D2.0: an example of use-case for casting invariant away

[Don Clugston]

Bruno Medeiros wrote:
> Don Clugston wrote:
>> Regan Heath wrote:
>>> Walter Bright Wrote:
>>>> Don Clugston wrote:
>>>>> Walter Bright wrote:
>>>>>> With D, you can cast away const-ness, that is legal. But if you 
>>>>>> subsequently modify the underlying data, that is undefined behavior.
>>>>> It sounds that in D, it will be too easy to cast away constness 
>>>>> accidentally.
>>>>> With C++, at least you can grep for const_cast and detect 
>>>>> potentially dangerous code, and you get a strong visual clue.
>>>>> Suppose I've written a D function like this:
>>>>>
>>>>> void f(int *b, uint c)
>>>>> {
>>>>>   // maybe I'm avoiding a compiler warning or something.
>>>>>   uint *d = cast(uint *)b;
>>>>>   d += c;
>>>>> }
>>>>>
>>>>> Months later, I'm refactoring the code, and I convert the int * 
>>>>> parameter to an invariant, without recognising that it's changing 
>>>>> the value of b. Oops.
>>>>>
>>>>> C++'s const would catch this mistake, but if I understand 
>>>>> correctly, D will compile it without error. Suddenly the function 
>>>>> has moved into the realm of undefined behaviour.
>>>>>
>>>>> I hope I'm wrong. Or did I miss something?
>>>> No, you're not missing something. It is a general problem with cast 
>>>> - cast is a blunt instrument which can easily hide problems.
>>>
>>> So.. we're going to have to put up with this potential nasty bug?
>>>
>>> What about a new cast which only removes 'const' and/or 'invariant 
>>> and prohibit normal cast from removing it.
>>>
>>> Perhaps calling it 'vary', eg.
>>>
>>> void f(const int *b, uint c)
>>> {
>>>    int *d = vary() b;
>>> }
>>
>> To avoid a new keyword...
>>
>> int* d = cast(break const) b;
>>
>> IMHO, we want something that looks really nasty.
> 
> New keyword? Why a new syntax a at all? This is the kind of stuff that 
> should be possible to do with D's (current or future) meta programming 
> capabilities. Here's a small proof of concept of what can currently be 
> done in D:
[snip]

Because as long as _any_ cast can remove const, you haven't fixed the problem.
The challenge is to make it extremely difficult to remove const, but still 
possible. And easy to grep for instances of it.