Discussion: Rvalue refs and a Move construtor for D

[kinke]

On Thursday, 5 September 2019 at 21:45:10 UTC, Manu wrote:
> On Thu, Sep 5, 2019 at 2:40 PM kinke via Digitalmars-d 
> <digitalmars-d at puremagic.com> wrote:
>>
>> On Thursday, 5 September 2019 at 19:38:57 UTC, Suleyman wrote:
>> > The whole program only calls the destructor for the lvalue, 
>> > and only once. You need a competitive alternative.
>>
>> Sry, I haven't focused on that - yes, rvalue refs would be 
>> even faster because of elided destruction + T.init reset. For 
>> hard-core guys wanting to optimize that away, I'd suggest to 
>> just use a regular ref in the function signature and enable 
>> `-preview=rvaluerefparam`. A regular ref (combined with either 
>> pragma(mangle) tricks or some UDA for C++ mangling) could also 
>> be used to represent C++ functions with rvalue refs.
>
> Please no. This is a terrible series of sentences >_< 
> `-preview=rvaluerefparam` naturally interacts benefitially with 
> the proposed `@rvalue` attribute. Don't mess with that.

Care to elaborate? Suleyman's example can be slightly adapted to 
this:

void foo(ref S value, int n = 0)
{
     if (n > 32)
         return;

     foo(value, n + 1);
}

With `-preview=rvaluerefparam`, we can feed it with an rvalue 
too, and get exactly the same behavior as Suleyman's original 
version, without `@rvalue ref` and the need to move the `value` 
parameter for the recursive call.