Raised by @CAD97:

I actually find the current behavior of #![feature(c_unwind)] unwinding in extern "C" somewhat strange. Specifically, any unwinding edges within the extern "C" fn get sent to core::panicking::panic_cannot_unwind, meaning that the unwind happens up to the extern "C" fn, but any locals in said function don't get dropped. I would personally not expect wrapping the body of an extern "C" function in an inner extern "Rust" function to change behavior, but it does.

Reproducing example:

#![feature(c_unwind)]

struct Noise;
impl Drop for Noise {
    fn drop(&mut self) {
        eprintln!("Noisy Drop");
    }
}

extern "C" fn test() {
    let _val = Noise;
    panic!("heyho");
}

fn main() {
    test();
}

I would expect "Noisy Drop" to be printed, but it is not.

IMO it'd make most sense to guarantee that with panic=unwind, this destructor is still called. @nbdd0121 however said they don't want to guarantee this.

What is the motivation for leaving this unspecified? The current behavior is quite surprising. If I understand @CAD97 correctly, we currently could make "Noisy Drop" be executed by tweaking the MIR we generate.

Tracking:

• Tracking Issue for "C-unwind ABI", RFC 2945 #74990