RCArray is unsafe

[Walter Bright via Digitalmars-d]

On 3/2/2015 9:27 PM, deadalnix wrote:
> On Tuesday, 3 March 2015 at 01:56:09 UTC, Walter Bright wrote:
>> On 3/2/2015 4:40 PM, deadalnix wrote:
>>> After moving resources, the previous owner can no longer be used.
>>
>> How does that work with the example presented by Marc?
>
> I'm not sure what you don't understand. The comment in the sample code seems
> clear to me.
>
> // "Move" `a` into `b`
> // Here's what happens under the hood: the pointer `a` gets copied (*not*
> // the data on the heap, just its address) into `b`. Now both are pointers
> // to the *same* heap allocated data. But now, `b` *owns* the heap
> // allocated data; `b` is now in charge of freeing the memory in the heap.
> let b = a;
>
> Now you have a variable b that owns the data. a is not usable anymore.
>
> // After the previous move, `a` can no longer be used
> // Error! `a` can no longer access the data, because it no longer owns the
> // heap memory
> //println!("a contains: {}", a);
> // TODO ^ Try uncommenting this line
>
> As explained here, if a is used, this is an error.
>
> In the example a is move to b. If you had let b = &a, then b would borrow from
> a. The same way, a would not be usable anymore. The difference with move is
> that, when b is destroyed, in the first case memory is freed. in the second
> case, memory is not freed and a is "reenabled".
>
> It is either possible to borrow once and disable who you borrow from, or borrow
> multiple time and everything become immutable for the time you borrow.
>
> This makes the problems mentioned in this thread effectively impossible happen.

What if 'a' is a field of a struct in some non-trivial data structure? How is 
the compiler going to statically keep track of which field instances in this 
structure are not usable?