repr(packed) calls drop with unaligned pointer

[RalfJung]

The following test fails:

struct NeedAlign(u64);

impl Drop for NeedAlign {
    fn drop(&mut self) {
        assert_eq!(self as *mut _ as usize % 8, 0);
    }
}


#[repr(packed)]
#[derive(Copy, Clone)]
struct Unalign<T>(T);

struct Breakit {
    x: u8,
    y: Unalign<NeedAlign>,
}

fn main() {
    println!("before");
    {
        let x = NeedAlign(0);
    }
    println!("middle");
    {
        let x = Breakit { x : 0, y : Unalign(NeedAlign(0)) };
    }
    println!("after");
}

This could lead to UB, because code for drop is generated under the assumption that the pointer is aligned.

Related to #27060, but not solved by making taking references to field of packed structs unsafe.
We furthermore have to forbid using types which implement Drop in a packed struct.

Open question:
How is this supposed to handle generic types, like in the example above? Error on declaration (T may implement Drop), or only error when Unalign is actually used with a type that implements Drop?
IIRC unions also have a no-drop restriction, so this should probably use the same rules.

[arielb1]

The union/destructor restriction is more like a lint than a strong restriction.

I think we might want a "finer" solution here, because people might have unaligned Box<T>/Rc<T>/etc., and we won't want them to be "use unsafe for instant UB".

[RalfJung]

people might have unaligned Box/Rc/etc., and we won't want them to be "use unsafe for instant UB".

Well but these are already UB right now, aren't they? What is the alternative to forbidding them?

[RalfJung]

This is fixed by #44884, isn't it?

[arielb1]

Sure. Fixed, with a test: src/test/run-pass/packed-struct-drop-aligned.rs