Parser stack overflow: deeply-nested source causes SIGSEGV instead of SyntaxError

Summary

Parsing deeply-nested Python source (brackets, parens, etc.) crashes the interpreter with SIGSEGV at ~4000 levels of nesting. CPython caps this at 200 via SyntaxError: too many nested parentheses from the parser's internal depth limit.

The crash is in the parser itself (rustpython-ruff_python_parser 0.15.8): compile(src, '<s>', 'exec') (parser + codegen, does NOT go through _ast::ast_to_object) and ast.parse() crash at the identical depth threshold, showing the shared component is the parser, not post-parse conversion.

This is the parser-layer root cause of the class of bugs tracked in #2796. #4857 also still reproduces on current main (closed as fixed in 2023).

Expected

Matches CPython 3.13.4 behavior:

$ python3 -c "import ast; ast.parse('[' * 5000 + '1' + ']' * 5000)"
Traceback (most recent call last):
  ...
SyntaxError: too many nested parentheses (<unknown>, line 1)

(CPython's parser rejects at depth 201 via a hard-coded limit, well before the native stack is exhausted.)

Actual

$ rustpython -c "import ast; ast.parse('[' * 5000 + '1' + ']' * 5000)"
[no output, exit 139 — SIGSEGV]

$ rustpython -c "src='['*5000+'1'+']'*5000; compile(src,'<s>','exec')"
[no output, exit 139 — SIGSEGV]

# #4857's original reproducer also still crashes:
$ rustpython -c "import ast; ast.parse('+chr(33)' * 1000000)"
[no output, exit 139]

Threshold

Identical thresholds confirm the crash is in the parser, not in post-parse conversion.

Relation to prior fixes

• Fix segfault on cyclic or deeply-nested AST in compile() #7630 guarded _ast::ast_from_object (AST object → Rust IR conversion).
• Fix stack overflow on deeply-nested JSON in json.loads() #7632 guarded json.loads() via vm.with_recursion.

Both fixes live downstream of the parser. The parser's own recursive descent has no depth guard, so any Python source deeply nested in brackets, parens, unary ops, or binary ops reaches the native stack limit first.

Environment

• RustPython main @ 5a45d41d (parser: rustpython-ruff_python_parser 0.15.8)
• Compared against: CPython 3.13.4
• macOS 15.1 / Darwin 24.1.0
• Rust release build