Choosing an Approach for the Template Lowering of _d_arrayctor

[Stanislav Blinov]

On Thursday, 25 November 2021 at 14:35:56 UTC, Teodor Dutu wrote:
> On Thursday, 25 November 2021 at 14:24:59 UTC, Stanislav Blinov 
> wrote:
>> On Thursday, 25 November 2021 at 13:28:18 UTC, Teodor Dutu 
>> wrote:
>>
>>> A third approach, which I call the **hack** approach, is very 
>>> close to the one using the template. The difference is that 
>>> the hack adds a third, unused, pointer-type parameter...
>>
>> Hm... if you're going to have a pointer parameter, why not 
>> instead simply keep the two parameter variant but pass the 
>> first parameter by reference?
>
> The first iteration of `_d_arrayctor` was like this, as 
> implemented by [this 
> PR](https://github.com/dlang/druntime/pull/2655). The problem 
> with it was that it made the compiler issue some warnings when 
> testing phobos, because the lowering of something like `S[3] 
> dst = src;` looked like this: `_d_arrayctor(dst[], src[])`. 
> `_d_arrayctor` was strongly pure and, since the return value of 
> the function was ignored, the compiler deemed the whole 
> function call useless.

If you take the first parameter by reference, that shouldn't 
happen. I.e. in

```d
void _d_arrayctor(A,B)(return ref A to, scope B from)
if (is(A : X[]) && is (B : Y[]) && 
is(immutable(typeof(A.init[0])) == immutable(typeof(B.init[0])))) 
{ /* ... */ }
```

`A` would be a static array, taken by reference.