RfC for language feature: rvalue struct

[FeepingCreature]

On Tuesday, 31 January 2023 at 18:30:44 UTC, Ola Fosheim Grøstad 
wrote:
> On Tuesday, 31 January 2023 at 18:13:58 UTC, Ola Fosheim 
> Grøstad wrote:
>> E.g. C# introduced "record" in addition to "struct" and 
>> "class" to get slightly different value semantics, but as far 
>> as I can tell it is only a rewrite.
>
> Or maybe it isn't (shrug). Anyway, you might want to look at 
> the C# "with" statement:
>
> https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/tutorials/records#non-destructive-mutation

Yes, that's exactly what we want!

What we use right now for `with` is

```
immutable struct S { int value; mixin(GenerateThis); }
S s = S(5);
S butFour = s.rebuild!(a => a.value = 4);
```

Which seems to work fine.