Helping with __mutable (which will be renamed to __metadata)

[Timon Gehr]

On 14.04.19 22:55, Suleyman wrote:
> On Sunday, 14 April 2019 at 00:38:16 UTC, Timon Gehr wrote:
>> I don't think it should be @safe. Rather, `pure` and `immutable` 
>> should retain their meanings,
> 
> It cannot be strongly pure. Same as we have weakly pure this is weakly 
> immutable we're dealing with here.
> ...

No, the point is that whatever high-level rewrites you can deduce from 
`immutable` and `pure` shouldn't be restricted by code that modifies 
`__metadata`.

>> which implies that there are wrong ways to use `__mutable` (hence unsafe)
> 
> can you elaborate?
> 

struct S{
     private __metadata x;
}

void foo(immutable ref S s)pure{
     s.x += 1;
}

void main(){
     immutable S s;
     foo(s); // there is no reason for this call to happen
     assert(s.x==1); // can't rely on this, it might also be 0
}

struct S{
     private __mutable x;
}

int foo(immutable ref S s)pure{
     s.x += 1;
     return s.x;
}

void main(){
     immutable S s;
     int a=foo(s);
     int b=foo(s); // could just reuse the previous result
     assert(a!=b); // can't rely on this, might be the same
}