Add small validator utility for PEG grammars (GH-23519)

Tools/peg_generator/pegen/validator.py

@@ -0,0 +1,52 @@
+from pegen import grammar
+from pegen.grammar import (
+    Alt,
+    Cut,
+    Gather,
+    GrammarVisitor,
+    Group,
+    Lookahead,
+    NamedItem,
+    NameLeaf,
+    NegativeLookahead,
+    Opt,
+    PositiveLookahead,
+    Repeat0,
+    Repeat1,
+    Rhs,
+    Rule,
+    StringLeaf,
+)
+
+class ValidationError(Exception):
+    pass
+
+class GrammarValidator(GrammarVisitor):
+    def __init__(self, grammar: grammar.Grammar):
+        self.grammar = grammar
+        self.rulename = None
+
+    def validate_rule(self, rulename: str, node: Rule):
+        self.rulename = rulename
+        self.visit(node)
+        self.rulename = None
+
+
+class SubRuleValidator(GrammarValidator):
+    def visit_Rhs(self, node: Rule):
+        for index, alt in enumerate(node.alts):
+            alts_to_consider = node.alts[index+1:]
+            for other_alt in alts_to_consider:
+                self.check_intersection(alt, other_alt)
+
+    def check_intersection(self, first_alt: Alt, second_alt: Alt) -> bool:
+        if str(second_alt).startswith(str(first_alt)):
+            raise ValidationError(
+                    f"In {self.rulename} there is an alternative that will "
+                    f"never be visited:\n{second_alt}")
+
+def validate_grammar(the_grammar: grammar.Grammar):
+    for validator_cls in GrammarValidator.__subclasses__():
+        validator = validator_cls(the_grammar)
+        for rule_name, rule in the_grammar.rules.items():
+            validator.validate_rule(rule_name, rule)