Accesses to fields with packed type fail alignment check

[RalfJung]

The following function fails to execute in miri, raising an alignment error:

fn test_inner_packed() {
    // Even if just the inner struct is packed, accesses to the outer field can get unaligned
    #[repr(packed)]
    #[derive(Clone,Copy)]
    struct Inner(u32);
    struct Outer(u8, Inner);

    let o = Outer(0, Inner(42)); // error: tried to access memory with alignment 1, but alignment 4 is required
    let _x = o.1;
    let _y = (o.1).0;
}

The problem is that the 2nd field of Outer is unaligned because it's type is repr(packed). This came as a surprise to me, I thought repr(packed) would only influence the layout within this type, not the layout of other types that just involve this type.

How far does this go? For example, if I have a local of type Inner, do I even still have alignment guarantees for that local?

(It is amazing how much time I spend thinking about alignment problems in miri, and how many problems there are. Why is this all so complicated?!?)

[RalfJung]

The problem is that the 2nd field of Outer is unaligned because it's type is repr(packed). This came as a surprise to me, I thought repr(packed) would only influence the layout within this type, not the layout of other types that just involve this type.

In the mean time, I learned that repr(packed) actually has two effects -- it changes the layout inside the struct, and it changes the alignment of the struct to 1. So this somehow makes more sense to me now.