Preview features status?

[Mathias LANG]

On Tuesday, 13 July 2021 at 18:20:36 UTC, evilrat wrote:
> On Tuesday, 13 July 2021 at 15:51:26 UTC, Tejas wrote:
>>
>> Shouldn't this be a bug? Isn't this still ```const - 
>> non-const```?
>> Do you think this is worth filing?
>
> Definitely a bug from usability point. But...
>
> Well, seems like it is just yet another half baked feature 
> according to this GH issue, also it seems like it was supposed 
> to be superseded by relevant "in" qualifier for extern(C++) but 
> that DIP seems to be abandoned as well.
>
> https://github.com/dlang/dmd/pull/12064
>
>> Also, where did you find out how to represent rvalue 
>> references as parameters? I couldn't find any info on that. 
>> Just learned that these are available under a preview switch.
>
> Can't find it anywhere in release notes, it was just silently 
> showed up around Apr 2019.

Here's the full picture from my POV:

`-preview=rvaluerefparams` was [implemented by 
Walter](https://github.com/dlang/dmd/pull/9817) and later 
[documented](https://github.com/dlang/dmd/pull/10633). However 
when I tried to use it, I ran into the three issues that were 
linked in the PR you mentioned. I originally started working on 
fixing those, but realized that there were many unanswered 
questions. One of them was, what to do with mutable parameters ? 
This was one of the main rationale provided when [the 
DIP](https://github.com/dlang/DIPs/blob/master/DIPs/rejected/DIP1016.md) was rejected.

This brought me back to an old idea of mine, which was to have 
`in` means `const scope [ref]` (`ref` when it makes sense), but 
the fact that it would conflict with passing rvalues to a 
function was always off-putting. Adding rvalue ref matching just 
made sense, so I went ahead and [implemented 
it](https://github.com/dlang/dmd/pull/11000). I also made sure 
druntime/Phobos, Vibe.d, [and many other packages on 
Buildkite](https://github.com/dlang/dmd/pull/11632) were 
compatible.

I've been using it in production as soon as I could, which was 
v2.094.0 because [`-preview` flags and DUB didn't play along well 
before](https://github.com/dlang/dub/pull/2040). It's been a 
breeze, and doing everything exactly as we want it done, so I'm 
very happy with the result.

However, there has been some serious backlash from a few people. 
So I don't think it's going to be made the default any time soon 
(or at least, start the process), because it seems Walter is 
among those who doesn't like it. So unless we manage to convince 
the BDFLs that it's useful, it's not going out of `-preview`. 
However, I'll make sure it works in every release, as I use it 
everywhere.