-preview=in might break code

[Iain Buclaw]

On Sunday, 4 October 2020 at 09:15:36 UTC, Joseph Rushton 
Wakeling wrote:
>
> Platform-dependent, yes -- but the behavioural difference here 
> can be 100% anticipated from the language rules (and hence, 
> from the code), no?
>
> Isn't the issue with `-preview="in"` as currently implemented 
> that _there is no way_ for the reader of the code to anticipate 
> when ref will be used and when not?

As it is an optimization, I think that's best left to the 
interpretation of the compiler/compiler author, however 
optimizations should never break the semantic guarantee.

```
void fun1(const long[16] a, ref long[16] b, in long[16] c, out 
long[16] d);
fun1(var, var, var, var);

void fun2(in long[16] a, int b);
fun2(var, var[0]);
```

In the above, all parameters are passed in memory as per ABI 
requirements.  The difference lies in whether or not `var` is 
passed by reference directly or a via a temporary copy.

When it comes to `in` parameters, I'm of the opinion that a 
temporary should be passed if the type is trivially copy-able.  
But as an optimization, if we look at the signature and determine 
that it's not possible for any parameters to be aliasing each 
other, then why not pass the `in` parameter as `ref`?