RfC for language feature: rvalue struct

[Salih Dincer]

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