Deprecating this(this)

[Steven Schveighoffer]

On 4/2/18 11:57 AM, Andrei Alexandrescu wrote:
> On 04/02/2018 10:59 AM, ag0aep6g wrote:
>>
>> That wouldn't be possible if `innocent` were only head-mutable in the 
>> postblit function, instead of fully mutable as it is currently (bug).
>>
>> `innocent` would be typed as `immutable(int)[]`, and you could not 
>> assign it to the fully mutable `sneaky`.
> 
> Problem is we don't have head-mutable in the language.

This is the wrong way to look at it. Head mutable is a specialized 
concept already implemented in constructors:

struct S
{
    int x;
    int *ptr;
    int *ptr2;
    this(int xval, immutable int *ptrval, int *cantuse) immutable
    {
       x = xval; // ok, head mutable
       //x = x + 1; // error, immutable field initialized multiple times
       ptr = ptrval; // ok
       // ptr2 = cantuse; // error, can't assign mutable to immutable
    }
}

We could have the same mechanism for postblit. Essentially, you should 
be able to assign immutable fields once, but they shouldn't lose their 
const protections (note the cantuse example).

As was mentioned, because postblit on an immutable (or const) is ONLY 
allowed for new data, there shouldn't be an issue.

-Steve