version(ctfe) (was: The demise of T[new])

[dsimcha]

This discussion originated in the T[new] thread, but I think it deserves its own
thread.

== Quote from Denis Koroskin (2korden at gmail.com)'s article
> An Array!(T) is really just a different name to a T[new]. You'll have the
> same problem explaining difference between Array!(T) and T[].
> But you are also creating a nightmare for CTFE. Since you can't use "a ~=
> b;" anymore, you'll have to use "a = a ~ b;" which *always* allocates. Not
> only it is syntactically less pleasant, this way you render this function
> useless at run-time - who in the sane mind will use such an inefficient
> stuff?

Maybe what we need is a version(ctfe) statement.  Stuff inside such a block would
be executed only if a function is being compile time evaluated.  When code is
generated for runtime evaluation the else block would be used.  This would allow
problems like this to be solved in a well-encapsulated way.  Example:

uint[] findPrimes(uint maxPrime) {
    version(ctfe) {
        uint[] ret;
    } else {
        ArrayBuilder!uint ret;
    }

    foreach(i; 0..maxPrime) {
        if(!isPrime(i)) {
             continue;
        }

        version(ctfe) {
            ret = ret ~ i;
        } else {
            ret ~= i;
        }
    }
}

Given that CTFE will likely never support everything that is supported at runtime,
this will likely make it much more useful.