Fix unsafe unaligned loads in test.

[solson]

r? @eddyb
cc @Aatch @nikomatsakis

The #[derive(PartialEq, Debug)] impls on a packed struct contain undefined behaviour. Both generated impls take references to unaligned fields, which will fail to compile once we correctly treat that as unsafe (see #27060).

This UB was found by running the test under Miri which rejects these unsafe unaligned loads. 😄

Here's a simpler example:

#[repr(packed)]
#[derive(PartialEq, Debug)]
struct Packed {
    a: u8,
    b: u64,
}

It expands to:

    fn fmt(&self, __arg_0: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
        match *self {
            Packed { a: ref __self_0_0, b: ref __self_0_1 } => { // BAD: these patterns are unsafe
                let mut builder = __arg_0.debug_struct("Packed");
                let _ = builder.field("a", &&(*__self_0_0));
                let _ = builder.field("b", &&(*__self_0_1));
                builder.finish()
            }
        }
    }

and

    fn eq(&self, __arg_0: &Packed) -> bool {
        match *__arg_0 {
            Packed { a: ref __self_1_0, b: ref __self_1_1 } => // BAD: these patterns are unsafe
            match *self {
                Packed { a: ref __self_0_0, b: ref __self_0_1 } => // BAD: these patterns are unsafe
                true && (*__self_0_0) == (*__self_1_0) &&
                    (*__self_0_1) == (*__self_1_1),
            },
        }
    }

[eddyb]

@bors r+

[bors]

📌 Commit 2589f4a has been approved by eddyb