futures::lock::Mutex is not implementing RefUnwindSafe trait (see https://docs.rs/futures/0.3.17/futures/lock/struct.Mutex.html#impl-RefUnwindSafe )
Why is not safe to pass a futures::lock::Mutex inside std::panic::catch_unwind?
Reproducible code:
use std::panic::catch_unwind;
use futures::lock::Mutex;
fn main() {
let m = Mutex::new(String::new());
catch_unwind(|| {
m.lock()
});
}
Could futures::lock::Mutex implement the RefUnwindSafe trait ?
The documentation for the UnwindSafe trait hints at a reason for this:
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.
futures::lock::Mutex provides mutability like &mut T and RefCell<T>, but does not implement the poisoning feature of std::sync::Mutex<T>, so it does not implement UnwindSafe.
Though as the documentation points out, the UnwindSafe is less about memory safety and more about upholding logical invariants - hence why neither UnwindSafe nor AssertUnwindSafe are unsafe.
