Safe reference counting cannot be implemented as a library

[Jonathan M Davis via Digitalmars-d]

On Wednesday, 28 October 2015 at 02:08:18 UTC, Sebastiaan Koppe 
wrote:
> On Tuesday, 27 October 2015 at 11:41:52 UTC, Andrei 
> Alexandrescu wrote:
>> The crux of the matter is modular typechecking. Consider the 
>> following example:
>>
>> // module widget.d
>> @safe class Widget {
>>   void fun() {
>>     g_widget = this;
>>   }
>> }
>> static Widget g_widget;
>> // end of module widget.d
>>
>> Now, once the typechecker OKs module widget.d, the summary 
>> that all other typechecking "sees" is:
>>
>> @safe class Widget {
>>   void fun();
>> }
>
> Isn't it a shame that that kind of information gets tossed 
> aside? Seems to be very valuable and the loss of it the cause 
> of several issues.

Perhaps, but that assignment is perfectly @safe. What isn't @safe 
is when you then free the Widget later because a ref-count hit 0. 
And that code already has to be @system or @trusted given what 
it's doing. So, really, the problem is that some @safe operations 
can lead to problems when @system code does stuff that assumes 
that those operations did not occur. So, unfortunately, whether 
that code can be @trusted depends on what some @safe code did 
rather than @system code like you'd normally expect. But the 
compiler simply doesn't understand enough about why something is 
@system to be able to figure out where @safe code could foul it 
up even if it has all of the source code to look at.

- Jonathan M Davis