[Linter panic] RUF027 preview rule can cause linter panic on specific string

[skraeven]

Ruff has crashed while running on our code base, due to a specific string/rule combination.

Given this (minimal repro version) string in a python file:

foo = "{'bar': {\t'"

Running ruff check . --preview --select=RUF027 --isolated will cause linter panic.

Playground: https://play.ruff.rs/486e305d-99f7-4dcb-88fb-60d8fe45f002

Here's RUST_BACKTRACE=1 output:

info: Backtrace:
   0: <unknown>
   1: <unknown>
   2: <unknown>
   3: <unknown>
   4: <unknown>
   5: <unknown>
   6: <unknown>
   7: <unknown>
   8: <unknown>
   9: <unknown>
  10: <unknown>
  11: <unknown>
  12: <unknown>
  13: <unknown>
  14: <unknown>
  15: <unknown>
  16: <unknown>
  17: <unknown>
  18: <unknown>
  19: <unknown>
  20: <unknown>
  21: <unknown>
  22: <unknown>
  23: <unknown>
  24: <unknown>
  25: <unknown>
  26: <unknown>
  27: start_thread
             at ./nptl/pthread_create.c:447:8
  28: clone3
             at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0

[MichaReiser]

Thanks. The playground has a more useful stacktrace and it points to: crates/ruff_python_parser/src/parser/expression.rs:1619:25 with the message internal error: entered unreachable code: f-string: unexpected token TStringMiddle at 14..15 (Sorry @dylwil3)

[MichaReiser]

This is actually valid Python syntax :)

[dylwil3]

Yes but the panic is caused not from parsing the expression in the OP, but from calling parse_expression on

f"{'bar': {\t'"

Recall that RUF027 checks to see whether turning a string into an f-string would be correct syntax before suggesting it. Unfortunately this has lead to errors in the past, mainly because our error recovery is not nearly as battle-tested for parse_expression as it is for parsing a module.

[MichaReiser]

Ah right. Too bad, my other change also doesn't fix that

[dogukancagatay]

I think #23232 already fixed this one.