RfC for language feature: rvalue struct

[FeepingCreature]

On Sunday, 29 January 2023 at 17:41:26 UTC, Salih Dincer wrote:
> On Sunday, 29 January 2023 at 03:29:45 UTC, FeepingCreature 
> wrote:
>> No, I mean
>>
>> ```
>> num1 = num2;
>> onlineapp.d(22): Error: cannot modify struct instance `num1` 
>> of type `Foo!int` because it contains `const` or `immutable` 
>> members
>> ```
>
> Sorry for writing an answer without doing the necessary unit 
> tests.  I forgot to put the type in parentheses.  Here is the 
> corrected version:
>
> ```d
> struct S(T)
> {
>   import std.conv   : to;
>   import std.traits : ImmutableOf;
>
>   immutable(T)[] data;
>
>   this(R)(R[] data)
>   {
>     this.data = data.to!(ImmutableOf!T[]);
>   }
>
>   string toString() const
>   {
>     import std.format : format;
>     return format("%s: %s", typeid(data), data);
>   }
> }
>
> unittest
> {
>   S!char test1, test2;
>   alias TestType = typeof(test1.data);
>
>   import std.traits : isSomeString;
>   assert(is(TestType == string) &&
>             isSomeString!TestType);
>
>   test1 = S!char("bcd");
>   test1.data ~= "234";
>
>   test2 = test1;
>   assert(test1 == test2);
> }
>
> void main()
> {
>   import std.stdio;
>
>   string str = "bcd";
>
>   auto str1 = S!char(str.dup);
>   auto str2 = S!char("abc");
>
>   str1 = str2;
>
>   str1.writeln; // const(immutable(char)[]): abc
>   str2.writeln; // const(immutable(char)[]): abc
> }
> ```
>
> SDB at 79

I think you're missing the concept a bit:

```
test1 = S!char("bcd");
test1.data ~= "234";

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".