Drop skipped on panic

[Stebalien]

I was working on a patch for @bluss's arrayvec and ran into the following issue. In the code below, I'm trying to keep the Test struct from dropping twice even if it panics on drop. If I explicitly drop the Test when reading it out of the array (using ptr::read), everything works. However, if I don't explicitly drop it (see XXX), it drops (good), panics (good), but then guard isn't dropped (bad) so the Test is dropped a second time (bad).

Sorry for the generic title.

use std::{mem, ptr};

struct Test;
impl Drop for Test {
    fn drop(&mut self) {
        println!("drop");
        panic!();
    }
}

struct Container(Option<[Test; 1]>);

impl Drop for Container {
    fn drop(&mut self) {
        /// Prevents Container's contents from dropping.
        struct Guard<'a>(&'a mut Container);
        impl<'a> Drop for Guard<'a> {
            fn drop(&mut self) {
                unsafe {
                    ptr::write(&mut (self.0).0, None);
                }
            }
        }
        {
            let guard = Guard(self);
            {
                unsafe {
                    let v = ptr::read(&mut (guard.0).0.as_mut().unwrap()[0]);
                    drop(v); // XXX: If removed, guard is never dropped and Test gets dropped twice.
                }
            }
        }
    }
}
fn main() {
    let c = Container(Some([Test]));
}

[bluss]

cc @pnkfelix since it's drop related

[alexcrichton]

I believe this is a dupe of #14875, so closing in favor of that

[Stebalien]

@alexcrichton Looks like it. Thanks.

[bluss]

@Stebalien did you find a bug in arrayvec?

[Stebalien]

@bluss bluss/arrayvec#3

[bluss]

FWIW, that feels like a panic safety bug in arrayvec, not really a bug in rust?

[Stebalien]

The bug in rust is that my PR doesn't work unless the item is dropped explicitly by calling drop on it.

Steven Allen