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

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Sep 25, 2023 at 05:30:05PM +0200, Alice Ryhl wrote:
> On 9/25/23 17:17, Boqun Feng wrote:
> > On Mon, Sep 25, 2023 at 03:00:47PM +0000, Alice Ryhl wrote:
> > > > > > I'm concerned about this change, because an `&WithRef<T>` only has
> > > > > > immutable permissions for the allocation. No pointer derived from it
> > > > > > may be used to modify the value in the Arc, however, the drop
> > > > > > implementation of Arc will do exactly that.
> > > > > 
> > > > > That is indeed a problem. We could put the value in an `UnsafeCell`, but
> > > > > that would lose us niche optimizations and probably also other optimizations.
> > > > > 
> > > > 
> > > > Not sure I understand the problem here, why do we allow modifying the
> > > > value in the Arc if you only have a shared ownership?
> > > 
> > > Well, usually it's when you have exclusive access even though the value
> > > is in an `Arc`.
> > > 
> > > The main example of this is the destructor of the `Arc`. When the last
> > > refcount drops to zero, this gives you exclusive access. This lets you
> > > run the destructor. The destructor requires mutable access.
> > > 
> > > Another example would be converting the `Arc` back into an `UniqueArc`
> > > by checking that the refcount is 1. Once you have a `UniqueArc`, you can
> > > use it to mutate the inner value.
> > > 
> > > Finally, there are methods like `Arc::get_mut_unchecked`, where you
> > > unsafely assert that nobody else is using the value while you are
> > > modifying it. We don't have that in our version of `Arc` right now, but
> > > we might want to add it later.
> > > 
> > 
> > Hmm.. but the only way to get an `Arc` from `&WithRef` is
> > 
> > 	impl From<&WithRef<T>> for Arc<T> {
> > 	    ...
> > 	}
> > 
> > , and we clone `Arc` in the that function (i.e. copying the raw
> > pointer), so we are still good?
> > 
> 
> No, the raw pointer in the Arc was created from the immutable reference, so
> the raw pointer has the same restrictions as the immutable reference did.
> 

I see, this was the part I was missing. Thanks!

Looks like the only fix is replacing `&WithRef<T>` with
`&UnsafeCell<WithRef<T>>`? But that's a bit wordy and I'm not sure
whether it's better than `ArcBorrow<'_, T>`...

Regards,
Boqun

> Alice