RfC for language feature: rvalue struct

[Salih Dincer]

On Sunday, 29 January 2023 at 20:48:11 UTC, FeepingCreature wrote:
>
> This should emphatically *not* work. You're mutating a field of 
> `test1`. That's what we don't want to ever happen!
>
> Instead, you should have to write `test1 = test1(test1.data ~ 
> "234");`
>
> This looks extremely similar, but the key is that the 
> assignment has to go through the constructor. That way, `test1` 
> can only go from "valid value" to "valid value".

In order to use it as you say, the following opCall() should be 
implemented:

```d
   this(R)(R[] data) { opCall(data); }

   alias opCall this;
   @property opCall() inout { return data; }
   @property opCall(R)(R[] data)
   {
     return this.data = data.to!(ImmutableOf!T[]);
   }
```

Then it works like this:

```d
   immutable arr = [ 1, 2, 3];

   auto num1 = S!int(arr.dup);
   auto num2 = S!int(arr);

   assert(is(typeof(num1) == S!int));
   assert(is(typeof(num1.data) == immutable(int)[]));

   num1(num1 ~ 4); // or:
   num1 = num1 ~ 5;

   assert(num1.length == 5);
   assert(num1.length > num2.length);

   num2 = num1;
   assert(num1.length == num2.length);
```

But `num1 ~= 6` doesn't work because it doesn't have an overload:

> Error: cannot append type `int` to type `S!int`

SDB at 79