How about some __initialize magic?

[vit]

On Saturday, 27 November 2021 at 21:56:05 UTC, Stanislav Blinov 
wrote:
> ...

Nice idea, placement new really missing from D.

I have reverse problem with emplace then you.
emplaceRef wrongly infers @safe attribute for non ctfe code if 
assignment is @system because of this:

```d
//emplaceRef:
if (__ctfe)
     ///...
     chunk = forward!(args[0]);
     ///...
}
```

Example:
```d
struct Foo{

     this(scope ref const typeof(this) rhs)@safe{}

     void opAssign(scope ref const typeof(this) rhs)@system{}
}


void main()@safe{
     import core.lifetime : emplace;

     Foo foo;
     {
         const Foo* ptr;
         emplace(ptr, foo);	//OK __ctfe path doesn't exists
     }
     {
         Foo* ptr;
         emplace(ptr, foo);	//ERROR __ctfe path exists and call 
@system opAssign
     }
}
```
Error: `@safe` function `D main` cannot call `@system` function 
`core.lifetime.emplace!(Foo, Foo).emplace`


D has one good thing, you can create custom emplace which run 
your own code between emplaceInitialize and ctor. You can 
initialize your own vptr before ctor.