RfC for language feature: rvalue struct

[FeepingCreature]

On Monday, 30 January 2023 at 20:15:48 UTC, Dukc wrote:
> I don't see a need for fields to be non-referenceable since 
> immutability protects them if the data in question is already 
> committed.

Yes, again, the idea is that rvalue allows *overwriting mutable 
variables* of this type.

```
rvalue struct S {
   int a;
}

S s;
s = S(5);
s = S(6);
```

Which `immutable` does not allow.

And this is important because this idiom is inescapable, it will 
always crop up (all over std.algorithm causing compile errors for 
our immutable structs, cough), and in fact I increasingly think 
it's just *good.* It's an inherent part of D's C heritage.