addr_of_enum's Issues
Unsoundness caused by different `GhostEnum` layout
Unfortunately this crate currently relies on the assumption that an enum containing fields of type T
has the same layout as an enum containing MaybeUninit<T>
. However, this is not always the case. It is mentioned in the rust docs of MaybeUninit
that
types containing a field of type
T
may be laid out (and sized) differently than if that field wereMaybeUninit<T>
Here is a test that fails for that reason:
#[derive(AddrOfEnum, PartialEq, Eq)]
enum MyOption<T> {
None,
Some(T),
}
#[test]
fn test_option() {
let some = MyOption::Some(false);
match &some {
MyOption::Some(item) => {
assert_eq!(item as *const bool, addr_of_enum!(&some, Some, 0)); // <- this will fail (off by one)
}
_ => panic!(),
}
}
In this specific example, the compiler does niche optimization because it knows that only 0
and 1
are valid values for bool
, so it can use one of the remaining values as None
. However, MaybeUninit<bool>
could, by definition, have any bit pattern, so the same optimization is not possible there.
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