ref parameters: there is no escape

[Andrei Alexandrescu]

Walter and I have had a long discussion and we thought we'd bring an 
idea for community review.

We believe it would be useful for safety purposes to disallow escaping 
addresses of ref parameters. Consider:

class C {
   int * p;
   this(ref int x) {
     p = &x; // escapes the address of a ref parameter
   }
}

Such code is accepted today. We believe it is error-prone and dangerous, 
particularly because the caller has no syntactic cue that the address of 
the parameter is passed into the function (in this case constructor). 
Worse, such a function cannot be characterized as @safe.

So we want to make the above an error. The workaround is obvious - just 
take int* as a parameter instead of ref int. What a function can do with 
a ref parameter in general is:

* use it directly just like a local;

* pass it down to other functions (which may take it by value or reference);

* pass its address down to pure functions because a pure function cannot 
escape the address anyway (cool insight by Walter);

* take its address as long as the address doesn't outlive the frame of 
the function.

The third bullet is not easy to implement as it requires flow analysis, 
but we may start with a conservative version first. Probably there won't 
be a lot of broken code anyway.

Please chime in with any comments you might have!


Thanks,

Andrei