[3.9] bpo-42123: Run the parser two times and only enable invalid rules on the second run (GH-22111) (GH-23011)

* Implement running the parser a second time for the errors messages

The first parser run is only responsible for detecting whether
there is a `SyntaxError` or not. If there isn't the AST gets returned.
Otherwise, the parser is run a second time with all the `invalid_*`
rules enabled so that all the customized error messages get produced.

(cherry picked from commit bca701403253379409dece03053dbd739c0bd059)

1 files changed, 4 insertions, 1 deletions

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 @@ 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('"', '\\"')