Closed
Description
Spawned from rust-lang/rust#58684
The following test is likely to fail in the current code base, at least for many versions of rustc
.
#[test]
fn test_option_encoding() {
let tester: ArrayDeque<[Box<()>; 100], Wrapping> = ArrayDeque::new();
assert!(Some(tester).is_some());
}
The reason it will fail is because ArrayDeque::new()
is using mem::uninitialized
to create the array itself, which is discouraged because the resulting value will tend to yield undefined behavior unless the type is valid for all possible bit patterns. (And an easy example of a type that is not valid for all possible bit patterns is Box<T>
, since it must be non-null.)
The test above is demonstrating how violating this rule can break invariants like Some(E).is_some()
for any E
.
- One potential fix for this has been pointed out by oli: Use an untagged union instead of
mem::uninitialized
. - Another potential fix would be to use
MaybeUninit
instead ofManuallyDrop
, as pointed out by nagisa.
Neither of these fixes is available in stable Rust today, though. (untagged unions only support Copy
data if you're on stable.)
Metadata
Metadata
Assignees
Labels
No labels