Affected versions of this crate called some methods using auto-ref. The affected code looked like this.

let mut arr = $crate::__core::mem::MaybeUninit::uninit();
let mut vec = $crate::__ArrayVec::<T>::new(arr.as_mut_ptr() as *mut T);

In this case, the problem is that as_mut_ptr is a method of &mut MaybeUninit, not MaybeUninit. This made it possible for traits to hijack the method calls in order to cause unsoundness.

trait AsMutPtr<T> {
    fn as_mut_ptr(&self) -> *mut T;
}
impl<T> AsMutPtr<T> for std::mem::MaybeUninit<T> {
    fn as_mut_ptr(&self) -> *mut T {
        std::ptr::null_mut()
    }
}
array![0; 1];

The flaw was corrected by explicitly referencing variables in macro body in order to avoid auto-ref.

References

• https://github.com/rustsec/advisory-db/blob/main/crates/array-macro/RUSTSEC-2020-0161.md
• https://rustsec.org/advisories/RUSTSEC-2020-0161.html
• KamilaBorowska/array-macro@0194063
• https://gitlab.com/KonradBorowski/array-macro/-/commit/01940637dd8f3bfeeee3faf9639fa9ae52f19f4d