`std::sync::barrier::BarrierState` is not `RefUnwindSafe` anymore

[weiznich]

Code

I tried this code:

git clone https://github.com/diesel-rs/diesel
cd diesel
git checkout 29a13b3
cargo +nightly check -p diesel -F sqlite -F returning_clauses_for_sqlite_3_35 --all-targets

I expected to see this happen: Compilation suceeds

Instead, this happened: Compilation fails with this error:

error[E0277]: the type `UnsafeCell<std::sync::barrier::BarrierState>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
   --> diesel/src/sqlite/connection/row.rs:383:48
    |
383 |           sleep_utils::register_impl(&mut conn2, move |a: i32| {
    |  _________--------------------------_____________^
    | |         |
    | |         required by a bound introduced by this call
384 | |             barrier.wait();
385 | |             std::thread::sleep(Duration::from_secs(a as u64));
386 | |             a
387 | |         })
    | |_________^ `UnsafeCell<std::sync::barrier::BarrierState>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    |
    = help: within `Barrier`, the trait `RefUnwindSafe` is not implemented for `UnsafeCell<std::sync::barrier::BarrierState>`
note: required because it appears within the type `std::sync::nonpoison::Mutex<std::sync::barrier::BarrierState>`
   --> /home/weiznich/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sync/nonpoison/mutex.rs:50:12
    |
 50 | pub struct Mutex<T: ?Sized> {
    |            ^^^^^
note: required because it appears within the type `Barrier`
   --> /home/weiznich/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sync/barrier.rs:28:12
    |
 28 | pub struct Barrier {
    |            ^^^^^^^
    = note: required for `Arc<Barrier>` to implement `UnwindSafe`
note: required because it's used within this closure
   --> diesel/src/sqlite/connection/row.rs:383:48
    |
383 |         sleep_utils::register_impl(&mut conn2, move |a: i32| {
    |                                                ^^^^^^^^^^^^^
note: required by a bound in `sleep_utils::register_impl`
   --> diesel/src/sqlite/connection/row.rs:346:5
    |
346 |     #[crate::declare_sql_function]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `register_impl`
    = note: this error originates in the attribute macro `crate::declare_sql_function` (in Nightly builds, run with -Z macro-backtrace for more info)

Version it worked on

It most recently worked on: Rust nightly-2025-08-30

Version with regression

rustc --version --verbose:

rustc 1.91.0-nightly (07d246fc6 2025-08-31)
binary: rustc
commit-hash: 07d246fc6dc227903da2955b38a59e060539a485
commit-date: 2025-08-31
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Cargo bisect-rustc:

Regression in rust-lang-ci@4de4acb

The PR introducing the regression in this rollup is #144651: Implementation: #[feature(nonpoison_condvar)]

searched nightlies: from nightly-2025-08-30 to nightly-2025-08-31
regressed nightly: nightly-2025-08-31
searched commit range: fe55364...523d399
regressed commit: e95db59

bisected with cargo-bisect-rustc v0.6.8

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start 2025-08-30 --end 2025-08-31 -- check -F sqlite -F returning_clauses_for_sqlite_3_35 --all-targets -p diesel 

cc @connortsui20 as author of the relevant PR

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

[connortsui20]

Oh I think I need to impl that for the nonpoison condvar and mutex (maybe even the rwlock too), I mistakenly thought that it is auto implemented...

I'll try to fix this by tomorrow but if someone else wants to take it, it should be an easy fix.

[bjorn3]

I don't think non-poison mutex should implement RefUnwindSafe. The reason regular Mutex is unwind safe is precisely because it gets poisoned on panics.

https://doc.rust-lang.org/stable/std/panic/trait.UnwindSafe.html#who-implements-unwindsafe

Who implements UnwindSafe?

Types such as &mut T and &RefCell are examples which are not unwind safe. The general idea is that any mutable state which can be shared across catch_unwind is not unwind safe by default. This is because it is very easy to witness a broken invariant outside of catch_unwind as the data is simply accessed as usual.

Types like &Mutex, however, are unwind safe because they implement poisoning by default. They still allow witnessing a broken invariant, but they already provide their own “speed bumps” to do so.

Implementing RefUnwindSafe for Barrier would be fine however. No panics can happen while the internal state is locked.

[connortsui20]

Ok I don't think I will be able to make this change by today if anyone else wants to take this up

[weiznich]

I opened #146322 to address this

[connortsui20]

@weiznich thanks for doing that! I was travelling the past week so I didn't have time.