[Issue 19268] BetterC turns off DRuntime for CTFE

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19268

--- Comment #12 from Adam D. Ruppe <destructionator at gmail.com> ---
(In reply to Steven Schveighoffer from comment #9)
> Except this is not the problem. The CTFE part DOES compile.

eeeeh, it is the problem that brought me to this bug.

private string makefoo() {
        if(__ctfe) {
                string a = "b";
                a ~= "c\0";
                return a;
        }
        assert(0);
}

enum foo = makefoo();

extern(C) int main() {
        import core.stdc.stdio;
        printf("%s", foo.ptr);
        return 0;
}

Here that private function is obviously intended to be CTFE only - the
`private` and `if(__ctfe)` make that extremely clear to a human reader, but the
compiler doesn't get the hint.

We could also do a like `pragma(ct_only)` maybe just I was hoping this pattern
would work since it is in the existing language. But nope :(

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