Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

[Benno Lossin <benno.lossin@proton.me>]

On 25.09.23 18:16, Boqun Feng wrote:
> On Mon, Sep 25, 2023 at 03:07:44PM +0000, Benno Lossin wrote:
>> ```rust
>> struct MutatingDrop {
>>       value: i32,
>> }
>>
>> impl Drop for MutatingDrop {
>>       fn drop(&mut self) {
>>           self.value = 0;
>>       }
>> }
>>
>> let arc = Arc::new(MutatingDrop { value: 42 });
>> let wr = arc.as_with_ref(); // this creates a shared `&` reference to the MutatingDrop
>> let arc2: Arc<MutatingDrop> = wr.into(); // increments the reference count to 2
> 
> More precisely, here we did a
> 
> 	&WithRef<_> -> NonNull<WithRef<_>>
> 
> conversion, and later on, we may use the `NonNull<WithRef<_>>` in
> `drop` to get a `Box<WithRef<_>>`.

Indeed.

> 
>> drop(arc); // this decrements the reference count to 1
>> drop(arc2); // this decrements the reference count to 0, so it will drop it
>> ```
>> When dropping `arc2` it will run the destructor for `MutatingDrop`,
>> which mutates `value`. This is a problem, because the mutable reference
>> supplied was derived from a `&`, that is not allowed in Rust.
>>
> 
> Is this an UB? I kinda wonder what's the real damage we can get, because
> in this case, we just use a reference to carry a value of a pointer,
> i.e.
> 
> 	ptr -> reference -> ptr
> 
> I cannot think of any real damage compiler can make, but I'm happy to be
> surprised ;-)

This is UB, so anything can happen :)

-- 
Cheers,
Benno