var;
@@ -14507,7 +14507,7 @@ incorrect_arguments_rule(Parser *p)
             (for_if_clauses_var = for_if_clauses_rule(p))  // for_if_clauses
         )
         {
-            D(fprintf(stderr, "%*c+ incorrect_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "args ',' expression for_if_clauses"));
+            D(fprintf(stderr, "%*c+ invalid_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "args ',' expression for_if_clauses"));
             _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "Generator expression must be parenthesized" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
@@ -14517,7 +14517,7 @@ incorrect_arguments_rule(Parser *p)
             goto done;
         }
         p->mark = _mark;
-        D(fprintf(stderr, "%*c%s incorrect_arguments[%d-%d]: %s failed!\n", p->level, ' ',
+        D(fprintf(stderr, "%*c%s invalid_arguments[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "args ',' expression for_if_clauses"));
     }
     { // args ',' args
@@ -14525,7 +14525,7 @@ incorrect_arguments_rule(Parser *p)
             D(p->level--);
             return NULL;
         }
-        D(fprintf(stderr, "%*c> incorrect_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "args ',' args"));
+        D(fprintf(stderr, "%*c> invalid_arguments[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "args ',' args"));
         Token * _literal;
         expr_ty a;
         expr_ty args_var;
@@ -14537,7 +14537,7 @@ incorrect_arguments_rule(Parser *p)
             (args_var = args_rule(p))  // args
         )
         {
-            D(fprintf(stderr, "%*c+ incorrect_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "args ',' args"));
+            D(fprintf(stderr, "%*c+ invalid_arguments[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "args ',' args"));
             _res = _PyPegen_arguments_parsing_error ( p , a );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
@@ -14547,7 +14547,7 @@ incorrect_arguments_rule(Parser *p)
             goto done;
         }
         p->mark = _mark;
-        D(fprintf(stderr, "%*c%s incorrect_arguments[%d-%d]: %s failed!\n", p->level, ' ',
+        D(fprintf(stderr, "%*c%s invalid_arguments[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "args ',' args"));
     }
     _res = NULL;
diff --git a/Parser/pegen.c b/Parser/pegen.c
index c7343f7f047..827d4dace1f 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -1102,15 +1102,28 @@ _PyPegen_Parser_New(struct tok_state *tok, int start_rule, int flags,
     p->feature_version = feature_version;
     p->known_err_token = NULL;
     p->level = 0;
+    p->call_invalid_rules = 0;
 
     return p;
 }
 
+static void
+reset_parser_state(Parser *p)
+{
+    for (int i = 0; i < p->fill; i++) {
+        p->tokens[i]->memo = NULL;
+    }
+    p->mark = 0;
+    p->call_invalid_rules = 1;
+}
+
 void *
 _PyPegen_run_parser(Parser *p)
 {
     void *res = _PyPegen_parse(p);
     if (res == NULL) {
+        reset_parser_state(p);
+        _PyPegen_parse(p);
         if (PyErr_Occurred()) {
             return NULL;
         }
diff --git a/Parser/pegen.h b/Parser/pegen.h
index 9a280ae240a..841f1e5eb43 100644
--- a/Parser/pegen.h
+++ b/Parser/pegen.h
@@ -73,6 +73,7 @@ typedef struct {
     growable_comment_array type_ignore_comments;
     Token *known_err_token;
     int level;
+    int call_invalid_rules;
 } Parser;
 
 typedef struct {
diff --git a/Tools/peg_generator/pegen/c_generator.py b/Tools/peg_generator/pegen/c_generator.py
index 1a814aad11c..52bdb844e6b 100644
--- a/Tools/peg_generator/pegen/c_generator.py
+++ b/Tools/peg_generator/pegen/c_generator.py
@@ -736,7 +736,10 @@ class CParserGenerator(ParserGenerator, GrammarVisitor):
     def visit_Alt(
         self, node: Alt, is_loop: bool, is_gather: bool, rulename: Optional[str]
     ) -> None:
-        self.print(f"{{ // {node}")
+        if len(node.items) == 1 and str(node.items[0]).startswith('invalid_'):
+            self.print(f"if (p->call_invalid_rules) {{ // {node}")
+        else:
+            self.print(f"{{ // {node}")
         with self.indent():
             self._check_for_errors()
             node_str = str(node).replace('"', '\\"')