-preview=in might break code

[Walter Bright]

On 10/3/2020 7:48 AM, Adam D. Ruppe wrote:
> But maybe like you said later, the spec should say any `in` is treated at the 
> language level as a `ref` (except for rvalue issues of course) just optimized to 
> value in defined ABI places. That'd probably be good enough.

But if the user *expected* to change that const ref view of the data, and relied 
on it, then that behavior randomly breaks.