thread 'main' panicked at 'internal error: entered unreachable code', src/tools/tidy/src/pal.rs:224:9

[glandium]

Adding some #[cfg()] with unbalanced parens in some file in the rust directory makes python x.py build fail with the following error and backtrace:

thread 'main' panicked at 'internal error: entered unreachable code', src/tools/tidy/src/pal.rs:224:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
   5: std::panicking::begin_panic
   6: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
   7: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
   8: tidy::pal::check::{{closure}}
   9: tidy::walk
  10: tidy::walk
  11: tidy::pal::check
  12: tidy::main
  13: std::rt::lang_start::{{closure}}
  14: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:293
  15: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:87
  16: std::rt::lang_start_internal
             at src/libstd/panicking.rs:272
             at src/libstd/panic.rs:388
             at src/libstd/rt.rs:48
  17: main
  18: __libc_start_main
  19: _start

Which is rather unhelpful.

[hellow554]

Looking at

rust/src/tools/tidy/src/pal.rs

Lines 206 to 225 in 08bfe16

	cfgs.map(|i| {
	let mut depth = 0;
	let contents_from = &contents[i..];
	for (j, byte) in contents_from.bytes().enumerate() {
	match byte {
	b'(' => {
	depth += 1;
	}
	b')' => {
	depth -= 1;
	if depth == 0 {
	return (i, &contents_from[..=j]);
	}
	}
	_ => { }
	}
	}
	unreachable!()
	}).collect()

means, that unreachable should be

panic!("An uneven amount of `(` and `)` has been discovered")

(or similar)

If that wording is okay, I would go and file a PR

[varkor]

Rather than panicking, it would be better to throw an error (e.g. using tidy_error!).
(I think unbalanced parentheses in `#[cfg]` would read slightly better.)

[glandium]

Note that there are other cases where there is an error message like:

    |
207 | #[cfg(not(stage0)]
    |  -   -           ^ incorrect close delimiter
    |  |   |
    |  |   un-closed delimiter
    |  close delimiter possibly meant for this

It seems like we should get the same here too.

Edit: Come to think of it, this one might be coming from the compiler.

[varkor]

We could just ignore #[cfg]s with unbalanced parentheses in tidy, as they'll be picked up during compilation. In that case, removing the unreachable!() and adding a comment to that effect should be enough.