Enforcing static closure

[Steven Schveighoffer]

On Thu, 19 May 2011 15:39:12 -0400, Piotr Szturmaj <bncrbme at jadamspam.pl>  
wrote:

> I want to make a delegate of blocking I/O statement and pass it to a  
> function. Then it will be called immediately. This delegate never  
> escapes its creation scope, so I don't want heap closure allocation.
>
> Will compiler create dynamic closure (e.g. with allocation) or static  
> closure (with pointer to the stack)?
>
> void recv(ubyte[] buf, out size_t len)
> {
>      // block until data is received
> }
>
> int numWaiters;
>
> // will that "scope" enforce static closure?
> void wait(scope void delegate() dg)
> {
>      numWaiters++;
>      dg();
>      numWaiters--;
> }
>
> void test()
> {
>      ubyte[] buf = new ubyte[1500];
>      size_t len;
>
>      wait( { recv(buf, len); } );
> }

In this case, a closure should *not* be created.  A closure is created  
when you create a delegate to an inner function UNLESS:

a. The expression which takes the delegate is being used to pass the  
delegate to a function

AND

b. The function's delegate parameter is marked 'scope' or 'in'

Note that the following still would create a closure:

auto dg = {recv(buf, len);}; // forces a closure.
wait(dg);

because you aren't passing the delegate to the scope function at the time  
of delegate creation.

dmd is kind of silly about that.

-Steve