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

[Alice Ryhl]

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.

Alice