Documentation re -betterC compatibility with standard library functions

[Jonathan M Davis]

On Tuesday, January 28, 2025 6:01:56 AM MST DLearner via Digitalmars-d-learn wrote:
> Is there a definitive list somewhere of standard library
> functions that work with -betterC?
>
> For example, the following code fragment (taken from the library
> docs) does not work with -betterC.
> ```
> extern(C) void main() {
>     import std.container.array;
>
>     auto arr = Array!int(0, 2, 3);
>
>
> }
> ```

No, certainly not an official one, and it arguably wouldn't make sense to
make one, because we do not support -betterC with druntimte or Phobos, and
for the most part, anywhere it works is accidental. In a few cases, someone
went in and explicitly made something work with -betterC, but we provide no
guarantees that any code that works with -betterC right now will continue to
do so, and we do not intend to add -betterC support. So, if you use
something from Phobos that happens to work with -betterC today, it could
stop working with -betterC tomorrow if a change is made to fix or improve
something, and it's not compatible with -betterC.

-betterC was originally intended to be a tool for making it easier to port C
code to D code (after which, the idea was that -betterC would no longer be
used, and you would just have a normal D program). It was never the goal to
use it to create a secondary version of D. Some folks have elected to use it
that way, and that's fine, but you're basically on your own if you're using
-betterC. The compiler is really the only thing that explicitly supports it,
and of course, it doesn't support all D features with it, because it can't.

The closest that you're likely to ever get for -betterC support is that we
want to try to make it so that stuff in druntime is more pay as you go -
that is, if it really doesn't need something, it doesn't pull it in, so you
then hopefully you don't pull in code that you're not using - and that would
naturally make more stuff work with -betterC, but we're not going to
explicitly support -betterC with druntime or Phobos.

- Jonathan M Davis