Escaped scoped

[Don]

Steven Schveighoffer wrote:
> On Mon, 04 Jan 2010 05:52:19 -0500, bearophile 
> <bearophileHUGS at lycos.com> wrote:
> 
>> If I compile the following code with DMD with and without the scope 
>> annotation I can see that both versions compile and the version with 
>> scope deletes the object. Is the compiler acting correctly here? I'd 
>> like the compiler to refuse to compile this code when the scope 
>> attribute is present (this is a reduced example from a bug I've just 
>> removed from a program of mine):
>>
>> class Foo {}
>> class Bar {
>>     Foo x;
>>     void spam() {
>>         scope Foo temp = new Foo();
>>         this.x = temp;
>>     }
>> }
>> void main() {}
> 
> Scope is not a type constructor, so once the compiler passes the line
> 
> scope Foo temp = new Foo();
> 
> the compiler sees temp as type Foo, not scope Foo.  So it is ignorant on 
> the next line to know to stop you from doing something foolish.

I don't think that's true, actually. The compiler has a SCOPE storage 
class internally. Not sure what it's used for, though.

> When using scope, it is on you to ensure that it doesn't escape.

Yes, I suspect it's impossible for the compiler to do perfect escape 
analysis. But it probably wouldn't be difficult for it to do some.

> I think the compiler cheats a little bit on delegates, but it has severe 
> limitations that I think make using delegates without accidentally 
> allocating closures difficult.

It could probably catch simple cases like this one, though.