[Issue 19268] BetterC turns off DRuntime for CTFE

[d-bugmail at puremagic.com]

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

--- Comment #6 from Adam D. Ruppe <destructionator at gmail.com> ---
I think my proposal would be that *specifically* the `if(__ctfe)` construct
overrides the betterC checks, since that can be easily factored out.

Just like how we can write

---
int* do_something() {
   if(__ctfe) {
       return new int;
   } else {
       return cast(int*) malloc(int.sizeof);
   }
}
enum e = *do_something();
---

today (which works in real D, including without linking druntime btw), which
bypasses the normal "malloc cannot be evaluated at compile time), we could
conceivably use it in the other direction to allow something in a CT context
that isn't allowed in a RT context.

I'm not sure how to write that in spec language... it might be able to just say
specifically that betterC's restrictions do not apply inside that specific ast
node and like specifically hack the hack that is betterC.


I think that would be fine with `__traits(compiles)` as well. The `if(__ctfe)`
switch keeps everything compiling the same way - it is specifically a runtime
branch in terms of formal semantics. So the compiles thing passes because that
if branch is encapsulated still.

--