DMD Symbol Reference Analysis Pass

[via Digitalmars-d]

On Tuesday, 26 May 2015 at 15:21:04 UTC, Marc Schütz wrote:
> On Tuesday, 26 May 2015 at 14:59:38 UTC, Per Nordlöw wrote:
>> On Tuesday, 26 May 2015 at 10:19:52 UTC, Marc Schütz wrote:
>>> ... to be used in templates and for enforcing these rules:
>>> http://wiki.dlang.org/User:Schuetzm/scope3#.40safe-ty_violations_with_borrowing
>>
>> There's at least a plan. Nice!
>>
>> One thing, though. I'm lacking a section in the document 
>> linked above on how `foreach` could be `scope`-enhanced so 
>> that an element reference of an aggregate doesn't escape its 
>> foreach scope.
>>
>>    char[] saved_line;
>>    string saved_str;
>>    foreach (scope line; File("foo.txt").byLine)
>>    {
>>        saved_line = line; // should give error
>>        saved_line = line.dup; // should be ok
>>        saved_str  = line.to!string; // should be ok
>>    }
>>
>> provided that `byLine` returns a reference to a volatile 
>> internal buffer.
>
> Assuming you mean by "volatile" that the buffer is released 
> upon destruction:

No, with volatile I mean that the buffer contents changes with 
each iteration.

> The compiler is supposed to do that automatically, i.e. `scope` 
> annotations on local variables are always inferred. In your

No, DMD cannot currently handle scope on foreach elements. It 
errors as

      Error: basic type expected, not scope

> example, it would figure out that you're assigning a reference 
> to a value with shorter lifetime (i.e. the slice to the buffer) 
> to a value with longer lifetime (saved_line), which it would 
> disallow. (Btw, I don't think to!string is enough, because it 
> is probably a no-op in this case: string -> string).

No to!string is not a no-op in this case. It allocates but it 
needs to create an immutable char array that is: char[] -> string

> However, byLine has another problem, which boils down to the 
> same cause as the problem with RCArray, namely that the content 
> of the buffer is reused in each iteration.

This is what I meant with volatile. Is there a better word for 
this?

> the "owner" can be modified while references to it exist. For 
> byLine, this is not a safety violation, but for RCArray it is. 
> A solution applicable to both is to detect this and then either 
> treat such a situation as @system, or make the owner `const` as 
> long as the references are alive.

AFAIK: Allowing scope in foreach would solve this problem in my 
case.