`restricted` member variables

[The Zealot]

On Tuesday, 21 June 2022 at 11:20:46 UTC, Ola Fosheim Grøstad 
wrote:
> On Tuesday, 21 June 2022 at 10:36:32 UTC, The Zealot wrote:
>> if anything, i'd like to see more control over when invariants 
>> are actually checked.
>>
>> something like
>> ```
>> invariant(default)
>>     {
>>         assert(1 <= day && day <= 31);
>>         assert(0 <= hour && hour < 24);
>>     }
>> invariant(always) {
>>         assert(1 <= sec && sec <= 60);
>> }
>> ```
>
> You can already do this, just add an or-clause to a compile 
> time boolean constant.
>
> ```d
> assert((!precise_invariant) || (1 <= day && day <= 31));
> ```
>
> But the class invariant should only be tested on 
> member-function entry in debug-mode.
>
> In release mode the invariant should be assumed to hold on 
> entry and can be used for optimizations (it should still be 
> tested on exit).
>
> So this:
>
> ```d
>
> class always_even {
>    private int x;
>    invariant { assert(x%2==0); }
>    void triple(){
>       x *= 3;
>       if (x&1) writeln("found one");
>    }
>    // more code
> }
>
> ```
>
> should become this:
>
> ```d
>
> class always_even {
>    private int x;
>    tripple(){
>       // assume(x%2==0)
>       x *= 3;
>       assert(x%2==0);
>    }
>    …
> }
>
> ```
>
> Or even this (with a decent optimizer):
> ```d
>
> class always_even {
>    private int x;
>    tripple(){
>       // assume(x%2==0)
>       x *= 3;
>       // assert(x%2==0);
>    }
>    …
> }
>
> ```
>
> With invariants turned off, the result will be worse as there 
> is no indication that the lowest bit of x is constant 0:
>
> ```d
> class always_even {
>    private int x;
>    void triple(){
>       x *= 3;
>       if (x&1) writeln("found one");
>    }
>    …
> }
>
> ```

sorry, i wasn't clear what i meant. i meant add the ability to 
control when invariants checks are inserted by the compiler. for 
example a way to specify that you want the invariant checked 
after each access to any member, as well as "the class invariant 
should only be tested on member-function entry in debug-mode".
but it's mainly a suggestion for performance reasons.