Summary

'[' * 5000 + '1' + ']' * 5000 makes the parser recurse until the native stack runs out, so the process dies with SIGSEGV instead of raising a Python error.

CPython stops this in the tokenizer rather than the parser: it rejects an opening bracket once tok->level reaches MAXLEVEL (200), so the parser never recurses that deep. This does the same, reusing the nesting counter the lexer already keeps — one comparison per opening bracket, no extra pass. As in CPython, (, [ and { share the counter and report the same message.

Deep recursion without bracket nesting, such as long operator chains, still overflows. That needs a parser-level stack guard and is out of scope here.

Refs RustPython/RustPython#7655
CPython: https://github.com/python/cpython/blob/main/Parser/lexer/lexer.c#L582-L600

Test Plan

Verified through RustPython against CPython 3.14.6:

• depth 200 parses; depth 201 raises SyntaxError: too many nested parentheses
• brackets inside strings, comments, f-strings, raw/bytes literals and triple-quoted strings are ignored (12 cases, all matching CPython)
• CPython suites pass: test_grammar (including test_max_level), test_syntax, test_compile, test_exceptions, test_fstring, test_ast, test_tokenize, test_symtable, test_unparse, test_string_literals, test_codeop

This crate's own tests don't build in this fork — a pre-existing serde failure in rustpython-ruff_python_ast, reproducible on main without this change — so verification was done through RustPython instead.

Investigated and implemented with Claude Code; reviewed and verified manually before submitting.