Unaligned raw references are considered UB

[RalfJung]

The following program should pass, but is incorrectly considered UB:

#![feature(raw_ref_op)]

fn main() {
    #[repr(packed)]
    struct S {
        fill: u8,
        a: i32,
        b: i64,
    }

    let mut x = S {
        fill: 0,
        a: 42,
        b: 99,
    };

    assert_eq!(unsafe { (&raw const x.a).read_unaligned() }, 42);
    assert_eq!(unsafe { (&raw const x.b).read_unaligned() }, 99);
}

The error source seems to be related to Stacked Borrows.

[RalfJung]

The last two executed MIR statements are:

[INFO  rustc_mir::interpret::step] _170 = &raw const (_1.1: i32)
[INFO  rustc_mir::interpret::step] Retag([raw] _170)

So the issue is that raw retags incorrectly assert proper alignment.