Miri does not check all offset_from conditions

[RalfJung]

offset_from currently requires that:

Both the starting and other pointer must be either in bounds or one byte past the end of the same allocated object.

However, Miri fails to check this condition -- the following program should error, but it does not:

fn main() {
    let start_ptr = &() as *const ();
    let length = 10;
    let end_ptr = (start_ptr as *const u8).wrapping_add(length) as *const ();
    unsafe { (end_ptr as *const u8).offset_from(start_ptr as *const u8); }
}

[RalfJung]

This UB also makes it impossible to do some things during CTFE that should be possible (Cc slightlyoutofphase/staticvec#48), so possibly we should change the UB rules here. At least it should be possible to write code like the above example in a way that can be executed at compile time without UB. Also see the discussion on Zulip.