Destroying structs (literally)

[via Digitalmars-d]

On Saturday, 30 August 2014 at 03:54:41 UTC, Orvid King wrote:
> On 8/29/2014 2:52 PM, "Marc =?UTF-8?B?U2Now7x0eiI=?= 
> <schuetzm at gmx.net>" wrote:
>> On Friday, 29 August 2014 at 19:01:51 UTC, Andrei Alexandrescu 
>> wrote:
>>> On 8/29/14, 3:53 AM, "Marc Schütz" <schuetzm at gmx.net>" wrote:
>>>> Jacob Carlborg just recently brought this up in another 
>>>> thread. Isn't it
>>>> kind of consensus that calling a destructor from the GC is 
>>>> not a good
>>>> idea because of the restrictions that apply in this context? 
>>>> Andrei even
>>>> wanted to deprecate destructors for classes because of this. 
>>>> Maybe a
>>>> better direction would be to separate the concepts of 
>>>> destruction and
>>>> finalization, and introduce two kinds of "destructors" for 
>>>> them.
>>>
>>> I think we need to stay with what we have. Adding a distinct 
>>> kind of
>>> destructor might be interesting. -- Andrei
>>
>> Our idea was that an additional destructor (let's call it a 
>> finalizer)
>> would be helpful because it is backward compatible. The 
>> compiler could
>> make some validity checks on it, at the least make it nothrow, 
>> maybe
>> @nogc (but I believe we can relax this restriction), pure (?).
>> Disallowing access to references (because they could pointer 
>> to already
>> destroyed objects) is unfortunately not feasible, because we 
>> can't
>> distinguish GC pointers from other ones. To avoid the need for 
>> code
>> duplication, finalizers could always be called implicitly by 
>> destructors
>> (assuming everything that is allowed in finalizers is also 
>> permitted in
>> destructors).
>>
>> Calling destructors from the GC could later be phased out. It 
>> is
>> technically not a breaking change, because there never was a 
>> guarantee
>> that they'd be called anyway.
>
> I would say that all of those restrictions, except for nothrow, 
> are dependent on the current GC implementation. It is possible 
> to write the GC in such a way that you can do GC allocations in 
> a destructor, as well as access any GC references you want. The 
> only thing with the GC references is that there's no way to 
> guarantee that the referenced objects won't have already had 
> their destructor called when the current destructor is being 
> called.

Hmmm... could the GC zero those references that it already 
destroyed, before calling the finalizer? Don't know how this 
would affect performance, but it would only be necessary if a 
finalizer exists (could even be restricted to those references 
that are accessible from non-trivial finalizers, i.e. if a struct 
has GCed pointers and an embedded struct member with a finalizer, 
but no finalizer of its own, the compiler would probably generate 
one that only calls the member's finalizer, but this would have 
no access to its parent's pointers).

You're right that many of the restrictions are only necessary 
because of the current GC implementation. Even the fact that 
garbage collection can happen in any thread could theoretically 
be changed. Even more complicated: I can imagine that with the 
upcoming allocator work there could be several different GC 
implementations, even used in parallel in the same program, each 
with different capabilities and restrictions. It's clear that 
this requires coordination.