Construct fat pointer to struct?

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9

This should be well-defined and doesn't assume the fat pointer layout:

#[derive(Debug, Fatten)]
#[repr(C)] // repr for what I put in main, not required in general
struct Struct {
    head: u8,
    tail: [u8],
}

// derive(Fatten) expands to:
impl Struct {
    fn fatten(data: *const u8, len: usize) -> *const Self {
        // Requirements of slice::from_raw_parts.
        assert!(!data.is_null());
        assert!(len <= isize::max_value() as usize);

        let slice = unsafe { core::slice::from_raw_parts(data as *const (), len) };
        slice as *const [()] as *const Self
    }
}

fn main() {
    let array = [2u8, 3u8, 4u8, 5u8, 6u8];
    let ptr = Struct::fatten(array.as_ptr(), 4);
    println!("{:?}", unsafe { &*ptr });
}

If the caller's struct has anything other than a dynamically sized slice as the tail, for example tail: dyn ToString or tail: T where T: ?Sized, it correctly won't compile:

error[E0606]: casting `*const [()]` as `*const Struct` is invalid
  --> src/main.rs:18:9
   |
18 |         slice as *const [()] as *const Self
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: vtable kinds may not match