A thread_local! with a const initializer and a panicking drop impl unwinds through an extern "C" frame

[saethlin]

Miri detects UB when running the following program:

pub struct NoisyDrop {}

impl Drop for NoisyDrop {
    fn drop(&mut self) {
        panic!("ow");
    }
}

thread_local! {
    pub static NOISY: NoisyDrop = const { NoisyDrop {} };
}

fn main() {
    NOISY.with(|_| ());
}

With --target=x86_64-unknown-linux-gnu and --target=aarch64-apple-darwin

thread '<unnamed>' panicked at 'ow', src/main.rs:5:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: Undefined Behavior: unwinding past a stack frame that does not allow unwinding
  --> /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/thread_local_dtor.rs:43:17
   |
43 |                 dtor(ptr);
   |                 ^^^^^^^^^ unwinding past a stack frame that does not allow unwinding
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE:
   = note: inside `std::sys_common::thread_local_dtor::register_dtor_fallback::run_dtors` at /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/thread_local_dtor.rs:43:17: 43:26

With --target=x86_64-pc-windows-msvc

thread 'main' panicked at 'ow', src/main.rs:5:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: Undefined Behavior: unwinding past a stack frame that does not allow unwinding
  --> /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/windows/thread_local_dtor.rs:28:9
   |
28 |         (dtor)(ptr);
   |         ^^^^^^^^^^^ unwinding past a stack frame that does not allow unwinding
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE:
   = note: inside `std::sys::windows::thread_local_dtor::run_keyless_dtors` at /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/windows/thread_local_dtor.rs:28:9: 28:20
   = note: inside `std::sys::windows::thread_local_key::on_tls_callback` at /home/ben/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/windows/thread_local_key.rs:246:9: 246:54

[thomcc]

I see the bug. We have #![feature(c_unwind)] (which means extern "C" fns in std automatically trap aborts) but that doesn't apply to extern "C" functions which are present in the macro expansion (some of which can't really be moved outside the macro).

Ill fix in like an hour or two. We just need to guard panics in a couple places.

In practice I think this should not call problems and we'd just hit an abort guard in an extern "C" defined in libstd a few frames up, although perhaps that's not true in 100% of cases (I can update on this once I implement the fix).

[workingjubilee]

The apparent consensus is that the fix to this may need to be backported thus I am tagging this with P-high.

[Noratrieb]

the fix was merged and a backport was rejected: #112292 (comment)