When D feels unfinished: union initialization and NRVO

[Mathias Lang]

On Thursday, 19 March 2020 at 13:34:11 UTC, kinke wrote:
>
> Another simple workaround:
>
> import core.lifetime : move;
>
> ...
> case Good.Baguette:
>     Foo f = { type_: type, f1: typeof(Foo.f1)("Hello World") };
>     return move(f);
> ...

 From my tests, it seems to do NRVO on LDC, so I suppose you 
recognize it ?
On DMD, it just does a blit, which can still be expensive and 
breaks interior pointers.

Another major advantage, which I also realized recently, is that 
having NRVO all the way means that the stack usage is bounded. 
Beside the obvious embedded code, it matters a lot when using 
Fibers (we have a Vibe.d style handling of connection, with 1 
connection == 1 fiber).

I also found out that one of the earlier case I mentioned 
(https://gist.github.com/Geod24/61ef0d8c57c3916cd3dd7611eac8234e#file-nrvo_struct_ctor-d) only does NRVO on LDC. DMD (and GDC 9.3.0) just silently move it.