Zero-sized data structures are not initialised in MIR

[jberthold]

I played around a bit with empty data structures ((), [usize;0] , and struct EmptyStruct(), which all don't require any stored bytes...
Interestingly, the MIR I am getting somewhat relies on the fact that there are no stored bytes for these. I have a program like this (example: (), but the same happens for the other types):

const EMPTY: () = ();
fn main() {
    let e = ();
    assert!(e == EMPTY);
}

The first statement in main (block 0, first statement) becomes _3 <- &_1. which assumes there is nothing to do to initialise _1 to be ().
That assumption is untrue for our semantics, though, where () is represented as Aggregate(variantIdx(0), .List) but we get newValue(...) in _1 and end up with a reference pointing to it in _3.
For my 3 programs with empty data I did not find any point where that reference would be Dereferenced but I cannot be sure. The eq instances for () and EmptyStruct() just return constant true but the one for [usize;0] blindly calls the raw_eq intrinsic.
We should adapt our forthcoming implementation of raw_eq to this.

[dkcumming]

Also related to ZSTs and pointers is:

• Offsetting a pointer to a collection of ZSTs does not increment the pointer;
• Therefore we can ptr::offset and then ptr::read without any problem;
  Here is example program.

fn main() {
    let arr: [(); 3] = [(), (), ()];
    assert_eq!(std::mem::size_of::<[(); 3]>(), 0);
    let r_arr = &arr;
    let p_arr = r_arr as *const ();
    
    unsafe {
        let p_arr_offset = p_arr.offset(20);
        assert!(*p_arr_offset == ());
        
        assert_eq!(p_arr, p_arr_offset);
    }
}