diff --git a/Grammar/python.gram b/Grammar/python.gram
index 6c36b6a..b709d3d 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -535,7 +535,7 @@
 
 arguments[expr_ty] (memo):
     | a=args [','] &')' { a }
-    | incorrect_arguments
+    | invalid_arguments
 args[expr_ty]:
     | a=','.(starred_expression | named_expression !'=')+ b=[',' k=kwargs {k}] { _PyPegen_collect_call_seqs(p, a, b, EXTRA) }
     | a=kwargs { _Py_Call(_PyPegen_dummy_name(p),
@@ -620,7 +620,7 @@
 
 
 # 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 0000000..6461efd
--- /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/pegen/parse.c b/Parser/pegen/parse.c
index 2eb9e03..48a2443 100644
--- a/Parser/pegen/parse.c
+++ b/Parser/pegen/parse.c
@@ -218,7 +218,7 @@
 #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
@@ -537,7 +537,7 @@
 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);
@@ -2219,7 +2219,7 @@
             return NULL;
         }
     }
-    { // invalid_assignment
+    if (p->call_invalid_rules) { // invalid_assignment
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -2892,7 +2892,7 @@
         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;
@@ -3243,7 +3243,7 @@
         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;
@@ -4036,7 +4036,7 @@
             return NULL;
         }
     }
-    { // invalid_for_target
+    if (p->call_invalid_rules) { // invalid_for_target
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -4337,7 +4337,7 @@
         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;
@@ -5072,7 +5072,7 @@
         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;
@@ -5127,7 +5127,7 @@
     }
     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;
@@ -5602,7 +5602,7 @@
         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;
@@ -6305,7 +6305,7 @@
         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;
@@ -6799,7 +6799,7 @@
         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;
@@ -7193,7 +7193,7 @@
     }
     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;
@@ -7670,7 +7670,7 @@
         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;
@@ -10330,7 +10330,7 @@
     UNUSED(_start_lineno); // Only used by EXTRA macro
     int _start_col_offset = p->tokens[_mark]->col_offset;
     UNUSED(_start_col_offset); // Only used by EXTRA macro
-    { // invalid_primary
+    if (p->call_invalid_rules) { // invalid_primary
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11209,7 +11209,7 @@
             return NULL;
         }
     }
-    { // invalid_comprehension
+    if (p->call_invalid_rules) { // invalid_comprehension
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11340,7 +11340,7 @@
         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;
@@ -11434,7 +11434,7 @@
             return NULL;
         }
     }
-    { // invalid_comprehension
+    if (p->call_invalid_rules) { // invalid_comprehension
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11593,7 +11593,7 @@
             return NULL;
         }
     }
-    { // invalid_comprehension
+    if (p->call_invalid_rules) { // invalid_comprehension
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -11745,7 +11745,7 @@
         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;
@@ -12064,7 +12064,7 @@
             return NULL;
         }
     }
-    { // invalid_for_target
+    if (p->call_invalid_rules) { // invalid_for_target
         if (p->error_indicator) {
             D(p->level--);
             return NULL;
@@ -12190,7 +12190,7 @@
     return _res;
 }
 
-// arguments: args ','? &')' | incorrect_arguments
+// arguments: args ','? &')' | invalid_arguments
 static expr_ty
 arguments_rule(Parser *p)
 {
@@ -12235,24 +12235,24 @@
         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:
@@ -12589,7 +12589,7 @@
         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;
@@ -12709,7 +12709,7 @@
         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;
@@ -14421,14 +14421,14 @@
     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) {
@@ -14442,7 +14442,7 @@
             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;
@@ -14454,7 +14454,7 @@
             (_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;
@@ -14464,7 +14464,7 @@
             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]
@@ -14472,7 +14472,7 @@
             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
@@ -14488,7 +14488,7 @@
             (_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;
@@ -14498,7 +14498,7 @@
             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
@@ -14506,7 +14506,7 @@
             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_seq* for_if_clauses_var;
         if (
@@ -14515,7 +14515,7 @@
             (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;
@@ -14525,7 +14525,7 @@
             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
@@ -14533,7 +14533,7 @@
             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;
@@ -14548,7 +14548,7 @@
             (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;
@@ -14558,7 +14558,7 @@
             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
@@ -14566,7 +14566,7 @@
             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;
@@ -14578,7 +14578,7 @@
             (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;
@@ -14588,7 +14588,7 @@
             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/pegen.c b/Parser/pegen/pegen.c
index a7643fc..78891af 100644
--- a/Parser/pegen/pegen.c
+++ b/Parser/pegen/pegen.c
@@ -1101,15 +1101,28 @@
     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/pegen.h b/Parser/pegen/pegen.h
index bd35d4f..2fea84f 100644
--- a/Parser/pegen/pegen.h
+++ b/Parser/pegen/pegen.h
@@ -73,6 +73,7 @@
     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 aee668c..d0abc12 100644
--- a/Tools/peg_generator/pegen/c_generator.py
+++ b/Tools/peg_generator/pegen/c_generator.py
@@ -730,7 +730,10 @@
     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('"', '\\"')