std::sync::Once: Should Once be UnwindSafe or RefUnwindSafe?

[joshlf]

Currently, the following program will not compile because Once isn't RefUnwindSafe. Is this the desired behavior, or should we consider making it RefUnwindSafe or UnwindSafe?

use std::sync::Once;
use std::panic::catch_unwind;

fn main() {
    let o = Once::new();
    catch_unwind(|| o.call_once(|| panic!("foo")));
}

[alexcrichton]

It also won't compile because call_once requires 'static self?

[joshlf]

Ah, right you are. This works:

use std::sync::Once;
use std::panic::catch_unwind;

static o: Once = Once::new();

fn main() {
    catch_unwind(|| o.call_once(|| panic!("foo")));
}

[lilyball]

Once::call_once() does not require &'static self anymore (tested with rustc 1.55.0). The original issue is still valid. Since Once poisons itself on panic, it should be considered unwind safe.

[m-ou-se]

We discussed this very briefly in the libs api meeting. It seems fine to add this impl.

[Milo123459]

@rustbot claim

Going to take a stab at this one.